The Arabidopsis Book 2006 American Society of Plant Biologists
Introductionn
Meiosis is a specialized cell division characteristic of
eukaryotes and is essential for sexual reproduction. The
function of meiosis is to generate cells that contain exactly
half of the genetic materials of the parental cells and that
develop into germ cells. Consequently, the fusion of germ
cells can then restore to the offspring the ploidy and level of
genetic complexity of the parent(s). In mitosis, DNA replica-
tion is followed by one round of nuclear division, which sep-
arate sister chromatids. In contrast, in meiosis, a single
round of DNA replication is followed by two rounds of
nuclear division, meiosis I and meiosis II. Meiosis I is unique
and involves the segregation of homologous chromosomes
(homologs); this division reduces ploidy by half, thus is also
called reductional division. Meiosis II is very similar to mito-
sis in that it results in the segregation of sister chromatids;
therefore, it is also called equational division. Both meiosis I
and II are further divided into four phases: prophase,
metaphase, anaphase, and telophase, roughly similar to
those in mitosis.
Meiotic prophase I is a long and complex process and is
further divided into five substages according to cytological
features of the chromosomes: leptotene, zygotene,
pachytene, diplotene, and diakinesis (Dawe, 1998; Zickler
and Kleckner, 1999; Stack and Anderson, 2001; Armstrong
and Jones, 2003). Very early in prophase I, chromosomes
begin to condense, forming thin thread-like structures called
the axial element; this substage is called leptotene. Because
the purpose of meiosis I is to properly separate homologs, it
is essential for the homologs to recognize each other during
early prophase I. The recognition between homologs is
achieved through homolog pairing. The term "pairing" has
been used to describe various interactions or associations
between homologs (Zickler and Kleckner, 1999). In this
chapter, it is used with a narrow definition for the transient
interaction at localized regions of homologs that occurs
prior to synapsis (Zickler and Kleckner, 1999). In most
organisms, pairing depends on DNA homology to verify that
the interacting chromosomes are indeed homologous.
Pairing then progresses to homolog juxtaposition, which is
also homology-dependent and refers to the coming togeth-
AA Molecularr Portraitt off Arabidopsiss Meiosis
Hongg Ma
Department of Biology, the Huck Institutes of the Life Sciences, and the Institute of Molecular Evolutionary Genetics, The
Pennsylvania State University, University Park, PA 16802; e-mail: hxm16@psu.edu
Keyy words:: pairing, recombination, synapsis, crossover formation, interference, spindle assembly
Abbreviations:: DAPI, 4',6-diamidino-2-phenylindole; DSB, double strand break; DSBR, double strand break repair; SC,
synaptonemal complex; TEM, transmission electron microscopy
Abstract
Meiosiss iss essentiall forr eukaryoticc sexuall reproductionn anndd importantt forr geneticc diversityy amongg individuals.. Effortss dur-
ingg thee lastt decadee inn Arabidopsiiss havee greatlyy expandedd ourr understandingg off thee molecularr basiss off plantt meiosis,, which
hass traditiionallyy providedd muchh informationn aboutt thee cytologicall descriptionn off meiosis.. Throughh bothh forwardd genetic
analysiss off mutantss withh reducedd fertilityy andd reversee geneticc studiess off homologss off knownn meioticc genes,, wee noww havee a
basicc knowledgee aboutt geness importantt forr meioticc recombinationn andd ittss relationshipp too pairingg andd synapsis,, critical
processess thatt ensuree properr homologg segregation.. Inn addition,, severall geness affectingg meioticc progression,, spindle
assembly,, chromosomee separation,, aandd meioticc cytokinesiss havee alsoo beenn uncoveredd andd characterized.. Itt iss worthh not-
ingg thatt Arabiddopsiss molecularr geneticc studiess aree alsoo revealingg secretss off meiosiss thatt havee nott yett beenn recogniizedd else-
wheree amongg eukaryotes,, includingg genee functionss thatt mightt bee uniquee too plantss andd thosee thatt aree potentiallyy sharedd with
animalss andd fungi.. Ass wee enterr thee post-genomicss eraa off plantt biollogy,, theree iss noo doubtt thatt thee nextt tenn yearss willl see
ann evenn greaterr numberr off discoveriess inn thiiss importantt areaa off plantt developmentt andd celll biology.
The Arabidopsis Book 2006 American Society of Plant Biologists
First published on June 6, 2006; doi: 10.1199/tab.0095
The Arabidopsis Book 2 of 39
er of homologs after the initial transient interaction (Zickler
and Kleckner, 1999).
Synapsis is an extensive and stable interaction between
chromosomes that involves the formation of a complex
proteinacious structure called the synaptonemal complex
(SC) (Roeder, 1990). When homologs are partial synapsed,
parts of homologs can be visibly detected as "fused"
together, at a substage called zzyyggootteennee. Using electron
microscopy, the SC can be seen as a tripartite proteina-
ceous structure (Zickler and Kleckner, 1999): two parallel
filementous "lateral elements" are from the axial elements.
A central element forms in between and parallel to the lat-
eral elements and is connected to the lateral elements by
the transverse elements (Zickler and Kleckner, 1999;
Schwarzacher, 2003; Higgins et al., 2005). The central ele-
ment consists of the heads of the proteins that form the
transverse elements (Zickler and Kleckner, 1999; Higgins
et al., 2005). When the homologs are fully synapsed with
the completion of the SC, the stage is called the ppaacchhyytteennee
substage, with each pair of synapsed homologs appearing
under a light microscope as a thick thread-like structure,
although the SC proper requires electron microscopy to
observe. During normal meiosis, SC is formed between
homologs and is closely coupled with homolog pairing and
juxtaposition; however, in some yeast and maize mutants,
SC can also form between non-homologous chromo-
somes (Leu et al., 1998; Pawlowski et al., 2004) suggest-
ing that synapsis per se does not require homology and
can be uncoupled from pairing. After pachytene, the SC is
disassembled and homologs partially separate, except at
the chiasmata, which are cytological features correspon-
ding to crossing over points between non-sister chro-
matids; this substage is recognized as ddiippllootteennee.
Subsequent further chromosome condensation results in
highly compacted bivalents (attached pairs of homologs),
reaching the ddiiaakkiinneessiiss substage, the end of prophase I.
In addition to pairing, juxtaposition, and synapsis,
homologs also undergo recombination during prophase I,
resulting in crossovers (exchange of chromosomal regions)
and gene conversion (copying of sequence from one
homolog to the other). Under an electron microscope,
electron dense bodies called meiotic nodules can be
detected along the chromosome; they are also called
recombination nodules because their number and distribu-
tion are correlated with those of recombination (Carpenter,
1975, 1987; Sherman et al., 1992; Sherman and Stack,
1995; Anderson et al., 2001). Recombinational crossover,
along with sister chromatid cohesion, ensures the forma-
tion of chiasmata, which are the physical links between
homologs in a bivalent at late prophase I through the
metaphase I/anaphase I transition. In the absence of
crossovers, homologs would separate following the disas-
sembly of the SC, resulting in detached chromosomes
called univalents, which cannot align properly at the
metaphase plate and segregate correctly at anaphase I.
Therefore, at the level of an individual organism, recombi-
nation is critical for the reproductive success, because it
maintains homolog association until the metaphase
I/anaphase I transition and allows correct segregation of
homologs at anaphase I. At the population and species
levels, meiotic recombination, along with independent
assortment of chromosomes, facilitates the redistribution
of genetic material between generations, and increases
genetic diversity among individuals in a population. This
obviously has great ecological and evolutionary implica-
tions, making meiosis and particularly meiotic recombina-
tion important genetic mechanisms that have likely con-
tributed to the enormous success of eukaryotes.
In flowering plants, meiosis occurs in both the male
organ anther, and the female structure called the ovule,
which is housed within the ovary. Both male and female
meioses produce haploid spores, which then develop fur-
ther through mitoses and cell differentiation into gameto-
phytes. Because of the ease with which one can obtain
male meiocytes and observe chromosome behaviors in a
number of flowering plants, male meiosis in these plants
has been studied extensively using cytogenetics and
genetics (Golubovskaya, 1979; Kaul and Murphy, 1985;
Dawe et al., 1994; Neuffer et al., 1997; Dawe, 1998; Zickler
and Kleckner, 1998, 1999; Hamant et al., 2006).
Chromosomal behavior, including condensation, pairing,
and synapsis have been described for a number of plants
using light and electron microscopy. Traditionally, plants
with many meiocytes and very large chromosomes, such
as lily, tomato, and maize, have been favored (Moens,
1969; Stack, 1973; Anderson et al., 1985; Anderson et al.,
1988; Sherman and Stack, 1995; Anderson et al., 1997;
Peterson et al., 1999; Stack and Anderson, 2001). In par-
ticular, maize has been a favorite organism for decades
and is an excellent system for genetic and cytological
analyses (Golubovskaya, 1979; Anderson et al., 1988;
Golubovskaya, 1989; Staiger and Cande, 1991, , 1992;
Golubovskaya et al., 1993; Dawe et al., 1994; Dawe and
Cande, 1996; Golubovskaya et al., 1997; Yu et al., 1997;
Franklin et al., 1999; Golubovskaya et al., 2002; Anderson
et al., 2003; Pawlowski et al., 2003; Anderson et al., 2004;
Pawlowski et al., 2004).
In recent years, cytological procedures have been devel-
oped for Arabidopsis thaliana (Peirson et al., 1996; Ross et
al., 1996; Klimyuk and Jones, 1997; Ross et al., 1997;
Yang et al., 1999; Caryl et al., 2000; Armstrong and Jones,
2001; Grelon et al., 2001; Azumi et al., 2002; Chen et al.,
2002; Caryl et al., 2003), even though it is well-known for
its small chromosomes. These technical advances in com-
bination with the molecular genetic properties of
Arabidopsis (Meyerowitz, 1987, , 1989) and the availability
of the complete genomic sequence and publicly available
collections of insertional lines (Parinov et al., 1999; AGI,
A Molecular Portrait of Arabidopsis Meiosis 3 of 39
2000; Alonso et al., 2003) have greatly facilitated the iden-
tification and characterization of meiotic genes using both
forward and reverse genetics. This chapter will first briefly
describe normal meiosis in Arabidopsis, and then present
a series of discussions of genes that are important for
meiosis (Table 1), roughly according to the progression of
the meiotic process. For studies in others plants, the read-
er is referred to a number of excellent reviews (Dawe,
1998; Zickler and Kleckner, 1999; Schwarzacher, 2003;
Harper et al., 2004; Pawlowski and Cande, 2005; Hamant
et al., 2006). Much information about meiotic genes has
been obtained from studies in the budding yeast
Saccharomyces cerevisiae; unless otherwise indicated, in
this chapter yeast refers to S. cerevisiae. Because meiosis
is a conserved process common to eukaryotes, other
reviews on meiosis in yeast and other non-plant systems
also offer valuable references (Zickler and Kleckner, 1999;
Haber, 2000; Page and Hawley, 2003; Bishop and Zickler,
2004; Page and Hawley, 2004).
NNoorrmmaall AArraabbiiddooppssiiss MMeeiioossiiss
In Arabidopsis, male meiosis occurs in the anther at anther
stage 6 as define by Sanders et al. (1999), when the meio-
cytes are surrounded by four layers of somatic cells,
including the tapetum (Ma, 2005). This is within stage 9 of
flower development (Smyth et al., 1990). Each Arabidopsis
flower has six anthers, each of which contains four lobes;
within each lobe, there are approximately 30 meiocytes,
giving rise to about 700 meiocytes per flower. To visualize
meiotic chromosomes, a procedure called chromosome
spreading is used to partially separate the chromosomes
after limited digestion of the meiocytes to loosen the cell
walls; this procedure has been widely used to study
Arabidopsis meiotic mutants (Peirson et al., 1996; Ross et
al., 1996; Klimyuk and Jones, 1997; Ross et al., 1997;
Yang et al., 1999; Caryl et al., 2000; Siddiqi et al., 2000;
Armstrong and Jones, 2001; Grelon et al., 2001; Sanchez
Moran et al., 2001; Azumi et al., 2002; Chen et al., 2002;
Caryl et al., 2003; Grelon et al., 2003; Yang et al., 2003a;
Yang et al., 2003b; Bleuyard and White, 2004; Higgins et
al., 2004; Li et al., 2004; Li et al., 2005).
Following spreading, the chromosome preparations are
stained with DAPI (4',6-diamidino-2-phenylindole) before
viewing under a fluorescence light microscope (Figure 1).
At leptotene, partially condensed meiotic chromosomes
are seen as thin lines (Figure 1A). From late leptotene to
early zygotene (Figure 1B), homologs pair and become
juxtaposed and then synapsed from zygotene through
pachytene (Figure 1C). At diplotene (Figure 1D), the SCs
are disassembled and the homologs separate in regions
other than the chiasma(ta). At diakinesis (Figure 1E), further
condensation results in very short chromosomal pairs,
seen as five bivalents. The bivalents move to the center of
the cell and are aligned at the equatorial plane at
metaphase I (Figure 1F), due to the forces of the spindle
microtubules. At the metaphase I/anaphase I transition,
the removal of cohesins along chromosomal arms allows
the separation of homologs, which are then pulled to the
opposite poles of the spindle (not seen here) at anaphase
I (Figure 1G). At telophase I (Figure 1H), five chromosomes
decondense and form a cluster at each pole. The chromo-
somes re-condense during prophase II (Figure 1I), and the
two groups are separated from each other by an organelle
band. The two groups of chromosomes are aligned sepa-
rately at two division planes at metaphase II (Figure 1J).
Subsequently at anaphase II, separation and segregation
of sister chromatids form four clusters of new chromo-
somes (Figure 1K). At telophase II, chromosome decon-
densation results in four haploid nuclei (Figure 1L), which
are then packaged into four microspores by cytokinesis.
Unlike male meiosis, Arabidopsis female meiosis occurs
in only about 50 cells per flower, one in each ovule. The rel-
atively small number of female meiocytes and the fact that
each female meiocyte is surrounded by somatic cells of
the ovule make the analysis of female meiosis much hard-
er than that of male meiosis. This is generally true for mei-
otic studies in other plants, such as maize. Much greater
amounts of efforts are needed to obtain female meiotic
images at various stages (Figure 2), which are generally
very similar to the male counterparts. One important dis-
tinction is that female meiosis is asymmetric, resulting in
one functional megaspore, which develops into the female
gametophytes, and three others that degenerate. Unless
specifically indicated, meiosis refers to male meiosis in this
chapter.
In addition to DAPI-stained chromosome spreads, fluo-
rescence in situ hybridization (FISH) methods have been
applied to plant meiotic chromosomes, and are important
for studying homolog pairing and juxtaposition (Zhong et
al., 1996; Bass et al., 1997; Yu et al., 1997; Fransz et al.,
1998; Peterson et al., 1999; Fransz et al., 2000; Armstrong
et al., 2001). Furthermore, SCs and recombination nodules
have been observed using transmission electron
microscopy (TEM) (Armstrong et al., 2002; Schwarzacher,
2003; Higgins et al., 2004; Li et al., 2004; Li et al., 2005)
(Figure 3). Using TEM, it seems that full-length axial ele-
ments are formed prior to homolog synapsis (Figure 3),
unlike some organisms, in which SC formation is initiated
before the axial element is completely formed (Zickler and
Kleckner, 1999). The axial elements become the lateral ele-
ments of the SC following synapsis (Figure 3). The central
element is formed from the heads of the transverse ele-
ments, which are connected to the lateral elements (Zickler
and Kleckner, 1999; Schwarzacher, 2003). The light and
electron microscopic methods provide the means to
obtain a morphological basis for genetic and molecular
analyses of Arabidopsis meiosis.
TTaabbllee 11.. Arabidopsis meiotic genes
Gene
symbol a ID number Protein features Function References b
ASK1 At1g75950 SCF subunit Homolog separation Yang et al. 1999
Inhibit recombination Wang et al., 2005
ASK2 At5g42190 SCF subunit Homolog separation (?) Zhao et al., 2003
ASY1 At1g67370 HOP1 homolog Homolog pairing Caryl et al., 2000
Armstrong et al., 2002
ATK1 At4g21270 C-terminal kinesin Spindle assembly Chen et al, 2002
ATM At3g48190 ATM homolog Regulation of DNA repair Garcia et al., 2000
Garcia et al., 2003
BRCA2a At4g00020 BRCA2 homolog Meiotic recombination Siaud et al., 2004
BRCA2b At5g01630 BRCA2 homolog Meiotic recombination Siaud et al., 2004
CAP-E1=TTN3 At5g62410 SMC2 homolog Condensation Siddiqui et al., 2003
CAP-E2 At3g47460 SMC2 homolog Condensation Siddiqui et al., 2003
CDC45 At3g25100 CDC45 homolog Meiotic progression Stevens et al., 2004
DIF1=SYN1 At5g05490 REC8 homolog Sister chromatid cohesion Blatt et al., 1999
DMC1 At3g22880 DMC1 Meiotic recombination Doutriaux et al., 1998
Couteau et al., 1999
DUET=MMD1 At1g66170 PHD finger Meiotic progression Reddy et al., 2003
DYAD=SWI1 A5g51330 Novel Sister chromatid cohesion Siddiqi et al., 2000
Agashe et al., 2002
MEI1 t1g77320 BRCT motifs Meiotic DNA repair Grelon et al., 2003
MER3=RCK At3g27730 MER3 homolog Crossover formation Mercier et al., 2005
MMD1=DUET At1g66170 PHD finger Meiotic progression Yang et al., 2003b
MRE11 At5g54260 MRE11 homolog Meiotic DNA repair Hartung and Puchta, 1999
Puizina et al., 2004
MS5=TDM At4g20900 Novel Meiotic progression Glover et al., 1998
MSH2 At3g18524 MSH2 homolog Inhibit recombination Emmanuel et al., 2006
MSH4 At4g17380 MSH4 homolog Crossover formation Higgins et al., 2004
PTD At1g12790 Similar to ERCC1 Crossover formation Wijeratne et al., 2006
RAD50 At2g31970 RAD50 homolog Homolog pairing, synapsis, Gallego et al., 2001
recombination Bleuyard et al., 2004b
RAD51 At5g20850 RAD51 homolog Homolog pairing, synapsis, Doutriaux et al., 1998
recombination Li et al., 2005
RAD51C At2g45280 RAD51C homolog Homolog pairing, Abe et al., 2005
synapsis, recombination Bleuyard et al., 2005
Li et al., 2005
RCK=MER3 At3g27730 MER3 homolog Crossover formation, Chen et al., 2005
synapsis
SCC3 At2g47980 SCC3 homolog Sister chromatid cohesion Chelysheva et al., 2005
SDS At1g14750 Novel cyclin Homolog pairing, synapsis, Azumi et al., 2002
recombination Wang et al., 2004a
SMC1=TTN8 At3g54670 SMC1 homolog Highly expressed in Lam et al., 2005b
floral buds
SMC3=TTN7 At2g27170 SMC3 homolog Localized to chromosomes Lam et al., 2005b
and the spindle
SPO11-1 At3g13170 SPO11 homolog Homolog pairing, synapsis, Grelon et al., 2001
recombination
The Arabidopsis Book 4 of 39
TTaabbllee 11 ((ccoonnttiinnuueedd)).. Arabidopsis meiotic genes
Gene
symbol a ID number Protein features Function References b
STD=TES At3g43210 Kinesin Meiotic cytokinesis Hulskamp et al., 1997
SWI1=DYAD At5g51330 Novel Sister chromatid cohesion Mercier et al., 2001, 2003
SYN1=DIF1 At5g05490 Rec8 homolog Sister chromatid cohesion Bai et al., 1999
Cai et al., 2003
SYN2 At5g40840 Rec8 homolog Highly expressed in Dong et al., 2001
dividing cells
SYN3 At3g59550 Rec8 homolog Highly expressed in Dong et al., 2001
dividing cells
TAM=CycA1,2 At1g77390 Cyclin A Meiotic progression Magnard et al., 2001
Wang et al., 2004b
TDM=MS5 At4g20900 Novel Meiotic progression Ross et al., 1997
TES=STD At3g43210 Kinesin Meiotic cytokinesis Spielman et al., 1997
Yang et al., 2003a
XRCC3 At5g57450 XRCC3 homolog Homolog pairing, synapsis, Bleuyard and White, 2004
recombination Bleuyard et al., 2004a
ZYP1a At1g22260 Similar to ZIP1 Synapsis, meiotic Higgins et al., 2005
progression
ZYP1b At1g22275 Similar to ZIP1 Synapsis, meiotic Higgins et al., 2005
progression
aThe "At" in gene names for Arabidopsis thaliana is not shown here.
bFor additional references, please see text.
A Molecular Portrait of Arabidopsis Meiosis 5 of 39
CChhrroommoossoommee CCoonnddeennssaattiioonn aanndd
SSiisstteerr CChhrroommaattiidd CCoohheessiioonn
In both mitosis and meiosis, chromosome condensation
from long interphase fibers to highly compacted units is
essential for successful segregation. In meiosis, chromo-
somes condense twice, one for each of the meiotic divi-
sions. Analyses in animals and yeast have identified a
group of highly conserved proteins, called Structural
Maintenance of Chromosomes (SMCs) (Hirano, 2000;
Hirano, 2002). In eukaryotes, there are six distinct types of
SMC proteins, SMC1-6; among these, SMC2 and SMC4
form a heterodimer that is specifically required for conden-
sation, whereas others have different functions (Hirano,
2002). Because condensation is a common feature of both
mitosis and meiosis, genes required for condensation are
likely important for mitotic development, such as gameto-
phyte development and embryogenesis. At the same time,
due to the highly duplicated Arabidopsis genome, often a
specific function is fulfilled by two or more highly related
genes. Indeed, a recent report showed that Arabidopsis
possesses two highly similar SMC2 genes, called AtCAP-
E1 and AtCAP-E2 (for Chromosome Associated Protein
subunit E) (Siddiqui et al., 2003). AtCAP-E1 is expressed at
a level approximately six times that of AtCAP-E2. In addi-
tion, T-DNA insertional mutants in either gene are viable
and fertile, although the atcap-E1 mutant (called titan3 for
enlarged embryos) exhibits mild embryo development
defects (Liu and Meinke, 1998). In contrast, the atcap-E1
atcap-E2 double homozygous plants are embryonically
lethal; even the presence of one copy of AtCAP-E2 is not
sufficient for embryo viability. Therefore, possible meiotic
chromosome condensation defects of the double homozy-
gous plants could not be assessed. Nevertheless, the
authors reported that one copy of the AtCAP-E1 gene was
sufficient for viability and the male meiocytes in the mutant
plants exhibited abnormal chromosomal morphology at
metaphase I and anaphase I, consistent with a defect in
chromosome condensation. It is not known whether chro-
mosome condensation during early prophase I is abnormal
in these mutant meiocytes.
The Arabidopsis Book 6 of 39
Following DNA replication, the sister chromatids are
closely associated with each other via sister chromatid
cohesion (Michaelis et al., 1997; Nasmyth, 1999). In mito-
sis, sister chromatid cohesion is the primary means of
chromosomal association until the metaphase/anaphase
transition, when sister chromatid cohesion is removed to
allow sister separation. As mentioned in the introduction,
in meiosis I, sister chromatid cohesion, along with a
crossover, is important for maintaining the chiasmata,
which hold homologs together prior to the metaphase
I/anaphase I transition. Removal of meiotic cohesion along
the arm then allows the homologs to separate (Klein et al.,
1999; Orr-Weaver, 1999; Watanabe and Nurse, 1999; van
Heemst and Heyting, 2000). Therefore, sister chromatid
cohesion and its regulated removal is critical for meiosis.
Cohesion depends on a multiprotein complex called
cohesin, which has at least four proteins; two of the four
proteins, SMC1 and SMC3 are related to the SMC2 and
SMC4 condensins, whereas the other two, SCC1, and
SCC3 are specific subunits of cohesins (Losada et al.,
1998; Nasmyth, 1999; Orr-Weaver, 1999). In addition, there
is a meiosis-specific SCC1-like protein REC8; both SCC3
and REC8 in yeast are required for meiotic sister chromatid
cohesion (Klein et al., 1999; Orr-Weaver, 1999; Parisi et al.,
FFiigguurree 11.. Arabidopsis male meiosis. A, leptotene. B, zygotene; arrows indicate regions of pairing between homologs. C, pachytene. Fully synapsed chromosomes appear
as thick threads in light micrographs. D, diplotene. E, diakinesis; arrows point to the position of chiasmata and arrowheads indicate the centromeric regions. F, metaphase I.
G, anaphase I. H, telophase I. I, prophase II; the arrow points to the organelle band between the two groups of chromosomes. J, metaphase II. K, anaphase II. L, telophase II,
M. Four newly formed nuclei. The images were obtained from DAPI-stained chromosome spreads following the fixation of floral buds. These images were provided by W. Li
and were previously published in Ma (2005).
A Molecular Portrait of Arabidopsis Meiosis 7 of 39
1999; StoopMyer and Amon, 1999; Watanabe and Nurse,
1999).
In Arabidopsis, a gene named SYN1 or DIF1 has been
found to be a homolog of REC8 and mutant analysis indi-
cates it is essential for normal meiosis (Peirson et al., 1996;
Peirson et al., 1997; Bai et al., 1999; Bhatt et al., 1999). In
syn1/dif1 mutant meiocytes, sister chromatid cohesion is
abnormal; in addition, chromosome condensation is also
affected and mutant meiocytes produce chromosomal
fragments. Recent immunolocalization experiments (Cai et
al., 2003) using antibodies against the SYN1 protein indi-
cate that the SYN1 protein is localized to the meiotic chro-
mosomes from interphase to pachytene. At diplotene and
diakinesis, the amount of SYN1 signal on the chromo-
somes reduces dramatically and the signal seems to local-
ize to discrete regions of the chromosomes. Concurrent
with the reduction of the chromosomal SYN1 signals, there
is an increase in nucleoplasm SYN1 staining, suggesting
that SYN1 is released from the chromosome to the nucle-
oplasm. This is consistent with the idea that initially
cohesins are cleaved rather than completely degraded. At
metaphase I, the SYN1 signal is further reduced and limit-
ed to the chromosomes, only detectable at a few loci on
the highly condensed chromosomes; at anaphase I, SYN1
signals on chromosomes disappear. These results indicate
that SYN1 is localized to the arms, as is the case for Rec8
protein. Although SYN1 signal is not detected at the cen-
tromere during prophase I or metaphase I, it should still be
present at very low levels before anaphase I, as supported
by the loss of centromere cohesion at anaphase I in syn1
meiocytes (Cai et al., 2003).
From anaphase I to metaphase II, the sister chromatids
remain associated at the centromeres. Therefore, other
cohesins are likely responsible for sister chromatid cohe-
sion at the centromere responsible for sister association
until anaphase II. Arabidopsis has at least two other REC8
homologs, SYN2 and SYN3 (Dong et al., 2001). SYN2 and
SYN3 are distantly related to each other (18% amino acid
sequence identity) and to SYN1 (16% and 19% aa
sequence identity, respectively), with greater similarity near
the N- and C-terminal regions than the central region.
SYN2 and SYN3 are widely expressed, with elevated lev-
els in tissues with dividing cells, such as root and shoot
meristems, suggesting a role in mitosis. It is possible that
SYN2/3 may also be involved in sister cohesion during
meioses I and/or II, especially considering that SYN1 does
not seem to be localized to the centromere of meiotic
chromosomes.
Arabidopsis also has homologs for both SMC1 and
SMC3, AtSMC1 and AtSMC3, respectively, which are
expressed widely, consistent with a role in both mitosis
and meiosis (Lam et al., 2005b). Both AtSMC1 and
FFiigguurree 22.. Arabidopsis female meiosis. A. Leptotene. B. Zygotene. C.
Pachytene. D. Diakinesis. E. Metaphase I. F. Anaphase I. These images were pro-
vided by W. Li.
FFiigguurree 33.. Transmission electron microscopy of Arabidopsis male meiotic prophase I. A. A portion of a leptotene nucleus showing axial elements (arrow). B. A portion of a
zygotene nucleus shown a short SC (arrowhead) with a recombination nodule (arrow). C. A portion of a pachytene nucleus showing a long SC (arrowhead). Bar = 500 nm.
These images were provided by L. Timofejeva and were previously published in Ma (2005).
The Arabidopsis Book 8 of 39
AtSMC3 are expressed more highly in floral buds than
other tissues, with AtSMC1 at a higher level, and more
non-specifically, than AtSMC3. Both AtSMC1 and AtSMC3
are about 140 kD in size and have predicted domains sim-
ilar to other SMC1 and SMC3 proteins: an N-terminal ATP-
binding domain, two long coiled-coil regions flanking a
flexible hinge region, and a C-terminal DA domain (Hirano,
2002). Protein localization studies using cell fractionation
and antibodies indicate that AtSMC3 is found in both the
cytoplasm and the nucleus (Lam et al., 2005b). In particu-
lar, from prophase and anaphase, AtSMC3 is associated
with chromosomes in both mitotic and meiotic cells, sup-
porting the idea that AtSMC3 is involved in sister chro-
matid cohesion, as its homologs do in other organisms.
This hypothesis is further supported by the observation
that syn1 mutant meiocytes exhibit abnormal AtSMC3
localization, indicating that SYN1 is required for normal
chromosomal localization of AtSMC3 in meiocytes.
Surprisingly, AtSMC3 is also associated with the spindle
from metaphase to telophase in a SYN1-independent
manner, suggesting a function previously unknown from
studies in other systems (Lam et al., 2005b). Mutations in
the AtSMC1 and AtSMC3 genes were previously isolated
as titan8 and titan7 mutants, respectively; these mutants
have enlarged embryos and arrested embryo and
endosperm development (Liu et al., 2002a), indicating that
these genes are critical for mitotic development.
Recently, the Arabidopsis homolog of SCC3, AtSCC3,
has been characterized (Chelysheva et al., 2005). The
AtSCC3 protein was detected in mitotic and meiotic cells;
during meiosis, AtSCC3 was localized to the chromosome
from leptotene to anaphase I. In addition, atscc3-2, a null
mutation, causes embryonic lethality and a weak allele,
atscc3-1, also results in defects during mitotic develop-
ment, indicating that AtSCC3 is important for the mitotic
cell cycle. Furthermore, AtSCC3 is required for mainte-
nance of sister chromatid cohesion at centromeres at
anaphase I, similar to SYN1 (Cai et al., 2003; Chelysheva
et al., 2005). In normal meiosis, the kinetochores of the sis-
ter chromatids are oriented in the same direction to allow
attachment to microtubules from the same pole of the
spindle during the reductional meiosis I. Analysis of the
atscc3 mutant meiocytes indicates that this monopolar ori-
entation of the meiosis I kinetochores requires AtSCC3
function.
In addition to cohesin subunits, the Arabidopsis
SWI1/DYAD gene is also required for normal sister chro-
matid cohesion during male and female meiosis (Mercier et
al., 2001; Agashe et al., 2002). In mutant male meiocytes,
sister chromatid cohesion is lost, resulting in 20 separated
chromatids at late prophase I to metaphase I, in contrast
to the five bivalents at wild-type diakinesis. Detailed exam-
ination of the male meiotic chromosomes in the swi1-2
mutant indicates that formation of the axial element is
affected, suggesting that SWI1 is also important for con-
densation (Mercier et al., 2003). Furthermore, the swi1-2
mutation can suppress the chromosome fragmentation
phenotype of dif1-1 (allelic to syn1), suggesting that SWI1
controls an early step in meiosis, as supported by the
detection of the SWI1 protein in meiotic G1 and S phases.
In the swi1 mutant, the absence of sister chromatid cohe-
sion does not result in chromosome breakage, unlike the
syn1 mutant chromosomes. In female meiosis of the swi1
mutant, a mitosis-like division takes place, suggesting that
the centromeres behave like mitotic centromeres, although
it is not clear how this might have occurred.
The predicted SWI1 protein is a novel protein with 639
amino acids; Arabidopsis has another gene (At5G23610)
with a predicted protein of 360 amino acids that shares
38% identity and 52% similarity with SWI1 over the N-ter-
minal 205 aa regions of both proteins. In addition, there is
a rice gene (accession number: AK064367) encoding a
protein with 33% identity and 48% similarity to SWI1 over
nearly the entire protein. Therefore, SWI1 might represent
a conserved plant gene family. Although the SWI1 protein
lacks clear homologs among proteins with known func-
FFiigguurree 44.. Immunolocalization of ZYP1 protein (bright light blue) in wild-type male meiocytes. A, leptotene, showing a small number of foci. B, early zygotene, with some
expansion to larger chromosome regions. C, mid-zygotene, showing further expansion of ZYP1 regions. D, pachytene, with ZYP1 distribution to all chromosomes. DNA was
stained with DAPI (blue). Bar = 10 m. This figure was modified from Figure 2 of Higgins et al. (2005), with permission from Genes and Development.
A Molecular Portrait of Arabidopsis Meiosis 9 of 39
tions, it has limited sequence similarity to predicted pro-
teins in Schizosaccharomyces pombe and human.
HHoommoolloogg PPaaiirriinngg//JJuuxxttaappoossiittiioonn aanndd
tthhee SSyynnaappttoonneemmaall CCoommpplleexx
As mentioned earlier, homolog pairing/juxtaposition,
synapsis, and recombination promote correct recognition
and association of homologs, such that they can properly
segregate subsequently. It is thought that telomeres play
an important role in chromosome pairing (Dawe, 1998;
Scherthan, 2001). In early prophase I of several organisms,
including plants (such as maize) and yeasts, dispersed
telomeres move and attach to the nuclear envelope, form-
ing a configuration called bouquet with telomeres clus-
tered on one face of the nucleus. Bouquet formation may
be important for initiating homolog pairing (Dawe et al.,
1994; Scherthan et al., 1996; Bass et al., 1997). In
Arabidopsis, the telomeres seem to cluster during inter-
phase to the nucleolus rather than on the nuclear enve-
lope, then pair before synapsis (Armstrong et al., 2001).
The paired telomeres then dissociate from the nucleolus
and are dispersed during leptotene, suggesting that telom-
ere clustering might also facilitate homolog pairing in
Arabidopsis (Armstrong et al., 2001).
Following pairing and juxtaposition, homologs form the
synaptonemal complex (SC) between the two aligned
chromosomes. In light microscopy, juxtaposed chromo-
somes can be visualized as thick lines (Figure 1C), which
represent synapsed homologs in normal Arabidopsis mei-
otic cells. The SC consists of three parallel filamentous ele-
ments (Figure 3): the two lateral elements and a central ele-
ment. The lateral elements correspond to the two aligned
homologs, with regions of the chromatin in complex with
SC proteins and large loops of chromatin emanating away
from the SC. The central element is seen in TEM as a thin
filament in between the lateral elements; it is known to be
composed of the overlapping heads of the transverse ele-
ments, which are very fine filaments that are perpendicular
to and connecting the lateral elements.
Because pairing, juxtaposition and synapsis are highly
coordinated processes, mutants defective in one often are
also abnormal in another, and have generally been called
synaptic mutants. These can be classified as either asy-
naptic mutants, which fail to establish synapsis, or desy-
naptic mutants, which cannot maintain homolog associa-
tion after forming SC. At diakinesis, both asynaptic and
desynaptic mutants have univalents rather than bivalents
(Ross et al., 1997; Caryl et al., 2000; Cai and Makaroff,
2001; Armstrong et al., 2002; Bass et al., 2003; Schommer
et al., 2003; Nonomura et al., 2004). In Arabidopsis, the
asy1 mutant is asynaptic, unable to form SC in both male
and female meiocytes (Ross et al., 1997; Caryl et al.,
2000). The ASY1 protein is localized to the axial/lateral ele-
ments, suggesting that it plays critical role in SC formation
(Armstrong et al., 2002). The ASY1 protein share sequence
similarity in its N-terminal domain with the yeast HOP1
protein (Hollingsworth et al., 1990; Caryl et al., 2000),
which is known to be important in SC assembly and is
localized on axial elements (Hollingsworth et al., 1990). In
rice there is an ASY1 homolog called PAIR2; a mutation in
PAIR2 causes defects in homolog alignment at pachytene
and the formation of univalents instead of bivalents at
diakinesis (Nonomura et al., 2004). Therefore, ASY1,
PAIR2, and HOP1 are functionally conserved genes in
plants and yeast. Another Arabidopsis mutant, ahp2, is
defective in pairing and bivalent formation (Schommer et
al., 2003). The AHP2 protein is similar to the S. cerevisiae
HOP2, S. pombe Meu13p, and human TBPIP (Tat-binding
protein 1-interacting protein) proteins. Both HOP2 and
Meu13p are important for homolog pairing during meiosis.
FFiigguurree 55.. Models for homologous recombination. The double stranded break
repair model (left) and the synthesis-dependent strand anneal model (right). Both
start with a DSB, 5' to 3' resection to 3' single strands, the invasion of one end
into the intact recombination partner to form a D-loop, and some DNA synthesis.
In the traditional DSBR model, the second end invades, leading to the formation
of the double Holliday junction, which expand and then can be resolved to either
crossover or noncrossover. Recently studies in yeast indicate that double Holliday
junctions usually resolve into crossovers. In addition, a number of results support
the synthesis-dependent strand annealing model, which branch off following D-
loop formation. This alternative model can generate noncrossovers, but not
crossovers. This figure was adopted from Figure 1 of Maloisel et al. (2004).
The Arabidopsis Book 10 of 39
In addition, TBPIP is expressed in the testis. Therefore,
AHP2 is a likely functional homolog of HOP2/Meu13p in
Arabidopsis.
Although SC is structurally similar among eukaryotes
under TEM, SC proteins from yeast and animals do not
share any obvious sequence similarity. Nevertheless, anal-
ogous protein components do show similarity in second-
ary structures. For example, in yeast, the ZIP1 protein is a
subunit of the transverse element; its counterparts in ani-
mals are SCP1 from mammals, SYP1 from C. elegans, and
C(3)G from Drosophila (Zickler and Kleckner, 1999; Page
and Hawley, 2004). They lack amino acid sequence simi-
larities, but share secondary structural properties, each
with globular heads and a long coiled-coil central region.
The lack of strong sequence similarity has made it difficult
to identify Arabidopsis genes encoding SC proteins.
However, recently, Higgens et al. (Higgins et al., 2005)
have used a combination of various bioinformatics tools to
identify an adjacent pair of divergently transcribed genes,
ZYP1a and ZYP1b. They encode highly similar (90% sim-
ilarity) proteins with structural features of ZIP1 and other
related proteins.
Immunolocalization experiments indicate that the ZYP1
proteins are located at 20-25 foci on leptotene chromo-
somes, and expand during zygotene to cover the entire
length of the pachytene chromosomes, consistent with the
hypothesis that ZYP1 is a SC component (Higgins et al.,
2005) (Figure 4). The appearance of ZYP1 foci is later than
the appearance of the ASY1 foci, which are found on the
axial element at leptotene. At pachytene, ASY1 signals
flank both sides of the ZYP1 signal, both along the
synapsed homologs, suggesting that ZYP1 forms part of
the central region of the SC. Single mutants in either
ZYP1a or ZYP1b genes have reduced fertility, slight reduc-
tion in the number of chiasmata and a delay in prophase I.
Because these two genes are adjacent, it was not possi-
ble to generate the double mutant. Localization analysis in
the single mutants indicates that the distributions of the
ZYP1a and ZYP1b proteins are indistinguishable from
each other and from that in the wild-type. Several RNAi
lines in single mutant backgrounds were obtained that had
no detectable ZYP1 proteins; these lines produced meio-
cytes that usually did not have aligned chromosomes but
occasionally did have fully aligned chromosomes,
although these aligned chromosomes are not synapsed
fully. The great increase of the number of cells with
unaligned chromosomes suggested a severe delay of mei-
otic progression. This is further supported by an increase
in the period between DNA replication and the completion
of meiosis I from ~30 hours in normal meiocytes to ~50
hours in cells of the ZYP1 RNAi lines.
The ZYP1 RNAi lines also display evidence of abnor-
mality in recombination (Higgins et al., 2005). The affected
meiocytes have a slightly reduced number of MLH1 foci
(7.3), which likely mark crossover sites, in comparison with
10 found in the wild-type cells. In addition, the average
number of chiasmata was estimated to between 7 and 8,
rather the normal ~10. These results suggest that recom-
bination is slightly affected. Nevertheless, the fact that
MLH1 foci and chiasmata are present in substantial num-
bers indicates that recombination was able to progress to
a relatively late stage. Strikingly, fluorescence in situ
hybridization experiments indicate that nonhomologous
chromosomes are closely associated, suggesting that
pairing might have been affected in the ZYP1 RNAi lines.
The results from the analysis of ZYP1 RNAi lines support
the idea that pairing, synapsis and recombination are
tightly coupled. Clearly, a severe reduction of the ZYP1
protein has a significant effect on pairing and recombina-
tion. It is possible that the RNAi lines still have some resid-
ual ZYP1 function, which may allow recombination at a
level greater than possible if the ZYP1 function were com-
pletely absent.
MMeeiioottiicc RReeccoommbbiinnaattiioonn:: IInniittiiaall EEvveennttss
In addition to pairing/juxtaposition and synapsis, homolo-
gous recombination occurs during prophase I between
homologs. It is initiated around late leptotene to early
zygotene and is completed at late pachytene. Using TEM,
electron dense bodies called meiotic nodules can be
found associated with the chromosome; they are also
called recombination nodules because they are thought to
be related to recombination (Carpenter, 1975, 1987;
Sherman et al., 1992; Sherman and Stack, 1995; Anderson
et al., 2001). Late recombination nodules are very well cor-
related with recombination crossovers (Anderson et al.,
2004), which are observed cytologically as the chiasmata.
The maintenance of chiasmata also requires sister chro-
matid cohesion.
A well-known model for recombination called the dou-
ble-strand break repair (DSBR) model was proposed pri-
marily on the basis on biochemical studies in yeast and
animal cells (Szostak et al., 1983). According to this model
(Figure 5, left), recombination is initiated by a double-
strand break (DSB) in one of two participating chromo-
somes, usually a non-sister chromatid during meiosis; 5'
resection of the DSB produces 3' single-stranded DNAs,
one of which then forms a D loop with the intact partner
chromatid. DNA synthesis ensues primed from the invad-
ing strand using the intact molecular as the template. The
invasion of the second broken end and further DNA syn-
thesis yield the double-Holliday junction, which can be
A Molecular Portrait of Arabidopsis Meiosis 11 of 39
resolved following cutting at alternative strands, effecting
either crossovers or non-crossovers.
Recent studies have yielded evidence supporting the
idea that the decision for crossover/non-crossover is made
very early during recombination, prior to the formation of
the double-Holliday junction, prompting a revision of the
DSBR model (Bishop and Zickler, 2004; Borner et al.,
2004; Haber et al., 2004). This revised model (Figure 5,
right) posits that the recombination pathway branches fol-
lowing the formation of the D loop. One branch leads to
the formation of the double-Holliday junction, which then
progresses to form a crossover, but usually not a non-
crossover. In the second branch of the pathway, the invad-
ing single strand, with its newly synthesized DNA, dissoci-
ates from the homolog and re-anneals with the other bro-
ken end; DNA synthesis and ligation then restore the chro-
mosome, resulting in a non-crossover.
Many genes important for meiotic recombination have
been identified by studies in the budding yeast
Sacchromyces cerevisiae, as well as in the fission yeast
Schizosaccharomyces pombe, C. elegans, Drosophila,
and others (Zickler and Kleckner, 1999). The relatively ease
with which to perform reverse genetics and the available
sequence information have allowed the testing of the func-
tion of Arabidopsis homologs of these known meiotic
genes. Because of the intimate relationship between
recombination, pairing and synapsis, studies of genes
important for meiotic recombination have also yielded
insights into their roles in pairing and synapsis. In budding
yeast, DSBs are generated by SPO11 (Keeney, 2001;
Lichten, 2001), which was discovered a number of year
ago as a mutant defective in sporulation (Esposito and
Esposito, 1969), the formation of haploid spores from a
diploid cell through meiosis. SPO11 encodes a member of
a novel family of type II topoisomerase and has a transes-
terase activity that creates DSBs along with several other
proteins (Bergerat et al., 1997; Keeney, 2001). SPO11
homologs have been identified in many organisms includ-
ing S. pombe, Drosophila, C. elegans, mammals, the
mushroom Coprinus cinereus, and Arabidopsis (Dernburg
et al., 1998; McKim et al., 1998; Keeney et al., 1999;
Romanienko and Camerini-Otero, 1999; Celerin et al.,
2000; Grelon et al., 2001), suggesting a highly conserved
function in eukaryotes.
Genetic studies using mutations in the SPO11 gene in
various organisms have revealed differences in the rela-
tionship between recombination initiation in the form of
DSBs and synapsis. In both budding and fission yeasts, as
well as mouse, spo11 mutants are defective in SC forma-
tion, indicating a requirement of DSBs for homolog synap-
sis (Cervantes et al., 2000; Romanienko and Camerini-
Otero, 2000; Zenvirth and Simchen, 2000). In contrast,
SPO11-dependent DSBs are not required for homolog
synapsis in C. elegans and Drosophila (Dernburg et al.,
1998; McKim et al., 1998). In Arabidopsis, there are three
SPO11 homologs (Hartung and Puchta, 2000, 2001). A
mutant in one of these SPO11 homologs, AtSPO11-1, was
shown to be defective in bivalent formation in prophase I
(Figure 6) and has a sharply reduced recombination rate
between homologous chromosomes, indicating that
AtSPO11-1 is important for recombination (Grelon et al.,
2001). The atspo11-1 mutant phenotypes also suggest
that AtSPO11-1 is required for homolog pairing, juxtaposi-
tion, and/or synapsis. Therefore, meiotic recombination
and its relationship to pairing and synapsis may be more
similar between yeast, mammals and plants, then between
these organisms and C. elegans or Drosophila (see below
for more discussion). The other two SPO11 homologs,
AtSPO11-2 and AtSPO11-3, are not known to be involved
in meiosis.
After the formation of SPO11-dependent DBSs in yeast,
then a complex of RAD50, MRE11, and NBS1 (XRS2) pro-
teins carries out the resection step to generate single-
stranded ends (Connelly and Leach, 2002). RAD50 and
MRE11 are similar in sequence to the bacterial sbcC and
sbcD subunits of a nuclease complex; in particular, the
MRE11 and sbcD proteins contain conserved phospho-
esterase motifs (Sharples and Leach, 1995). The
Arabidopsis homologs of RAD50 and MRE11 are known,
but a NBS1 homolog has not yet been identified in
Arabidopsis (Hartung and Puchta, 1999; Gallego et al.,
2001). An atrad50 mutant is completely sterile and is
hypersensitive to DNA-damaging radiation and chemicals
(Gallego et al., 2001). Further studies indicate atrad50 mei-
otic cells have numerous chromosome fragments in
prophase I, suggesting that the repair of DSBs is defective
(Bleuyard et al., 2004b). This is consistent with a role for
AtRAD50 just downstream of AtSPO11-1. The predicted
Arabidopsis MRE11 protein has 720 amino acids and
FFiigguurree 66.. Male meiosis in the atspo11-1 mutant. A, leptotene. B, early diakine-
sis, showing many univalents. C, prometaphase I, with 10 univalents. D,
metaphase I, with one bivalent (arrow) and 8 univalents. E, metaphase II to
anaphase II, showing abnormal distribution. F, anaphase II. This figure was modi-
fied from Figure 8 of Li et al. (2005).
The Arabidopsis Book 12 of 39
shares strong sequence similarity with yeast and human
MRE11 proteins in the N-terminal two thirds, which con-
tains four phosphoesterase motifs (Hartung and Puchta,
1999; Bundock and Hooykaas, 2002). A T-DNA insertional
mre11 mutant was found to have small and abnormally
shaped leaves and enlarged shoot apical meristem
(Puizina et al., 2004). In addition, the Arabidopsis mre11
mutant is sterile, has meiotic chromosome fragmentation,
is defective in homolog pairing and bivalent formation, and
exhibits abnormal chromosome segregation (Puizina et al.,
2004). The chromosome fragmentation phenotype is due
to a failure to process SPO11-induced DBSs because
spo11-1 mre11 double mutant meiocytes do not show
chromosome fragmentation. Therefore, similar to RAD50
and MRE11 in yeast, the Arabidopsis AtRAD50 and
MRE11 genes are required for normal meiotic recombina-
tion. In addition to its meiotic function, AtRAD50 and
MRE11 are also needed for normal mitotic growth. In
yeast, RAD50 and MRE11 are also important for repair of
DSBs by non-homologous end joining (Haber, 1998);
therefore, at least some of the somatic phenotypes in the
Arabidopsis atrad50 and mre11 mutants could be due
defects in this pathway, rather than that in the homologous
recombination pathway.
MMeeiioottiicc RReeccoommbbiinnaattiioonn:: TThhee RRAADD5511 GGeennee
FFaammiillyy aanndd DDSSBB RReeppaaiirr
Another yeast gene important for meiotic recombination is
RAD51, which encodes a eukaryotic homolog of the E. coli
RecA recombinase critical for DSB repair in homologous
recombination (Smith et al., 1987; Smith and Wang, 1989;
Cox, 1999). RecA is a 38-KDa protein that has activities in
single-stranded DNA (ssDNA) binding, ATP binding, and
ATP hydrolysis (McEntee et al., 1980; Weinstock et al.,
1981). The yeast RAD51 and E. coli RecA proteins are 30%
identical (54% conserved) over a ~210-amino acid region.
In addition, the meiosis-specific yeast protein DMC1 is
another RecA homolog and is highly similar to RAD51
(Bishop et al., 1992). The RAD51 gene is required for both
meiotic and mitotic recombination, whereas DMC1 is
specifically required for meiotic progression, synaptone-
FFiigguurree 77.. Male meiosis in wild-type (A-F, M-R) and atrad51-1 (G-L, S-X) plants. A and G, leptotene. B and H, zygotene. C and I, pachytene. D and J, diplotene. E and K,
diakinesis. F and L, metaphase I. M and S, anaphase I. N and T, telophase I. O and U, prophase II. P and V, metaphase II. O and W, anaphase II. R and X, telophase II.
Chromosome fragmentation is evident starting at diakinesis and through telophase II. In addition, pairing and juxtaposition is apparently defective at zygotene and pachytene.
This figure was modified from Figure 4 of Li et al. (2004).
A Molecular Portrait of Arabidopsis Meiosis 13 of 39
mal complex formation, and homologous recombination
(Bishop et al., 1992; Shinohara et al., 1992). In C. elegans
and Drosophila, the RAD51 homologs are required for nor-
mal meiosis (Rinaldo et al., 1998; Staeva-Vieira et al.,
2003). It is worth noting that Drosophila and C. elegans do
not seem to possess a DMC1 gene. In mouse, RAD51 has
been shown to interact with DMC1 (Masson et al., 1999;
Tarsounas et al., 1999), but its function in meiosis has not
been shown genetically because the rad51 mutant is
embryonic lethal. The mouse dmc1 mutant is defective in
meiosis (Habu et al., 1996; Pittman et al., 1998; Yoshida et
al., 1998).
Homologs of RAD51/DMC1 have been identified in sev-
eral plants, including Arabidopsis and maize (Sato et al.,
1995; Anderson et al., 1997; Klimyuk and Jones, 1997;
Doutriaux et al., 1998; Franklin et al., 1999; Ding et al.,
2001; Shimazu et al., 2001). Protein localization studies in
maize and Arabidopsis using antibodies indicate that the
RAD51 protein is located on the chromosome at a large
number of foci at the zygotene stage (Franklin et al., 1999;
Mercier et al., 2003). The number of early RAD51 foci is in
the hundreds, easily allowing more than a few sites per
pair of homologs. In addition, it was observed that pairs of
RAD51 sites are associated with pairing homologs and
that meiocytes of maize pairing defective mutants have
few or no RAD51 foci (Franklin et al., 1999; Franklin et al.,
2003; Pawlowski et al., 2003). These results led to the pro-
posal that RAD51 is important for homology search during
pairing (Franklin et al., 1999; Franklin et al., 2003;
Pawlowski et al., 2003).
The hypothesis that RAD51 is important for pairing/jux-
taposition is supported by analysis of a T-DNA insertional
mutant in the Arabidopsis RAD51 ortholog (AtRAD51) (Li et
al., 2004). The atrad51 mutant is male and female sterile;
using both light microscopy of DAPI-stain chromosome
spreads and TEM, it was found that mutant meiocytes are
defective in homolog juxtaposition, lacking the typical "thin
threads" seen at pachytene (Figures 7). Analysis using
TEM revealed that mutant meiocytes were able to form
axial elements, but they did not progress to form SC
(Figure 8). Moreover, consistent with a predicted role in D-
loop formation according to the DSBR model of recombi-
nation, atrad51 mutant meiotic cells have multiple chro-
mosomal fragments detectable beginning at diakinesis
and continuing through anaphase II, suggesting a severe
defect in repair of SPO11-induced DBSs. Indeed, meio-
cytes of the atspo11-1 atrad51 double mutant lack chro-
mosome fragments, indicating the fragmentation in the
atrad51 mutant requires the SPO11-1 function (Li et al.,
FFiigguurree 88.. Transmission electron micrographs of male meiocyte nuclei in wild
type and atrad51. A and B, wild type nuclei with SC (arrows) at zygotene (A) and
pachytene (B). C and D, atrad51-1 nuclei, with axial elements (arrowheads), which
are unpaired in nuclei at zygotene (C) and pachytene (D) stages. E, an occasional
fragmentary SC in the atrad51-1 mutant. Nu: nucleolus. Bar = 1 m. This figure is
Figure 5 of Li et al. (2004).
FFiigguurree 99.. A Neighbor-Joining phylogenetic tree of the RecA/RAD51 gene fami-
ly. Species designation: Arth: Arabidopsis thaliana; Dare: Danio rario; Esco:
Escherichia coli; Hosa: Homo sapien; Sace: Saccharomyces cerevisiae; Scpo:
Schizosaccharomyces pombe. Eukaryotic recA/RAD51 gene family members
form 7 clades; Arabidopsis genes are present in 6 of these clades, indicating an
orthologous relationship between each Arabidopsis gene and animal genes.
Bootstrap values are indicated near the stem of each clade. The tree in this figure
was generated by Z. Lin.
The Arabidopsis Book 14 of 39
2004). Therefore, AtRAD51 participates in the SPO11-1
dependent meiotic recombination, and the RAD51 meiotic
function is similar between yeast and plants.
Protein localization studies also indicate that at the
pachytene stage, the number of RAD51 foci is greatly
reduced (Franklin et al., 1999; Mercier et al., 2003), con-
sistent with the number of crossovers per cell (Copenhaver
et al., 1997). This suggests that RAD51 is also needed for
the branch of the recombination pathway that led to
crossover formation. In addition, studies of a T-DNA
knockout mutant of AtDMC1 indicate that the mutant
meiocytes have largely univalents instead of bivalents at
late prophase I (Couteau et al., 1999). Because crossovers
are needed for maintaining bivalents at diakinesis, the
atdmc1 phenotypes indicate that AtDMC1 is crucial for
Arabidopsis meiotic crossover formation. Indeed, the
recombination frequency is greatly reduced in the atdmc1
mutant (Couteau et al., 1999). Although both AtRAD51 and
AtDMC1 are RecA homologs and are required for normal
meiosis, particularly meiotic recombination, the meiotic
phenotypes of the corresponding mutants are very differ-
ent. Specifically, atdmc1 mutant meiocytes do not have
detectable chromosome fragmentation. It is possible that
atdmc1 is primarily functioning to promote crossover for-
mation, and is not required for the non-crossover branch
of homologous recombination.
DSBs in chromosomes can occur due to radiation and
chemical mutagens, even during DNA replication in a nor-
mal cell cycle. These DSBs must be repaired to maintain
chromosome stability and cell viability because a single
chromosomal break can cause cell death. One of the major
pathways of DSB repair is homologous recombination
(Masson and West, 2001). As mentioned above, the yeast
RAD51 is important for the repair of DSBs caused by DNA-
damaging agents, but is not required for mitotic cell viabil-
ity (Shinohara et al., 1992; Game, 1993). Similarly, in C. ele-
gans and Drosophila, the RAD51 gene is not essential for
mitotic cell viability, unless the animals are exposed to
DNA-damaging agents (Rinaldo et al., 2002; Staeva-Vieira
et al., 2003). On the other hand, the mouse RAD51 gene is
essential during embryogenesis (Lim and Hasty, 1996;
Tsuzuki et al., 1996). Consistent with this, RAD51 is impor-
tant for DNA repair and chromosome stability during the
mitotic cell cycle in mammalian and chicken cell cultures
(Li and Maizels, 1997; Sonoda et al., 1998; Kim et al.,
2001; Lambert and Lopez, 2002; Lundin et al., 2003).
Reduction or loss of RAD51 function in these cells cause
hypersensitivity to ionizing radiation and methyl methane-
sulfonate (MMS) and a deficiency in DSB repair and cell
death (Taki et al., 1996; Sonoda et al., 1998; Collis et al.,
2001). In Arabidopsis, the atrad51 mutant plants appear
healthy, identical to the wild type, when grown under nor-
mal conditions (Li et al., 2004). The size and number of
atrad51 leaves and floral organs all seem normal. In addi-
tion, mitosis in the atrad51 mutant appears to proceed
normally, unlike the mammalian rad51 knockout, which is
embryo lethal. Therefore, the Arabidopsis AtRAD51 gene is
not critical for DNA repair during vegetative and floral
development.
In addition to RAD51 and DMC1, yeast also has two
other distant homologs of recA: RAD55 and RAD57, which
were identified as mutants hypersensitive to radiation
(Kans and Mortimer, 1991; Lovett, 1994). Several lines of
evidence support the hypothesis that RAD57 interacts with
RAD51 to promote recombination. It was found that rad57
mutants (Kans and Mortimer, 1991; Game, 1993) can be
partially rescued by the overexpression of RAD51 (Hays et
al., 1995). In addition, RAD57 promotes RAD51-mediated
homologous pairing and strand exchange in vitro (Sung,
1997). Amino acid substitutions in RAD51 that increase its
DNA-binding affinity can suppress rad57 mutations (Fortin
and Symington, 2002). These observations are consistent
with the idea that RAD57 facilitates the binding RAD51 to
DNA. In S. pombe, hypersensitivity to DNA damaging
agent MMS and gamma-rays were used to identify muta-
tions in the RHP57 gene, which is the putative ortholog of
the S. cerevisiae RAD57 gene (Tsutsui et al., 2000).
Vertebrate animals also possess five other RAD51 par-
alogs: RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3
(Albala et al., 1997; Cartwright et al., 1998; Dosanjh et al.,
1998; Liu et al., 1998b; Pittman et al., 1998; Braybrooke et
al., 2000; Havre et al., 2000) (Figure 9). RAD51B, RAD51C,
and RAD51D were isolated as cDNAs with similarity to
RAD51, and XRCC2 and XRCC3 genes were identified by
their ability to counter the sensitivity of hamster mutant cell
lines, irs1 and irs1SF, to DNA-damaging radiation and
chemicals (Liu et al., 1998b). Phylogenetic analysis sug-
gests that XRCC3 is a putative ortholog of the budding
yeast RAD57 and fission yeast RHP57 (Tsutsui et al.,
2000). Similar to RAD57, the XRCC3 protein can also asso-
ciate with RAD51 in a yeast two-hybrid assay (Schild et al.,
2000). The human XRCC3 is required for the assembly of
RAD51 complexes in vitro (Bishop et al., 1998) and for
homology-directed repair of DNA DSBs in cell lines (Pierce
et al., 1999; Brenneman et al., 2000). In addition, bio-
chemical studies have detected two protein complexes
involving these proteins: one containing RAD51B,
RAD51C, RAD51D, and XRCC2 and called the BCDX2
complex; the other containing XRCC3 and RAD51C
(Masson et al., 2001; Liu et al., 2002b). The BCDX2 com-
plex binds to nicks in duplex DNA, and may play a role in
the recognition of nicks during repair of DSBs (Masson et
al., 2001). In the XRCC3-RAD51C complex, RAD51C facil-
itates homolog pairing, and XRCC3 contributes to prefer-
ential binding to ssDNAs (Kurumizaka et al., 2001).
Disruption of one of mouse RAD51, RAD51B, RAD51D,
or XRCC2 genes causes early embryo lethality (Lim and
Hasty, 1996; Tsuzuki et al., 1996; Shu et al., 1999; Deans
A Molecular Portrait of Arabidopsis Meiosis 15 of 39
et al., 2000; Pittman and Schimenti, 2000), indicating non-
redundant essential roles in embryogenesis, consistent
with their having important functions in DSB repair in the
mitotic cell cycle. Furthermore, the early lethality of rad51
and rad51b mutant embryos can be delayed due to partial
suppression by a mutation in p53, a checkpoint control
gene, suggesting that the loss of the p53 function can
allow cell cycle progression even with a defect in DNA
repair. However, the rad51 p53 double mutant embryos
eventually also die (Lim and Hasty, 1996; Shu et al., 1999).
Therefore, the RAD51-dependent function is linked to
mitotic DNA damage checkpoint control. Indeed, RAD51
has been found to interact genetically and physically with
tumor suppressor genes p53, ATM, BRCA1, BRCA2, and
others (Scully et al., 1997; Sharan et al., 1997; Wong et al.,
1997; Marmorstein et al., 1998; Chen et al., 1999), sug-
gesting that it is involved in the normal cellular process
that maintains chromosome integrity and stability.
In Arabidopsis, molecular cloning and genomic
sequence analysis indicate that there are at least six
Arabidopsis RAD51 homologs. In addition to AtRAD51 and
AtDMC1, Arabidopsis also has putative orthologs of
FFiigguurree 1100.. Male meiosis in wild-type (A-F) and atrad51c-1 (G-L) plants. A and G, leptotene. B and H, zygotene. C and I, pachytene. D and J, diplotene. E and K, diaki-
nesis. F and L, metaphase I. In atrad51c-1, multiple chromosome fragments were detected at diakinesis and metaphase I. This figure is modified from Figure 4 of Li et al.
(2005).
FFiigguurree 1111.. Male meiotic prophase I images from TEM. A-C, wild-type; D-F, atrad51c-1. A and B, zygotene; C, pachytene. Synaptonemal complexes (SC, arrows) can be
seen in these images. In atrad51c-1, at zygotene (D) or pachytene (F), the majority of the axial elements (arrowheads) remained unpaired; sometimes, an abnormal SC (thick
arrows) was also observed (D and E). Nu: nucleolus; ERN: early recombination nodule. Bar in A equals to 100 nm (same scale for C, D, and F); bar in B equals to 500 nm
(same scale for E). This figure is Figure 7 of Li et al. (2005).
The Arabidopsis Book 16 of 39
RAD51B, RAD51C, XRCC2, and XRCC3: AtRAD51B,
AtRAD51C, AtXRCC2, and AtXRCC3, respectively
(Doutriaux et al., 1998; Couteau et al., 1999; Osakabe et
al., 2002; Bleuyard and White, 2004; Li et al., 2004;
Bleuyard et al., 2005; Li et al., 2005), as supported by phy-
logenetic analysis (Figure 9). AtRAD51C is expressed at
low levels in somatic cells (Li et al., 2005). RAD51C is also
expressed in meiocytes, but at a lower level than that of
RAD51 (Li et al., 2004; Li et al., 2005). Similar to the
atrad51 mutant, an atrad51c T-DNA insertional mutant is
also completely sterile and defective in meiosis, with frag-
mented chromosomes (Abe et al., 2005; Bleuyard et al.,
2005; Li et al., 2005) (Figure 10). Moreover, the rad51c male
and female meiocytes display pairing/juxtaposition
defects, as indicated by fluorescence in situ hybridization
experiments (Li et al., 2005). Furthermore, TEM experi-
ments indicate that atrad51c meiocytes have abnormal
partial SCs, sometimes with more than two chromosomes,
suggesting that non-homologous chromosomes might
have partially synapsed (Figure 11). At late pachytene, no
SC was detected; therefore, the partial SCs at earlier
stages could not progress to complete SCs. Therefore,
RAD51C also is required for normal homolog juxtaposition,
synapsis, and recombination, following the formation of
DSBs during prophase I. Similar to the atrad51 atspo11-1
double mutant, the atrad51c atspo11-1 double mutant
also lacks chromosome fragments, indicating that
AtRAD51C also acts downstream of AtSPO11-1 to repair
DSBs. In addition to the meiotic defectives, the atrad51c
mutant is hypersensitive to gamma radiation and a DNA
crosslinking agent and is reduced in somatic recombina-
tion frequency (Abe et al., 2005; Bleuyard et al., 2005).
The function of the Arabidopsis XRCC3 homolog,
AtXRCC3, was revealed by the analysis of a T-DNA inser-
tional mutant, which showed sterility and fragmented mei-
otic chromosomes (Bleuyard and White, 2004), similar to
the phenotypes of the atrad51 mutant. In addition, the
appearance of chromosome fragments depends on the
function of AtSPO11-1, indicating that the fragmentation is
due to failure to repair SPO11-induced DSBs (Bleuyard et
al., 2004a; Bleuyard and White, 2004). When grown under
normal conditions, the atxrcc3 mutant can develop nor-
mally; in contrast, mutant cells in culture are sensitive to
DNA cross-linking agents but not to DSB-inducing chemi-
cals. Therefore, AtXRCC3 is not required for normal mitot-
ic development, unlike the mammalian XRCC3 gene. The
role of AtXRCC3 in DNA repair also seems to be different
from that of the XRCC3 gene, which is involved in DSB
repair. Insertions in the AtRAD51B and AtXRCC2 genes
have no visible developmental defects, but cause DNA
repair defects in somatic cells (Abe et al., 2005; Bleuyard
et al., 2005; Osakabe et al., 2005). Therefore, AtRAD51,
AtRAD51C, and AtXRCC3 are required for normal pro-
cessing of DSBs induced by AtSPO11-1 during meiosis; in
addition, AtRAD51B, AtRAD51C, AtXRCC2 and AtXRCC3
are involved in repair of damaged DNA during the mitotic
cell cycle.
Mutant phenotypes from DAPI analysis suggest that
atrad51c and atxrcc3 meiocytes might have fewer chro-
mosome fragments than that in the atrad51 mutant,
although the number of fragments is difficult to quantify
and the possible meiotic phenotypic difference between
these three mutants needs to be further investigated. If
atrad51c and atxrcc3 meiocytes do have weaker pheno-
types than does atrad51, there might be a functional dif-
ference between these genes. EM results also support the
idea that these genes are not functionally identical,
atrad51 cells had almost no SC (Figure 8), except very
occasional abnormal ones (Li et al., 2004), additional to
axial elements, whereas atrad51c meiocytes had occa-
sional polycomplexes (Figure 11), and xrcc3 cells had
abnormal SC, at a frequency greater than those in the
atrad51 cells (L. Timofejeva, W. Li, and H. Ma, unpublished
data). The fact that each of the three single mutants has
such severe meiotic defects indicates that these genes are
not genetically redundant. In addition, the fact that sepa-
rate RAD51, RAD51C, and XRCC3 evolutionary lineages
have been maintained and conserved in both animals and
plants supports the hypothesis that RAD51, RAD51C, and
XRCC3 have distinct essential functions (Figure 9).
Immunolocalization studies indicate that there are
numerous (more than 200) AtRAD51 foci in early
Arabidopsis prophase I cells (leptotene to zygotene) and
this number is reduced to fewer than 20 at pachytene
(Mercier et al., 2003). From DAPI-stained images, the num-
bers of chromosome fragments in the atrad51, atrad51c,
and atxrcc3 seem to be far fewer than the number of early
AtRAD51 foci (Bleuyard and White, 2004; Li et al., 2004;
Abe et al., 2005; Bleuyard et al., 2005; Li et al., 2005). It is
possible that each of these proteins is present at a subset
of the DSBs. This is consistent with the observation that
RAD51 labeled 44% of early meiotic nodules and 79% of
late nodules in lily (Anderson et al., 1997). It is possible that
AtRAD51, AtRAD51C, and AtXRCC3 are localized to non-
identical but possibly overlapping subsets of DSBs, so
that some AtRAD51 foci also have AtRAD51C and/or
AtXRCC3 proteins. It is also possible that AtRAD51C and
AtXRCC3 are localized to fewer foci than AtRAD51.
RAD51 and XRCC3 are essential for mitotic growth in
mammals and birds, but their putative orthologs are not
required for cellular viability in yeast, Drosophila, or
Arabidopsis. Clearly, AtRAD51, AtRAD51C, and AtXRCC3
are each required for meiosis and fertility, indicating that
they cannot replace each other during meiosis. As men-
tioned above, AtRAD51C and AtXRCC3 are also important
for DNA repair when the plant is exposed to DNA-damag-
ing chemicals, as are AtRAD51B and AtXRCC2. Therefore,
even though these genes are members of the same fami-
A Molecular Portrait of Arabidopsis Meiosis 17 of 39
ly, they are not redundant genetically. It is possible that
these genes are needed when the numbers of DSBs are
large, as in meiosis or when caused by mutagens. The fact
these genes are highly conserved suggests that they are
also important for meiosis in animals, even though the
embryo lethality has thus far prevented a direct test of this
hypothesis. The fact that atrad51, atrad51c, and atrxcc3
mutants all appear normal during mitotic development
under standard growth conditions suggests two different
possibilities. First, DSB repair is not important for normal
mitotic cell cycle, although this is not likely because
Arabidopsis rad50 and mre11 mutants have vegetative
phenotypes. Second, DSBs might be repaired by other
pathways, such as those involving other RAD51 paralogs,
or the non-homologous end joining pathways.
In vitro studies of mammalian RAD51 indicate that it
binds to ssDNA and catalyzes the formation of a D loop by
promoting the invasion of the ssDNA into a double strand-
ed DNA (Petukhova et al., 2000). In addition, mammalian
XRCC3 and RAD51C form a complex that promotes the
loading of RAD51 onto DNA (Masson et al., 2001;
Sigurdsson et al., 2001; Wiese et al., 2002; Lio et al., 2003).
Also, xrcc3 and rad51c mutant animal cell lines have
reduced number of RAD51 foci; furthermore, RAD51C, but
not RAD51, is needed for the formation of XRCC3 foci on
chromosome (Yoshioka et al., 2003; Lio et al., 2004).
Because the Arabidopsis AtRAD51, AtRAD51C, and
AtXRCC3 proteins are orthologous to their mammalian
counterparts, the Arabidopsis proteins might have similar
biochemical properties, suggesting the following model for
how these genes function in meiosis. During meiosis,
AtRAD51C and AtXRCC3 form a complex that promotes
the formation of AtRAD51 foci. It is possible that formation
of AtRAD51 foci does not absolutely require AtRAD51C or
AtXRCC3; when AtRAD51C or AtXRCC3 are eliminated by
mutations, the number of AtRAD51 foci is reduced. This
model can explain the residual homolog interaction in the
atrad51c-1 mutant. In addition, the observation of a small-
er number of chromosome fragments in atrad51c-1 meio-
cytes than in atrad51-1 meiocytes supports the idea that
some DSBs are repaired in atrad51c-1 cells.
Besides the RAD51 homologs, the BRCA1, BRCA2 and
ATM tumor suppressor proteins have been linked with
DSB repair in animals and interact with each other (Khanna
and Jackson, 2001; Thompson and Schild, 2002; Henning
and Sturzbecher, 2003). ATM is the gene that is defective
in the human disease called ataxia telangiectasia (AT) and
encodes a very large protein of over 3000 amino acids with
a C-terminal protein kinase domain that is similar to phos-
phatidyl inositol-3 kinase (Savitsky et al., 1995; Rotman
and Shiloh, 1998). Mutations in ATM cause meiotic defects
associated with the inability to repair DSBs (Barlow et al.,
1998; Ashley et al., 2004). Mutations in the BRCA1 and
BRCA2 genes causes risk in breast cancer and are also
associated with other cancers (Khanna and Jackson,
2001). ATM has been shown to phosphorylate BRCA1,
which interacts with BRCA2; BRCA2 can interact with
RAD51, which is regulated via phosphorylation by c-Abl,
another target of ATM (Khanna and Jackson, 2001). Mice
deficient in BRCA1 usually die during embryo develop-
ment, but a small number of brca1 p53 double homozy-
gous mice survive; these mice exhibit multiple abnormali-
ties including growth retardation, DNA repair defects and
failure to complete meiosis (Cressman et al., 1999). The
brca2 knockout mutation causes embryonic lethality in
mouse and mutant cells have chromosome breaks and
rearrangement (Yu et al., 2000). In humans, the BRCA2
protein is localized to meiotic chromosomes, suggesting
that it plays a role in meiosis (Chen et al., 1998), although
this has not been demonstrated genetically in part
because of the embryo lethality of the mouse mutant.
The Arabidopsis ATM homolog, AtATM, is widely
expressed and encodes a protein of 3856 amino acids
(Garcia et al., 2000). The predicted AtATM protein has a
350-aa C-terminal phosphatidyl inositol-3 kinase like
domain (58% identity and 67% similarity to the same
domain in ATM), as well as a RAD3 domain just upstream
of the C-terminal domain, similar to other ATM homologs.
Two T-DNA insertional mutations in AtATM have been
identified and found to cause reduced fertility (Garcia et al.,
2003). Meiosis is abnormal in atatm mutants, with chro-
mosome fragmentation and chromosome bridges,
although pairing and synapsis seem to be normal. In addi-
tion, atatm mutants are partially fertile, but gametophytes
often arrest in development. Therefore, AtATM promotes
normal meiosis, possibly facilitate DNA repair, but it is not
essential for meiosis, as supported by the normal meiotic
recombination frequency in the mutant (Garcia et al.,
2003). The atatm mutants are hypersensitive to gamma
radiation and unable to induce elevated expression of DNA
repair genes, such as AtRAD51, in response to DNA dam-
age (Garcia et al., 2003), indicating it plays an important
role in regulating DNA damage response.
A true BRCA1 homolog has not yet been found in
Arabidopsis; nevertheless, the Arabidopsis MEI1 protein
bears some resemblance to BRCA1. The Arabidopsis
MEI1 gene was discovered originally for its mutant meiot-
ic defects and has been recently shown to encode a pro-
tein with five BRCT motifs (He and Mascarenhas, 1998;
Yang and McCormick, 2002; Grelon et al., 2003). BRCT
motifs are found in BRCA1 and other proteins involved in
DNA repair (Koonin et al., 1996; Grelon et al., 2003); the
presence of BRCT motifs in MEI1 suggests that it may
have related protein activities. Detailed analysis of mei1
mutants revealed that mutant meiocytes exhibit chromo-
some breakage during meiosis I (Grelon et al., 2003).
Unlike the atrad51, atrad51c, and atxrcc3 cells, mei1
mutant meiocytes exhibit variable phenotypes in terms of
The Arabidopsis Book 18 of 39
chromosome fragmentation (Grelon et al., 2003). In about
75% of mei1 male meiocytes, chromosome breakage is
severe, results in multiple fragmentation, whereas in about
20% of the mei1 cells, fragmentation is infrequent yet
detectable. In about 5% of the cells, the chromosomal
morphology appeared normal (Grelon et al., 2003). In the
mei1 spo11-1 double mutant, most meiocytes have a phe-
notype similar to the severe class of mei1 single mutant
cells, indicating that spo11-1 could not rescue the mei1
mutant phenotype and the DNA breaks in the mei1 mutant
are not SPO11-1 dependent. A small number of the dou-
ble mutant cells had a phenotype similar to spo11-1 single
mutant, suggesting that these cells did not have DNA
breaks, similar to the small number of mei1 cells with nor-
mal chromosomes. The mei1 somatic cells were indistin-
guishable from wild-type cells in terms of sensitivity to
gamma and UV-C radiations (Grelon et al., 2003).
There are two highly similar Arabidopsis homologs of
BRCA2 (97% nucleotide sequence identity) (Siaud et al.,
2004). Using a meiosis-specific RNAi construct, it was
found that reduction of AtBRCA2 expression causes the
formation univalents instead of bivalents and mis-segrega-
tion of chromosome at anaphase I, similar to those of the
atdmc1 mutant cells. An interaction with the DNA repair
process was further supported by the observation that the
AtBRCA2 protein can interact with AtRAD51 and AtDMC1
in a yeast two-hybrid assays. These results suggest that
AtBRCA2 is involved in meiotic recombination.
Furthermore, the lack of chromosome fragmentation in the
AtBRCA2 RNAi transgenic plants further suggests that
AtBRCA2 is similar to AtDMC1, not needed for the repair
of most DSBs, unlike the mutants defective in AtRAD51,
AtRAD51C, or AtXRCC3. The analysis of Arabidopsis
AtATM, MEI1, and AtBRCA2 genes suggests that they play
a role in the regulation of the repair of meiotic DSBs breaks
and may be functionally conserved compared with their
animal counterparts.
MMeeiioottiicc RReeccoommbbiinnaattiioonn:: CCrroossssoovveerr FFoorrmmaattiioonn
As mentioned before, crossing over is required to maintain
homolog association after disassembly of the SC and the
recombinational exchange of sequences flanking a
crossover is a major mechanism of re-distribution of
genetic information among meiotic products. Crossing
over in many organisms exhibits interference, which refers
to the phenomenon that the presence of a crossover alters
the probability of a nearby second crossover as expected
from random distribution. In the budding yeast S. cerevisi-
ae, several genes are required for normal crossing over,
including MSH4, MSH5, and MER3 (Hollingsworth et al.,
1995; Nakagawa and Ogawa, 1999; Novak et al., 2001;
Nakagawa and Kolodner, 2002a). MSH4 and MSH5 are
two of several eukaryotic homologs of the bacterial MutS
protein involved in DNA repair and are important for the
formation of double-Holliday junctions; they form a het-
erodimer in vitro that recognizes Holliday juctions (Bocker
et al., 1999; Snowden et al., 2004). MER3 was first discov-
ered as a mutant defective in meiosis and sequence com-
parison suggests that the MER3 protein is a DNA helicase
containing a DEXDc-box. The DNA helicase activity was
demonstrated by biochemical studies showing that MER3
can unwind double stranded DNA molecules, including
Holliday junctions (Nakagawa and Kolodner, 2002b;
Mazina et al., 2004).
The msh4, msh5, and mer3 mutant meiocytes produce
a small number of crossovers, about 10-15% of the normal
levels. The remaining crossovers are randomly distributed;
therefore, they are not sensitive to interference. In addition,
mutations in the MUS81 and MMS4/EME1 genes that
encode subunits of a heterodimeric endonuclease affect a
small portion of the crossovers that are not sensitive to
interference (Hollingsworth and Brill, 2004). Thus, there are
two genetically separable pathways in yeast for crossover
formation: one is responsible for the majority of
crossovers, is sensitive to interference, and is dependent
on the function of the MSH4, MSH5, and MER3 genes; the
other is less prominent, insensitive to interference, and
requires the function of the MUS81 and MMS4/EME1
genes (Nakagawa and Ogawa, 1999; de los Santos et al.,
2003; Bishop and Zickler, 2004; Hollingsworth and Brill,
2004; Stahl et al., 2004). In humans, statistical modeling
also supports the existence of two pathways for crossing
over, with only one of these being sensitive to interference
(Housworth and Stahl, 2003). It has been observed that the
numbers of crossovers dependent on the two different
pathways vary in different organisms (Hollingsworth and
Brill, 2004). In contrast to the budding yeast, in which the
MSH4-MSH5 dependent pathway accounts for the major-
ity of crossovers, fission yeast employs the MUS81-
MMS4/EME1 pathway to generate most of the crossovers
(de los Santos et al., 2003; Bishop and Zickler, 2004;
Hollingsworth and Brill, 2004). MSH4 and MSH5 homologs
are also important for meiosis in mammals (de Vries et al.,
1999; Kneitz et al., 2000).
Several recent studies indicate that Arabidopsis, like
yeast, also has two distinct pathways for crossover forma-
tion. Using genetic mapping studies with a quartet mutant
that produces attached pollen grains that are the 4 prod-
ucts of each meiosis, it was shown that the distribution of
crossovers in Arabidopsis is also consistent two different
kinds of crossovers: interference-sensitive and insensitive
ones (Copenhaver et al., 2002; Lam et al., 2005a). In addi-
tion, Higgins et al. (2004) reported that the MSH4 homolog
in Arabidopsis, AtMSH4, is essential for an interference-
sensitive pathway for crossover formation. The AtMSH4
A Molecular Portrait of Arabidopsis Meiosis 19 of 39
protein has 792 amino acids and is 35% identical and 57%
similar to the human MSH4 in amino acid sequence.
AtMSH4 expression is highest in floral buds containing
meiotic cells, reduced in mature flowers, and not
detectable in vegetative organs. The AtMSH4 protein was
found at numerous (~100) foci at mid-leptotene, gradually
decreasing through zygotene, to only several sites at early
pachytene, and completely disappearing by late
pachytene, except at the ends of chromosomes (Higgins
et al., 2004). This pattern of protein localization is consis-
tent with the idea that AtMSH4 functions in recombination.
This hypothesis is further supported by the observation
that AtMSH4 and AtRAD51 have similar spatial and tem-
poral patterns of distribution, although AtRAD51 might be
loaded onto the chromosomes slightly earlier that
AtMSH4. Furthermore, during synapsis in zygotene,
AtMSH4 does not seem to remain on regions of chromo-
somes that have synapsed, as indicated by colocalization
studies with ZYP1.
The importance of AtMSH4 in recombination, particular-
ly crossover formation, was demonstrated by the analysis
of atmsh4 mutant and RNAi lines (Higgins et al., 2004).
These plants have dramatically reduced fertility, producing
only 7% of the normal number of seeds, and defective in
bivalent formation. The mutant meiocytes have a small
number (1.55) of chiasmata per cell, compared the normal
~10 per cell. In addition, the remaining crossovers in the
atmsh4 mutant are distributed randomly, indicating that
Arabidopsis, like the budding yeast, also has a genetically
separate pathway for interference-insensitive crossovers,
which do not require the function of the AtMSH4 gene
(Higgins et al., 2004). Although juxtaposed chromosomes
are observed at pachytene in light microscopic images,
TEM analysis of chromosome spreads indicate that SC
formation is not complete, with regions of desynapsed
chromosomes, suggesting AtMSH4 is involved in promot-
ing synapsis.
The analysis of the AtMSH4 gene provides direct evi-
dence for an interference-sensitive pathway of crossover
formation in Arabidopsis. It is likely that other genes are
also needed for this pathway. In yeast, the MER3 gene is
also required for this pathway. Is there an Arabidopsis
MER3 homolog that is necessary for crossover formation?
Two recent studies have demonstrated that this is indeed
the case (Chen et al., 2005; Mercier et al., 2005). Chen et
al. (2005) identified a gene that is preferentially expressed
in the Arabidopsis anther at the time of meiosis and found
that it is expressed in meiocytes more than other tissues in
the developing anther. This gene was named ROCK-N-
ROLLERS (RCK) for the meiotic chromosome behavior
FFiigguurree 1122.. Male meiosis I in the wild type and ptd mutants. Shown are DAPI images from wild-type (A-E), ptd-1 (F-J), ptd-2 (K-O) cells. A, F, K, zygotene. B, G, L, pachytene,
wild-type and ptd cells appear similar. C, H, M, diplotene. D, I, N, diakinesis, showing a clear contrast between 5 bivalents in the wild type and 10 univalents in ptd mutants.
E, J, O, metaphase I. Unlike the 5 bivalents in the wild-type, the ptd cells have one bivalent (arrow) and 8 univalents. This figure is modified from Figure 4 of Wijeratne et al.
(2006).
The Arabidopsis Book 20 of 39
caused by mutations in this gene. The analysis of the full-
length RCK cDNA indicates that the predicted gene struc-
ture from the genomic sequence has a number of errors,
resulting in a putative protein with a wrong sequence. The
newly obtained sequence revealed that RCK is a highly
similar to MER3, with three conserved domains.
Several T-DNA insertional lines were available for the
RCK gene, and analysis of four insertional rck mutants
FFiigguurree 1133.. Analysis of meiotic prophase I in wild-type (A,D,G, J and M), the ptd-1 (B,E,H,K and N) and the ptd-2 (C,F,I,L and O) meiocytes (White arrowheads point to the
synaptonemal complexes; black arrows point to unsynapsed elements and white arrows point to the recombination nodules). A, early pachytene, with synapsed chromo-
somes. B, SCs are found in ptd-1at a nearly normal level. C, SCs in ptd-2 is present at a reduced level. Early recombination nodules seem to be present at normal levels in
the ptd-1 and ptd-2 mutants (A and D, B and E, C and F). G and H, at mid-pachytene, SCs in the wild type and ptd-1 have the same morphology. I, SC in ptd-2 is delayed.
J, K, L, late recombination nodules are found on mid-pachytene SCs. M, N, O, normal levels of SCs in both mutant alleles at late pachytene-early diplotene stages. D, E, F,
J, K and L are magnified images from ones A, B, C, G, H and I, respectively. Bars, (A-C, G-I and M-O) =1000 nm; (D-F and J-L) =250 nm. This figure is modified from Figure
6 of Wijeratne et al. (2006).
A Molecular Portrait of Arabidopsis Meiosis 21 of 39
indicates that they are reduced in fertility (Chen et al.,
2005; Mercier et al., 2005). Phenotypic characterization
using light and electron microscopy showed that rck
mutants were defective in meiotic homolog synapsis and
crossover formation, resulting in reduced number of biva-
lents. Instead of ~10 chiasmata per cell as seen in wild-
type meiocytes, rck-1 meiocytes had an average of 3.10
chiasmata per cell. Similar to the atmsh4 mutant, the
remaining chiasmata found in the rck-1 mutant had a dis-
tribution very close to the predicted Poisson distribution,
suggesting that the residual chiasmata of the rck mutant
are insensitive to interference. In contrast, in the wild type,
chiasma distribution deviates significantly from the pre-
dicted Poisson distribution. These results provide further
support for the presence in Arabidopsis of both interfer-
ence-sensitive and -insensitive pathways and indicate that
the Arabidopsis interference-sensitive pathway for
crossover formation also requires a MER3-dependent
function.
Crossing over via the interference sensitive pathway
requires not only the formation of the double-Holliday
junction, but also its resolution. Previous studies in yeast
or animals have not uncovered a gene specific involved in
a step after the Holliday junction. Nevertheless, a recent
study in Arabidopsis has identified a potential key player in
this process (Wijeratne et al., 2006). Using microarray
analysis with RNAs from immature Arabidopsis anthers
and other tissues, a number of putative meiotic genes
were identified. One of these, At1g12790, was annotated
as being similar to bacterial DNA ligases and is expressed
in the anthers at approximately the time of meiosis. This
gene was named as PARTING DANCERS (PTD) for its mei-
otic phenotypes (see below). RNA in situ hybridization
experiments indicate that PTD is expressed in the floral
meristem, the male and female meiocytes, and other tis-
sues. Analysis of two independent T-DNA insertional lines
in the PTD gene indicates that the mutants are normal in
vegetative and flower development, but have reduced fer-
tility. Reciprocal crosses indicate that both male and
female fertility are affected in the ptd mutants.
An examination of pollen development quickly estab-
lished that male meiosis is abnormal in the ptd mutants
(Wijeratne et al., 2006). Further investigation using DAPI-
stained chromosome spreads indicate that the ptd meio-
cytes exhibit normal meiotic chromosome morphologies
from the onset of meiosis I through pachytene, forming
juxtaposed chromosomes detected as "thick threads"
(Figure 12). However, at diakinesis, the numbers of biva-
lents in the mutants are clearly reduced, though not
absent, with an average of about 2.7 (ptd-1) or 1.8 (ptd-2)
bivalents per cell, instead of the normal 5. Therefore, ptd
mutants could not maintain the normal number of biva-
lents. TEM analysis further revealed that indeed the SC is
formed in the ptd meiocytes, indicating that synapsis is not
affected by the ptd mutations (Figure 13). Again using a
social dance as a metaphor for the meiotic homolog inter-
actions, the homologs in the ptd mutants initially dance in
pairs, able to pair, juxtapose, and synapse, but then sepa-
rating from each other prematurely. Thus the mutants were
named "parting dancers."
The ptd mutant phenotypes are rather similar to those of
the atmsh4 and rck/atmer3 mutants (Higgins et al., 2004;
Chen et al., 2005; Mercier et al., 2005), suggesting that
PTD might also be important for crossing over via the inter-
ference-sensitive pathway. This hypothesis was tested by
investigating the distribution of the remaining chiasmata in
ptd-1 and ptd-2 meiocytes (Figure 14) (Wijeratne et al.,
2006). Unlike the wild-type chiasmata distribution, which
deviated dramatically from the Poisson prediction (Figure
14A), the distribution of the residual chiasmata among
cells in ptd-1 and ptd-2 mutant alleles (Figures 14B and C)
were very similar to the predicted Poisson distribution.
Furthermore, distributions of chiasmata per single chro-
mosome are very different between the wild type (Figure
14D) and the ptd mutants (Figures 14E and 14F).
Therefore, the chiasmata remaining in the ptd mutants
appear not to be sensitive to interference, indicating that
PTD is important for the interference-sensitive pathway.
FFiigguurree 1144.. Distribution of chiasmata in wild type, ptd-1 and ptd-2. A-C, chias-
ma distribution per meiocytes. D-F, chiasma distribution per chromosome. Black
lines and closed circles indicate observed distributions whereas gray line and
closed squares represent predicted Poisson distribution. A and D, the observed
distributions of the chiasmata in the wild-type cells and on the wild-type chromo-
somes, respectively. B, C, E and F, the observed distributions of the chiasmata in
mutant cells and on mutant chromosomes are close to the predicted Poisson dis-
tributions. This figure is Figure 5 of Wijeratne et al. (2006), with permission from
Mol. Biol. Cell.
The Arabidopsis Book 22 of 39
Although ptd mutants are similar to the atmsh4 and
rck/atmer3 mutants in terms of reduction of chiasmata and
bivalents and the distribution of the remaining chiasmata,
ptd mutants differ from atmsh4 and rck/atmer3 mutants in
one critical aspect, the formation of complete SC in the ptd
mutants (Wijeratne et al., 2006). In other words, the onset
of meiotic defects in ptd is later than that in the atmsh4
and rck/atmer3 mutants, suggesting that PTD acts down-
stream of AtMSH4 and RCK/AtMER3. It is possible that
PTD may function after the formation of the double
Holliday junctions, for example, by promoting their resolu-
tion. As mentioned earlier, late recombination nodules are
correlated with crossing over and in Arabidopsis,
RCK/AtMER3 is involved in the formation of late recombi-
nation nodules. Therefore, the ptd mutants were examined
in detail for the formation of recombinational nodules using
serial TEM sections. At both mid-pachytene and late
pachytene, the ptd mutant meiocytes have similar num-
bers of late recombination nodules to that in the wild-type,
with associated normal appearing SCs. Therefore, late
recombination nodules are formed at normal levels, indi-
cating that PTD is not required for this step in the recom-
binational pathway and supporting the idea that PTD acts
downstream of AtMSH4 and RCK/AtMER3.
Clearly, PTD defines a critical step in the meiotic recom-
bination pathway. What clues might its sequence provide
in terms of the mechanisms of its actions? The PTD gene
encode a 250 amino acid protein that has a putative rice
homolog, OsPTD, with 63% amino acid sequence identity
and 78% similarity over a 223-aa region, as well as puta-
tive homologs in other plants. Therefore, PTD represents a
conserved function in plants, suggesting that its homologs
in others may also have a similar meiotic function.
Moreover, PTD shares low levels of sequence similarity in
its C-terminal region to several DNA repair/recombination
proteins, including ERCC1 from mammals, RAD10 from S.
cerevisiae, the UvrC subunit of the ABC exonucleases in
bacteria, and a bacterial NAD-dependent DNA ligase
(Wijeratne et al., 2006). The level of sequence similarity is
between 24 and 36% identity (45-58% similarity) over a
region of up to 80 amino acids; therefore, it is unlikely that
PTD has the same function as these proteins, although
there may be some resemblance in biochemical activities.
The ptd mutant phenotypes and the PTD protein sequence
suggest that PTD may act to facilitate the resolution of the
double Holliday junction, by cutting and/or ligating DNA.
Furthermore, it is possible that PTD acts to resolve the
double Holliday junction in favor of crossing over, rather
than non-crossing over.
RReellaattiioonnsshhiipp aammoonngg PPaaiirriinngg,, SSyynnaappssiiss,,
aanndd RReeccoommbbiinnaattiioonn
In meiotic prophase I, homologs pair and become juxta-
posed; the SC is formed between homologs, and recom-
bination also involve the same homologs. In addition,
these processes occur during overlapping time periods
and must be carefully coordinated and intimately coupled.
However, the relationship between these processes is not
always the same among different organisms. In yeast,
synapsis and recombination are interdependent and close-
ly coupled. In particular, genetic studies strongly support
the idea that SC is dependent on the initiation of recombi-
nation by DSBs. On the other hand, in C. elegans and
Drosophila, recombination, but not synapsis, is dependent
on DSBs, indicating a different relationship in these inver-
tebrate animals (Keeney et al., 1997; Dernburg et al., 1998;
McKim et al., 1998; Peoples et al., 2002). The analysis of
Arabidopsis genes provides direct evidence for the close
relationship between pairing, synapsis and recombination
in plants.
In plants, pairing is largely homology dependent; simi-
larly, recombination necessarily relies on homology. From
the above discussion of the phenotypes of several mutants
defective in recombination, it is more than plausible that
pairing and early recombination share the same molecular
machinery. SPO11 induces DSBs, which allow homology
verification between homologs. RAD51 is important for
DSBs repair; therefore, it is reasonable that early RAD51
foci mark DSBs. Analysis in maize wild-type and mutant
meiocyte have led to the proposal that early DSBs allow
homology search for homolog pairing (Franklin et al., 2003;
Pawlowski et al., 2003; Pawlowski et al., 2004). It has been
shown that RAD51 is part of some early meiotic nodules in
lily (Anderson et al., 1997) and that recombination nodules
may be involved in pairing and synapsis (Anderson et al.,
2001; Anderson et al., 2004). The analysis of the
Arabidopsis AtRAD51 gene provides genetic support for
its role in homolog pairing and/or juxtaposition (Li et al.,
2004). Furthermore, the phenotypes of atrad51c and atxr-
cc3 mutants also support the function of these genes in
both DSBs repair and pairing. It is known that early RAD51
foci, thus DSB sites, are more numerous than recombina-
tional crossovers in maize and Arabidopsis (Franklin et al.,
1999; Higgins et al., 2004). Therefore, most of SPO11-
induced DSBs are probably generated to allow pairing,
which involved base-pairing of DNA from two non-sister
chromatids. After homology has been established, most of
the DSBs are repaired without exchange of flanking
sequences (crossing over), whereas a small fraction of
DSBs remain to form crossovers and chiasmata, which
maintain bivalent. This hypothesis predicts that all genes
needed for the generation and repair of DSBs are also
A Molecular Portrait of Arabidopsis Meiosis 23 of 39
important for pairing, and that pairing is correlated with
repair of DSBs, or gene conversion. In addition to
AtSPO11-1, AtRAD51, AtRAD51C, and AtXRCC3,
AtMRE11, and AtRAD50 are also required for early recom-
bination steps. Although details of homolog pairing in the
atmre11 mutant meiocytes are not available, the
pachytene phenotypes are consistent with a pairing and/or
synapsis defect. Further analysis of atmre11 and other
mutants to fully investigate their pairing properties is need-
ed to verify the above proposed direct coupling of DSB
repair and pairing.
The interdependence of synapsis and recombination in
plants is very well supported. Although synapsis was not
directly observed in the atspo11-1 mutant, the atspo11-1
mutant phenotypes is consistent with synapsis being
SPO11-dependent (Grelon et al., 2001). Furthermore,
analysis using TEM on synapsis in the atrad51-1 and
atrad51c-1 mutants (Li et al., 2004; Li et al., 2005) argue
strongly for a close relationship between recombination
and synapsis, similar to that in yeast. Moreover, the
atmsh4 and rck/atmer3 mutants are also defective in
synapsis. During normal meiosis, it is likely that DSBs are
FFiigguurree 1155.. Spindle structures in wild-type and atk1-1 male meiocytes. The microtubules (green) were visualized with anti-tubulin antibodies; DNA was stained with DAPI
and the false red color was generated using Photoshop. A-D, I-L, wild-type. E-H, M-P, atk1-1. A and E, prophase I, with perinuclear microtubules. B and F, metaphase I; the
atk1 spindle is unfocused at the poles. C and G, anaphase I, showing abnormal chromosome segregation in the atk1 cell. D and H, telophase I. I and M, prophase II. J and
N, metaphase II, with multiple mini-spindles in the atk1 mutant. K and O, anaphase II. L and P, telophase II, with more than 4 nuclei in the mutant. This figure is Figure 5 of
Chen et al. (2002).
The Arabidopsis Book 24 of 39
the sites of early recombination nodules. In addition, a
subset of DSBs might be the sites for loading SC proteins,
as supported by the analysis of ZYP1 foci, which initially
number in 20-25 (Higgins et al., 2005). This can explain
why in the absence of SPO11-1 induced DSBs, synapsis
is defective. AtDMC1 is required for bivalent formation,
indicating a critical role in crossover formation. However,
unlike AtRAD51, AtRAD51C, and AtXRCC3, AtDMC1 does
not seem to be required for DSBs repair, suggesting that it
may have a more specialized function for processing those
DSBs targeted for crossover formation. It is possible that
these DSBs correspond to late recombination nodules that
require AtMSH4 and RCK/AtMER3, and that facilitate the
loading of ZYP1. However, the localization of ZYP1 in
atmsh4 meiocytes indicates that AtMSH4 is not required
for early loading of ZYP1 (Higgins et al., 2005). It is not
known what effect rck/atmer3 mutations might have on
ZYP1 localization to the chromosomes.
CChhrroommoossoommee SSeeppaarraattiioonn aanndd SSeeggrreeggaattiioonn
The elaborate process of prophase I involving homolog
pairing, synapsis and recombination results in formation of
the bivalents at diakinesis. The purpose of this highly com-
plex and tightly controlled "meiotic dance" is to make sure
that homolog segregation at anaphase I is accurate.
However, bivalent formation is not sufficient for proper
segregation. Before the homologs can separate, the biva-
lents need to move to the metaphase plate, located at the
center of the meiotic spindle. This process depends on a
FFiigguurree 1166.. Wild-type and atk1-1 male meiosis. A.-D., I., J., L., wild type; E.-H., K., M.-T. atk1-1. A. and E, zygotene. B and F, pachytene. C and G, diplotene. D and H, diaki-
nesis. Mutant cells during prophase I seem normal. I and M, metaphase I; the position of bivalents are not normal in the atk1-1 cell, with one bivalent (arrow) located slightly
away from the equator; three others (arrowheads) are not aligned in parallel. K and N, metaphase I to anaphase I transition in atk1, showing asynchrony in homolog separa-
tion (arrows). J and O, anaphase I; in atk1-1 meiocytes, some pairs of homologues are misaligned (arrowheads in K and O). L and P, telophase I; atk1-1 has uneven chromo-
some segregation. This figure is modified from Figure 4 of Chen et al. (2002).
A Molecular Portrait of Arabidopsis Meiosis 25 of 39
properly assembled spindle and other spindle associated
functions. Following the alignment of bivalents along the
equatorial plane of the spindle, sister chromatid cohesion
along chromosomal arms must be removed via the prote-
olytic degradation of specific cohesin subunits, to allow
homolog separation at the onset of anaphase I.
Subsequently in anaphase I, the newly separated
homologs are pulled by the forces of the spindle micro-
tubules towards the two spindle poles.
The meiotic spindle provides the force for chromosome
movement and is critical for normal chromosome segrega-
tion. During meiosis I, the spindle forms at prometaphase
I into a bipolar structure (Franklin and Cande, 1999). In
Arabidopsis, a mutant (atk1, for Arabidopsis thaliana
kinesin1) was found to have greatly reduced male fertility
and mutant male meiocytes form abnormal meiotic spin-
dles (Chen et al., 2002). The atk1 mutant meiotic spindles
have unfocused poles, with disorganized microtubules
(Figure 15). In addition, bivalents in mutant meiocytes fail
to align at the metaphase I plate perfectly, with some biva-
lents seemingly lagging behind in their movement to the
metaphase I plate (Figure 16). Therefore, a normal-shaped
spindle may be needed to facilitate the alignment of the
bivalents. The atk1 mutant meiocytes also exhibit slightly
asynchronous homolog separation (Figure 16). It is possi-
ble that uniform homolog separation requires an even dis-
tribution of microtubule forces from a properly formed
spindle. The ATK1 gene is the same gene as the previous-
ly cloned KATA gene, which encodes a kinesin protein
(Mitsui et al., 1993; Liu et al., 1996; Chen et al., 2002).
ATK1 has a predicted C-terminal motor domain and is
highly similar to the Drosophila NCD and budding yeast
KAR3 proteins; both NCD and KAR3 are important for mei-
otic and/or mitotic spindle morphogenesis (Hatsumi and
Endow, 1992; Roof et al., 1992). Therefore, ATK1, NCD and
KAR3 are structurally and functionally conserved proteins
FFiigguurree 1177.. DNA breaks in mmd1 microsporocytes detected by the TUNEL assay. A-D, wild type cells showing DNA (red), each with an inset in the upper right-hand corner
showing no signal detected for DNA breaks (green). E-H, Chromosomes in mmd1 cells; I-L, DNA breaks in the same mmd1 cells as in E-H, respectively. A, diakinesis. B,
prometaphase I. C, G and K, metaphase I. D, H and L, anaphase I. E and I, early diakinesis. F and J, late diakinesis. Chromosomes in mmd1 cells are labeled with TUNEL start-
ing at late diakinesis. Bar = 10mm. This figure is modified from Figure 4 in Yang et al. (2003b).
The Arabidopsis Book 26 of 39
important for meiotic spindle assembly. The abnormal
unfocused spindle in the atk1 mutant is also similar to that
of the maize mutant dv (divergent spindle) (Staiger and
Cande, 1990).
Both ncd and kar3 mutants also have mitotic defects
(Hatsumi and Endow, 1992; Roof et al., 1992; Endow et al.,
1994). ATK1 is also expressed in vegetative and floral
organs, suggesting that it may also have a mitotic function.
Careful examination of mitotically dividing cells of the atk1
mutant revealed that the mitotic spindle is also affected
and mitosis is prolonged in the mutant (Marcus et al.,
2003); however, the spindle defect is not severe enough to
cause gross developmental abnormalities (Chen et al.,
2002). The mild defect in mitotic cells might be due to
functional redundancy, because Arabidopsis has another
gene, ATK5, which encodes a protein highly similar to
ATK1, with an overall 83% amino acid sequence identity to
ATK1. Both ATK1 and ATK5 proteins can bind to and move
along microtubules in vitro (Marcus et al., 2002; Ambrose
et al., 2005), suggesting that they have very similar func-
tions. Whether they indeed have redundant in vivo func-
tions will be determined by genetic studies.
At the metaphase I/anaphase I transition, the separation
of homologs requires the resolution of sister chromatid
cohesion. In Arabidopsis, it is not clear how cohesins are
removed. Nevertheless, the ask1 mutant is defective in
chromosome separation and segregation at anaphase I
(Yang et al., 1999). In ask1 meiocytes, some chromosomes
remain associated when they are pulled by the spindle and
become stretched. In addition, the forces of the spindle
also seem to cause chromosome fragmentation. The fail-
ure of chromosome separation then results in grossly
uneven chromosome distribution. Cytokinesis then pro-
duces abnormal number of microspores contain abnormal
amounts of DNA. ASK1 encodes a homolog of the SKP1
protein (Yang et al., 1999), which is a critical component of
SCF complexes (for SKP1, cullin/CDC53, F-box protein)
(Zhang et al., 1995; Bai et al., 1996; Connelly and Hieter,
1996; Peters, 1998). As discussed before, the SCF com-
plexes are E3 ubiquitin-protein ligases that control a num-
ber of biological processes by targeting specific proteins
for degradation by the 26S proteosome (Zheng et al.,
2002). The chromosome behavior of the ask1 mutant
meiocytes suggests that an ASK1-containing SCF ubiqui-
tin ligase is involved in the removal of the cohesin complex
along chromosomal arms. Recently, it was found that the
ASK1 protein accumulates during early prophase I, prima-
rily at leptotene to pachytene, at levels higher than those
at other stages (Wang and Yang, 2005), suggesting that
the effect of ask1-1 mutation on chromosome separation
might be indirect consequence of an early function of
ASK1. In addition, the reduction of ASK1 function in the
ASK1/ask1-1 heterozygous plants is associated with an
increase in the rate of recombination, suggesting that
ASK1 negatively regulates recombination (Wang and Yang,
2005). Further investigation is needed to understand the
role of ASK1 and SCF complexes in meiosis.
In addition to ASK1, Arabidopsis has 20 other SKP1
homologs; among these, ASK2 is most similar to ASK1 in
sequence and expression patterns (Zhao et al., 2003b;
Kong et al., 2004). However, the fact that ask1 is com-
pletely defective in male meiosis, resulting in total male
sterility, indicates that ASK2 is not able to fulfill the
required function. ASK2 is expressed at a level lower than
that of ASK1; therefore, it is possible ASK2 is not
expressed sufficiently for the male meiotic function.
Alternatively, the ASK2 protein might be different from
ASK1 in sequence, such that ASK2 is not able to interact
with other proteins as well as ASK1. These ideas were
tested using transgenic plants overexpressing ASK2 (Zhao
et al., 2003a); when ASK2 is overexpressed in the ask1
mutant background, the meiotic defects are partially cor-
rected and the plants are partially fertile. However, the fact
that these plants are still partially defective indicates that
the ASK2 protein is functionally distinct from ASK1. This
idea is further supported by the phylogenetic finding that
ASK1 and ASK2 are members of separate clades which
each have representatives of many plants (Kong et al.,
2004). Therefore, ASK1 and ASK2 are not equivalent in
male meiosis, although they do have redundant functions
during embryo and seedling development (Liu et al., 2004).
MMeeiioottiicc PPrrooggrreessssiioonn:: PPootteennttiiaall RReegguullaattoorrss
To ensure the success of a complex process such as
meiosis, it is necessary to control the timing and sequence
of various meiotic events. For example, chromosome con-
densation and sister chromatid cohesion are regulated by
protein phosphorylation (Wei et al., 1998; Suja et al., 1999;
Wei et al., 1999; De Souza et al., 2000). In yeast and nem-
atode, the mitotic histone H3 phosphorylation is controlled
by the balanced activities of both a protein kinase
(Ipl1/Aurora) and a phosphatase (Glc7/PP1) (Francisco and
Chan, 1994; Francisco et al., 1994; Hsu et al., 2000).
Furthermore, it was shown that the maintenance of sister-
chromatid cohesion in maize is correlated with phosphory-
lation of histone H3 (Kaszas and Cande, 2000), suggesting
a role in the regulation of sister chromatid cohesion.
Another study indicates that dissociation of sister chro-
matid cohesin is correlated with phosphorylation of the
Xenopus XSA1/Scc3p cohesin subunit (Losada et al.,
2000). Furthermore, purified cyclin B-CDK has been
shown to phosphorylate XSA1 in vitro and reduce its abili-
ty to bind DNA or chromatin (Losada et al., 2000). These
results together suggest that phosphorylation of several
A Molecular Portrait of Arabidopsis Meiosis 27 of 39
proteins, at least in part mediated by cyclin-CDK, may be
involved in the separation of sister chromatids.
Cyclins and CDKs were originally discovered as factors
promoting the maturation of frog oocytes (MPF). In yeast,
cyclins and CDKs (cdc2 from S. pombe and CDC28 from
S. cerevisiae) play pivotal roles in controlling the mitotic
cell cycle (Futcher, 1991; Nasmyth, 1993; Murray, 1994;
Reed, 1996). In addition, they also function in regulating
meiosis. For example, Clb1, Clb3, and Clb4 regulate
events just before meiosis I (Grandin and Reed, 1993) and
also meiosis II (Dahmann and Futcher, 1995). Moreover,
the mitotic S-phase cyclins Clb5 and Clb6 are also
required for the meiotic S phase (Dirick et al., 1998; Stuart
and Wittenberg, 1998). Furthermore, Clb5 and CLb6 also
seem to be important for the meiotic prophase I, including
synapsis and recombination (Smith et al., 2001); however,
the fact that these proteins are required for pre-meiotic S
phase makes it different to determine whether the
Clb5/Clb6 function in prophase I is direct. In mouse, the
cyclin A1 is specifically expressed in the testis and mutant
mice are male sterile and defective in male meiosis (Liu et
al., 1998a). In the mutant, male meiosis arrests before
metaphase I, with desynaptic chromosomes, and then
undergo programmed cell death. Cyclin-CDK also plays
critical roles in regulating sister chromatid cohesion during
mitosis. Cyclin-CDK mediated phosphorylation regulates
the activity of the APC E3 ubiquitin ligase, thereby control-
ling the timing of cohesin removal (Heo et al., 1999;
Zachariae, 1999).
In Arabidopsis, a screen for fertility defects among a
transposon insertional population identified a mutant with
greatly reduced fertility (Azumi et al., 2002). This mutant
was named solo dancers (sds) because meiotic chromo-
somes progress through prophase I largely as individuals,
unlike the wild-type chromosomes, which undergo
prophase I as pairs following homolog pairing. The sds
mutant meiocytes are defective in homolog pairing/juxta-
position and recombination, resulting in greatly reduced
bivalent formation. The sds mutant phenotypes are nearly
identical to those of atspo11-1 and atdmc1 mutants, sug-
gesting that SDS is required for the SPO11-dependent
pathway for meiotic recombination. At the same time, in
contrast to the atrad51, atrad51c, or atxrcc3 mutants, the
sds mutant meiocytes do not have fragmented chromo-
somes, suggesting that the sds mutant cells either do not
have SPO11-induced DSBs or are able to repair the DSBs.
In addition, sds mutant meiocytes exhibit premature partial
separation of sister chromatids, suggesting a possible role
in regulating cohesion. In situ hybridization experiments
indicate that SDS is specifically expressed in male and
female meiotic cells; in addition, SDS encodes a putative
novel cyclin, which is phylogenetically distinct from other
cyclins (Azumi et al., 2002; Wang et al., 2004a). It is likely
that SDS activates one or more CDKs to regulate protein
activities via phosphorylation, thereby coordinating multi-
ple meiotic events, including pairing, synapsis, recombina-
tion, and possibly sister chromatid cohesion.
In addition, the Arabidopsis tardy asynchronous meiosis
(tam) mutant is abnormal in the timing of meiotic divisions,
resulting in the formation of intermediate structures such
as dyads, which are not produced by wild-type meiosis
(Magnard et al., 2001). In particular, in cells entering
metaphase I or metaphase II, there is a clear indication of
asynchronous meiosis (Magnard et al., 2001). Therefore,
TAM may control the entry into metaphase, as further sup-
ported by the fact that the tam mutation is in the gene
encoding the cyclin A CycA1,2 (Wang et al., 2004b). Using
a GFP fusion, it was observed that TAM/CycA1,2 is pres-
ent at its highest level at pachytene, becoming unde-
tectable at diakinesis and subsequent meiotic stages, sug-
gesting it regulates later stages of meiosis indirectly, pre-
sumably via the targets of TAM/CycA1,2-activated phos-
phorylation (Wang et al., 2004b). The function of
TAM/CycA1,2 and the mouse cyclin A1 (Liu et al., 1998a)
in meiosis suggests that plants and animals may share
conserved aspects of the control of meiotic progression.
Despite the asynchronous meiosis, the tam mutant is still
fertile; therefore, it is possible that mutant screens for a
reduction of fertility might have missed mutants that have
altered meiotic timing but are fertile.
Another possibility for the small number of mutants in
meiotic progression is that meiotic progression may share
regulators with the mitotic cell cycle and mutations in such
genes might have severe defects in mitotic growth, pre-
venting their recovery as meiotic mutants. For example,
the Arabidopsis homolog of the yeast CDC45 gene is
expressed in floral buds; a reduction of CDC45 expression
using an RNAi construct resulted in fertility defects
(Stevens et al., 2004). Phenotypic characterization of these
RNAi lines indicates that polyads instead of tetrad were
formed from meiosis and nonviable pollen grains were pro-
duced. Furthermore, meiotic chromosomes in the RNAi
transgenic plants are fragmented at late prophase I and
the fragmentation becomes more severe at metaphase I
and anaphase I. In yeast, CDC45 regulates mitotic DNA
replication (Zou and Stillman, 1998, 2000), suggesting that
CDC45 in Arabidopsis may regulate the pre-meiotic DNA
replication; a defect in DNA synthesis provides an expla-
nation for chromosome fragmentation during meiosis.
Another potential Arabidopsis regulator of meiosis was
discovered by two independent studies, called MALE
MEIOCYTE DEATH1 (MMD1) or DUET (Reddy et al., 2003;
Yang et al., 2003b). Mutations in the MMD1/DUET gene
cause severe defects in the progression of meiosis and the
The Arabidopsis Book 28 of 39
inability to produce any normal microspores. In prophase I,
the mmd1 mutant meiocytes appears normal up to diaki-
nesis (Yang et al., 2003b). Subsequently through telophase
II, mutant male meiocytes have chromosome fragmenta-
tion and cytoplasmic shrinkage; suggesting that the cells
are undergoing programmed cell death. In addition, these
mutant cells have positive TUNEL signal, indicating the
presence of many DNA breakage in the chromosomes
(Figure 17). The mmd1 mutant meiotic cells die before
cytokinesis, resulting in a failure to produce any
microspores. The mmd1 mutant was caused by a Ds
transposon insertion just downstream of the ATG codon,
resulting in a likely null allele (Yang et al., 2003b). In situ
hybridization experiments indicate that MMD1 is preferen-
tially expressed in the meiocytes. The predicted MMD1
protein contains a C-terminal PHD finger domain. PHD fin-
gers are cystein-containing domains, some of which have
been shown bind other proteins, DNA, or RNA (Aasland et
al., 1995; Kennison, 1995; Coscoy and Ganem, 2003;
Reddy et al., 2003; Yang et al., 2003b). The duet mutant
has a milder phenotype than that of the mmd1 mutant,
with delayed meiotic cytokinesis that often results in dyads
(two spores) instead of tetrads (Reddy et al., 2003). The
duet microspores usually proceed with one or two mitotic
divisions, and then undergo cell degeneration. The duet
mutation is also caused by a Ds insertion, which is approx-
imately 500 amino acid residues downstream of the N-ter-
minus and just upstream of the PHD finger. It is possible
that the duet allele can produce a truncated protein that is
partially functional. It is worth noting that in most plant
meiotic mutants, the abnormal meiocytes do not undergo
programmed cell death, unlike the meiotic mutants in ani-
mals. Therefore, the phenotypes of the mmd1 mutant are
rather unusually for a plant meiotic mutant, and may sug-
gest a general regulatory function of the MMD1 gene. The
hypothesis that MMD1 is a general regulator, rather a pro-
moter of a specific meiotic event, is also supported by the
lack of specific meiotic chromosomal defects in pairing,
juxtaposition, synapsis, or segregation in the mmd1
mutant.
Another potential regulatory gene was uncovered by the
mutants ms5 and tdm1, which are male sterile and defec-
tive in male meiosis (Chaudhury, 1993; Ross et al., 1997).
Detailed phenotypic analysis of tdm1 revealed that the
mutant cells undergo meiosis I and II in a way similar to the
wild-type cells; however, meiosis II is followed by an
abnormal third division without a new round of DNA repli-
cation, resulting in the formation of polyads of 5-8 abnor-
mal spores. Therefore, MS5 normally regulates the com-
pletion of meiosis II by presenting another division. The T-
DNA insertional ms5-2/tdm1 mutation was used to clone
the MS5 gene (Glover et al., 1998), which is a member of
a gene family conserved in plants, including multiple mem-
bers in Arabidopsis and rice. The MS5 protein sequence is
novel; although it has limited similarity to a synaptonemal
complex protein from rat, it is not clear what the physio-
logical significance of this similarity is, as the ms5 mutant
does not seem to have any defect in synapsis or other
abnormal chromosomal behavior during prophase I.
MMeeiioottiicc CCyyttookkiinneessiiss
At the end of meiosis II, four clusters of chromosomes are
formed and they become four new nuclei. Cell membranes
and wall materials are then transported to the space
between these nuclei and cytokinesis occurs to generate
four haploid microspores. Using immuno-electron
microscopy and electron tomography, Otegui and
Staehelin (2004) showed that meiotic cytokinesis entails
the formation of a distinctive post-meiotic-type cell plate.
The formation of the post-meiotic cell plates depends on
the function of mini-phragmoplasts and occurs simultane-
ously throughout the divisional plane. Presumably, vesicles
carrying materials for cell plate are transported along
microtubules of the mini-phragmoplast by microtubule
associated motor kinesins. Fusion of these vesicles results
in a tubular network, which fuses at its periphery with the
plasma membrane, prior to the formation of the cell plate.
The Arabidopsis stud (std) and tetraspore (tes) mutants
were found to have similar phenotypes and subsequently
shown to be allelic (Hulskamp et al., 1997; Spielman et al.,
1997). In the std/tes mutant meiocytes, meioses I and II
proceed normally, but cytokinesis is defective and cell wall
formation is incomplete. Consequently, a single
microspore is formed with four nuclei that are partially sep-
arated by incomplete walls and a single cytoplasm. The
abnormal microspore is able to continue through pollen
development. In normal pollen development, the
microspore first divides asymmetrically to form a large
vegetative cell and a small generative cell, which is then
internalized to be surrounded by the vegetative cells. The
generative cell divides again inside the vegetative cell to
form two sperm cells. During std/tes mutant pollen devel-
opment within the large abnormal microspore, the four
nuclei divide separately. The result is a very large pollen
grain with four vegetative nuclei and up to eight sperm
cells. Upon pollination, the large pollen grain produces a
single pollen tube, allowing the fertilization of one ovule,
resulting in greatly reduced male fertility (Hulskamp et al.,
1997).
The TES/STD gene has been cloned using a mapped-
based cloning strategy and it encodes a kinesin with an N-
terminal motor domain (Yang et al., 2003a). The TES pro-
tein is most similar to two closely related tobacco kinesins,
NACK1 and NACK2 (64% and 58% identity over the entire
proteins, respectively), that are required for cell plate
A Molecular Portrait of Arabidopsis Meiosis 29 of 39
expansion during cytokinesis (Nishihama et al., 2002). In
addition, TES is highly similar (57% identity/73% similarity
overall) to another Arabidopsis kinesin called HINKEL,
which is involved in mitotic cytokinesis (Strompen et al.,
2002). In the tes mutant meiocytes, the radial microtubule
arrays that are found between the four meiotic nuclei at the
end of meiosis II are not present, indicating that the TES
kinesin is required for the formation and/or maintenance of
such microtubule structures. In the absence of the micro-
tubule arrays, material for cell plate formation cannot be
properly distributed and cytokinesis is incomplete. This
hypothesis is consistent with the known functions of
NACK1/2 and HINKEL in mitotic cytokinesis (Nishihama et
al., 2002; Strompen et al., 2002). The TES gene is
expressed in both meiocytes and mitotically dividing cells,
suggesting that it may also play a role in mitotic cytokine-
sis and possibly other processes. One explanation that the
tes/std mutants are defective only in meiotic cytokinesis is
that other (partially) redundant genes may provide the
needed function in mitotic cell cycle, such as the HINKEL
gene.
CCoonncclluuddiinngg RReemmaarrkkss
The development of cytological tools for Arabidopsis by
pioneers of plant meiosis has opened the door for the
molecular genetic investigations of the meiotic machinery.
Efforts by a number of laboratories over the last decade
have identified key genes important for processes from
early prophase I to meiotic cytokinesis, not only demon-
strating conservation of gene functions between yeast and
plants, possibly also with vertebrate animals, but also
uncovering novel aspects of a molecular portrait of
Arabidopsis meiosis. These studies have firmly established
Arabidopsis as an excellent model system for understand-
ing not only plant meiosis, but also meiosis in general. As
it is already indicated by a number of genes, meiosis
depends on not only single-copy meiosis-specific genes,
such as AtSPO11-1, AtDMC1, and SDS, that are readily
discovered by forward genetics or examined using reverse
genetics, but also requires genes that are functionally
redundant or are also essential for mitotic growth, includ-
ing the BRCA2 genes. It is likely that future efforts will
greatly expand the ranks of this second category of genes
important for meiosis.
While currently available tools are invaluable in under-
standing meiotic gene functions, additional technology
need to be developed. Meiosis in Arabidopsis is usually
analyzed using chromosome spreading, which may distort
mutant phenotypes, including exaggeration and loss of
some details. In maize, meiotic chromosomes can be
observed in three-dimensional images using deconvolu-
tion microscopy (Franklin et al., 1999; Franklin et al., 2003;
Pawlowski et al., 2003; Pawlowski et al., 2004); such tech-
nology should also be applicable to Arabidopsis meio-
cytes. In addition, markers for observing meiosis in living
cells will go a long way to providing physiological informa-
tion about this still mysterious process. The tools from the
post-genomic era of Arabidopsis biology will also greatly
facilitate the process of identification and characterization
of new genes and proteins important for meiosis.
Therefore, whereas the past decade has been nothing
short of amazing, the future of meiosis research promises
to be even more exciting and satisfying.
AAcckknnoowwlleeddggmmeennttss
I greatly appreciate the contributions to the analyses of
Arabidopsis meiosis from former and current lab members,
including M. Yang, D. Liu, Y. Azumi, D. Zhao, X. Yang, W.
Li, W. Zhang, C. Chen, L. Timofejeva, G. Wang, H. Kong,
A. Wijeratne, L. Quan, Z. Lin, R. Xiao, and Y. Hu. In addi-
tion, I would like to thank W. Li, L. Timofejeva, and Z. Lin
for providing meiotic images and a phylogenetic tree, and
Z. Cande, O. Hamant, C. Makaroff, W. Pawlowski, A.
Villeneuve, and A. Wijeratne for helpful discussion and
comments. I am grateful to the John Simon Guggenheim
Memorial Foundation and the National Institutes of Health
(F33 GM72245-1) for their support. The research in my lab-
oratory has been supported by grants from USDA (2001-
35301-10570 and 2003-35301-13313), NSF (IBN-
0077832, MCB-9896340, MCB-0092075, and DBI-
0115684), NIH (RO1 GM63871), and DOE (DE-FG02-
02ER15332). The work in my lab was conducted using
plant materials generated withsupport from the National
Science Foundation. Further support for our research was
provided by the Biology Department and the Huck
Institutes of the Life Sciences, The Pennsylvania State
University.
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