B chromosomes
B chromosomes
rye (Secale cereale)
Brachycome
dichromosomatica Crepis capillaris
(1) dispensable
(2) usually smaller than A chromosomes
(3) do not pair/recombine with A chromosomes at meiosis
(4) their inheritance is irregular, non-Mendelian (univalents)
(5) meiotic elimination in some species is counter-balanced by
processes of drive at mitosis, and less frequently at meiosis
(equilibrium frequencies in populations)
(6) neutral effects; negative and quantitative effects on the
phenotype when present in high numbers (reduced fertility)
(7) they lack any known major gene loci, but rDNA sequences are
known in a few species
(8) they contribute greatly to intraspecific genome size variation
(9) they have no obvious adaptive properties
(10) their mode of origin remains a mystery
B chromosomes
14 A and 2 B chromosomes in rye
B
B
B chromosomes
The occurrence of Bs across angiosperms seems to be not random:
their presence is correlated with genome size - higher frequency in
families with large genomes
How frequent?
Bs in about 9% of angiosperm species
monocots: 8% (Poaceae, Liliales and Commelinales)
eudicots: 3% (Asteraceae)
How many?
▪ usually in low numbers (0–5)
▪ exceptions: Silene maritima (0–15), Brachycome lineariloba (0–
22) or Allium schoenoprasum
▪ more Bs than As in maize (2n = 20 As + 0–34 Bs)
▪ number can vary between tissues: grasses Aegilops speltoides
and Ae. mutica - Bs exist in aerial organs but not in roots
HGolczyk(2012)CGR
Tradescantia virginiana
(2n=24)
PNAS 2012
Mysterious origins (?)
▪ Seem to arose in different ways in different species
▪ Generally thought to originate from A chromosomes
▪ Proposed origins (examples): from centric fragments after an unequal
reciprocal translocation or by excision from A chromosomes
Multichromosomal Origin of Rye B Chromosomes and Sequences
Located on B Chromosomes
B chromosomes as a „genomic sponge“
Model of Origin and Evolution of Rye B Chromosomes
How a supernumerary B chromosome survives over time?
Post-meiotic chromosome drive in rye/Aegilops/maize
(Poaceae), pollen grain mitosis I:
➢ asymmetry of the mitotic spindle (vegetative and generative nucleus)
→ the equatorial plate is nearer to the generative pole → Bs into the
generative nucleus
➢ Nondisjunction: 2 chromatids of the B chromosome do not separate at
anaphase and are included in the generative nucleus.
Nondisjunction control factor on B chromosome (sequence unknown) →
? noncoding RNA influencing the differential sister-chromatid cohesion of As
and Bs (= B chromosome chromatids not separated)
➢ pollen grain mitosis II: both spermatids have an unreduced number of
Bs
➢ similar nondisjunction does not occur in the female gametophytes
❖ transmission higher than Mendelian → kept in populations
❖ drive (pre-meiotic, meiotic, post-meiotic) = preferential maintenance of Bs
❖ post-meiotic drive common in plants during gametophyte maturation (examples: rye, maize)
Alternative explanation (in some other species?):
B-specific centromeric repeats acting as a neocentromere → „stronger centromeres“→ higher pulling force on the B
centromere towards the generative pole
Post-meiotic B-chromosome drive
1stpollenmitosis
At late anaphase,
nondisjunction of B
sister chromatids
(lagging chromosomes).
Formation of
asymmetric spindle at
early anaphase. The
generative spindle
(Gen) is short and
blunt and the
vegetative spindle
(Veg) is long and
sharp.
Pollen at late
anaphase
forming an
asymmetric spindle
midzone. Note the
unequal number of
CENH3 signals.
Wu et al. (2019) New Phytol
Holocentric chromosomes
Juncaceae
Cyperaceae
Myristica fragrans (Myristicaceae)
Drosera (Droseraceae)
Angiosperm species with holokinetic chromosomes
Chionographis (Melanthiaceae)
• holocentric vs. monocentric chromosomes
• holocentrics: huge variation in chromosome numbers [the largest number
of chromosomes in animals (2n = 446) is found in the blue butterfly
Polyommatus atlantica with holokinetic chromosomes]
• in c. 5,500 angiosperm species
• chromosome numbers from n=2 up to n=110
• chromosome fission (agmatoploidy) and fusion (symploidy)
→ extensive chromosome number variation
Holocentric chromosomes
chromosome segregation
in anaphase
difuse kinetochor →
Variation in chromosome number in some holocentric plant genera
(Bureš et al. 2013)
Holocentric chromosomes, centromeres and microtubules
Microtubules (tubulin) attach at CENH3,
but not at H2AThr120ph. The microtubule
bundle formation is less pronounced at
holocentromeres.
Model of the centromere
organization of mono- and
holocentric plant chromosomes
active centromeres have H2AThr120ph phosphorylation
of threonine 120 of
histone H2A
▪ chromosomes are structurally and functionally holocentric throughout
meiosis
▪ an inverted sequence of sister chromatid segregation occurs during
meiosis
▪ terminal satellite DNA repeat-enriched chromatin threads assist the
pairwise movement and the linkage of homologous non-sister chromatids
up to metaphase II to enable the faithful formation of haploid gametes
…also in Rhynchospora pubera
and R. tenuis (Cyperaceae)
Chromosomes align at metaphase I in such a manner that sister chromatids rather than
homologous chromosomes are separated during meiosis I. Homologous non-sister chromatids
are terminally linked by satellite DNA-enriched chromatin threads until metaphase II to
ensure faithful transmission of holocentric chromatids.
Holocentric chromosomes in Eleocharis
CGR 122 (2008)
2n = 10
2n = 8 2n = 7 2n = 6