DOI: 10.1126/science.286.5441.964
, 964 (1999);286Science
Bruce T. Lahn and David C. Page
Four Evolutionary Strata on the Human X Chromosome
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mann, J. R. Ecker, Cell 72, 427 (1993)] (23). The
largest of 16 independent NPH3 cDNAs was sequenced
(24) completely (GenBank accession number
AF180390).
11. GenBank searches were accomplished with the
gapped BLAST program [S. F. Altschul et al., Nucleic
Acid Res. 25, 3389 (1997)].
12. The data are available at www.sciencemag.org/
feature/data/1042358.shl.
13. Single-letter abbreviations for the amino acid residues
are as follows: A, Ala; C, Cys; D, Asp; E, Glu; F,
Phe; G, Gly; H, His; I, Ile; K, Lys; L, Leu; M, Met; N, Asn;
P, Pro; Q, Gln; R, Arg; S, Ser; T, Thr; V, Val; W, Trp; and
Y, Tyr.
14. T. Patschinsky, T. Hunter, F. S. Esch, J. A. Cooper, B. M.
Sefton, Proc. Natl. Acad. Sci. U.S.A. 79, 973 (1982).
15. The BTB/POZ domain was identified with SMART [J.
Schultz, F. Milpetz, P. Bork, C. P. Ponting, Proc. Natl.
Acad. Sci. U.S.A. 95, 5857 (1998)]. The coiled-coil
structure was identified with COILS [A. Lupas, M. Van
Dyke, J. Stock, Science 252, 1162 (1991)].
16. O. Albagli, P. Dhordain, C. DeWeindt, G. LeCocq, D.
LePince, Cell Growth Differ. 6, 1193 (1995); L. Aravind
and E. V. Koonin, J. Mol. Biol. 285, 1353 (1999).
17. C. Cohen and D. A. D. Parry, Proteins 7, 1 (1990); A.
Lupas, Trends Biochem. Sci. 21, 375 (1996).
18. Structural analyses were performed with the Protean
program (DNASTAR, Madison, WI).
19. S. Fields, Methods 5, 116 (1993); S. Fields and R.
Sternglanz, Trends Genet. 10, 286 (1994).
20. M. Nagao and K. Tanaka, J. Biol. Chem. 267, 17925
(1992).
21. M. C. Faux and J. D. Scott, Cell 85, 9 (1996); T.
Pawson and J. D. Scott, Science 278, 2075 (1997);
E. A. Elion, Science 281, 1625 (1998).
22. S. D. Choi, R. Creelman, J. Mullet, R. A. Wing, Weeds
World 2, 17 (1995), http://genome-www.stanford.
edu/Arabidopsis/ww/home.html.
23. J. Sambrook, E. F. Fritsch, T. Maniatis, Molecular Cloning:
A Laboratory Manual (Cold Spring Harbor Laboratory
Press, Plainview, NY, 1989).
24. Sequencing templates were prepared by polymerase
chain reaction and sequenced with an ABI377 automated
sequencer (Perkin-Elmer, Norwalk, CT ).
25. Phototropism and hypocotyl growth was assayed as
described previously [E. L. Stowe-Evans, R. M. Harper,
A. V. Motchoulski, E. Liscum, Plant Physiol. 118, 1265
(1998)].
26. E. Liscum and R. P. Hangarter, Plant Cell 3, 685
(1991).
27. J. W. Reed, P. Nagpal, D. S. Poole, M. Furuya, J. Chory,
Plant Cell 5, 147 (1993).
28. C. Bell and J. R. Ecker, Genomics 19, 137 (1994).
29. H.-G. Nam et al., Plant Cell 1, 699 (1989).
30. Information about markers AM40 and AM80 is available
at http://www.biosci.missouri.edu/liscum/newmarkers.
html.
31. Y. Nakamura et al., DNA Res. 4, 401 (1997); http://
www.kazusa.or.jp/arabi/chr5/map/24-26Mb.html.
32. Soluble and total microsomal membrane fractions
were separated by ultracentrifugation, followed by
two-phase partitioning to enrich for plasma membranes,
as described previously [T. W. Short, P. Reymond,
W. R. Briggs, Plant Physiol. 101, 647 (1993)].
33. Antibodies against NPH1 were previously described
(7). Rabbit polyclonal antisera were raised (22)
against a COOH-terminal NPH3 fusion protein [CBDNPH3C2
(see Fig. 3A)]. CBD-NPH3 protein was expressed
from pET34-Ek/LIC in Escherichia coli and
purified according to manufacturer’s instructions
(Novagen, Madison, WI).
34. NPH1-NPH3 interaction was examined in yeast with
the Matchmaker Gal4 II System (Clontech, Palo Alto,
CA). Expression of fusion peptides was verified by
immunoblot analysis (9, 22) with monoclonal antibodies
raised against the Gal4 DNA binding domain
(GBD) and Gal4 activation domain (GAD) (Clontech).
35. J. H. Miller, Experiments in Molecular Genetics (Cold
Spring Harbor Laboratory, Plainview, NY, 1972).
36. We thank R. Harper for data in Fig. 1; J. M. Christie
and W. R. Briggs for GBD-NPH1 constructs and NPH1
antisera; D. Randall for production of NPH3 antisera;
the Arabidopsis Biological Resource Center in Columbus,
Ohio, for BAC clones and cDNA libraries; and
members of our laboratory for helpful comments on
the manuscript. This work was funded by USDA
National Research Initiative grant 96-35304-3709,
NSF grant MCB-9723124, and University of Missouri
Research Board grant RB96-055.
3 June 1999; accepted 17 September 1999
Four Evolutionary Strata on the
Human X Chromosome
Bruce T. Lahn* and David C. Page†
Human sex chromosomes evolved from autosomes. Nineteen ancestral autosomal
genes persist as differentiated homologs on the X and Y chromosomes.
The ages of individual X-Y gene pairs (measured by nucleotide divergence) and
the locations of their X members on the X chromosome were found to be highly
correlated. Age decreased in stepwise fashion from the distal long arm to the
distal short arm in at least four “evolutionary strata.” Human sex chromosome
evolution was probably punctuated by at least four events, each suppressing
X-Y recombination in one stratum, without disturbing gene order on the X
chromosome. The first event, which marked the beginnings of X-Y differentiation,
occurred about 240 to 320 million years ago, shortly after divergence of
the mammalian and avian lineages.
The human X and Y chromosomes, like those
of other animals, are thought to have evolved
from an ordinary pair of autosomes (1). The
pseudoautosomal regions at the termini of the X
and Y chromosomes still recombine during
male meiosis, ensuring X-Y nucleotide sequence
identity there. Elsewhere on the X and
Y chromosomes, however, X-Y recombination
has been suppressed. These nonrecombining
regions of the X and Y chromosomes have
become highly differentiated during evolution,
and only a few X-Y sequence similarities persist
within them. These modern X-Y gene pairs
are the remaining “fossils” where extensive sequence
identity between ancestral X and Y
chromosomes once existed. The recent discovery
of many X-Y genes has made it possible to
examine the entire group to search for patterns
of human sex chromosome evolution. Thus far,
the human sex chromosomes—the best characterized
mammalian sex chromosomes—have
been found to contain 19 X-Y gene pairs (2).
We first compared the locations of all 19
pairs of genes on the human X and Y chromosomes
(Fig. 1). We determined the relative
positions of the X-linked genes through radiation
hybrid analysis, in many cases confirming
previously published localizations (3). Map positions
of the Y-linked homologs were obtained
principally from the literature (4–6). On the X
chromosome, most of the X-Y genes map to the
short arm, where they are concentrated toward
the distal end. By contrast, the X-Y genes are
found as singletons or small clusters throughout
the euchromatic portion of the Y chromosome.
In general, the map order of the X-linked genes
corresponds poorly to that of the Y-linked homologs.
Local exceptions to this rule are provided
by three small gene clusters that are
present on both X and Y chromosomes (Fig. 1).
We next measured, for each of the 19 X-Y
gene pairs, synonymous nucleotide divergence
between the X-linked and Y-linked coding regions
(7). Because synonymous substitutions
do not alter the encoded protein, they are generally
assumed to be nearly neutral with respect
to selection. The statistic KS (the estimated
mean number of synonymous substitutions per
synonymous site) is often used to gauge evolutionary
time (8). In the present context, KS
values provide a measure of the evolutionary
time that has elapsed since the gene pairs started
differentiating into distinct X and Y forms.
The calculated KS values are given in Table 1,
where gene pairs are listed according to map
order on the X chromosome.
We noted that the 19 KS values appeared to
cluster into approximately four groups (Fig. 2):
0.94 to 1.25 (group 1), 0.52 to 0.58 (group 2),
0.23 to 0.36 (group 3), and 0.05 to 0.12 (group
4). Each X-Y gene pair’s KS value differed
significantly from those of all gene pairs in
other groups (P Յ 0.02). The most striking
observation was that, on the X chromosome,
the four KS-defined groups of genes are arranged
in an orderly sequence (Fig. 2). X-Y
genes are stratified by age along the length of
the X chromosome. By contrast, on the Y chromosome,
the KS-defined groups appear to be
scrambled (compare Table 1 and Fig. 1).
What might account for the orderly stratification
of X-Y genes by age on the human X
chromosome? We hypothesize that, during evoHoward
Hughes Medical Institute, Whitehead Institute,
and Department of Biology, Massachusetts Institute
of Technology, 9 Cambridge Center, Cambridge,
MA 02142, USA.
*Present address: Department of Human Genetics,
University of Chicago, 924 East 57th Street, Chicago,
IL 60637, USA.
†To whom correspondence should be addressed. Email:
dcpage@wi.mit.edu
R E P O R T S
29 OCTOBER 1999 VOL 286 SCIENCE www.sciencemag.org964
lution, differentiation of the X from the Y chromosome
was initiated one region, or stratum, at
a time. Regions were recruited in the order of
their physical position, with stratum 1 (containing
the genes of group 1) having been the first
to embark on X-Y differentiation, and stratum 4
having been the most recent. Genes in the same
stratum began differentiating into X and Y homologs
at about the same time, accounting for
their similar KS values.
X-Y differentiation would have occurred
only after X-Y recombination ceased (9). Our
findings suggest that during evolution, X-Y recombination
was suppressed regionally, beginning
with stratum 1 and subsequently expanding
in discrete steps to include strata 2, 3, and 4.
Chromosomal inversions, which are known to be
capable of suppressing recombination across
broad regions in mammals (10), would appear to
be the most likely mechanism. These inversions
must have occurred on the evolving Y chromosome,
where the strata have been scrambled, but
not on the X chromosome, where the order of
strata apparently has been preserved (Figs. 1 and
2). [Had the strata on the human X chromosome
been extensively shuffled during evolution—as
may have occurred on the mouse X chromosome
after divergence of the human and murine lineages
(11)—we would have observed no correlation
between the age of X-Y gene pairs and the
map positions of their X-chromosomal members.]
In the modern human sex chromosomes,
the proximal boundary of the pseudoautosomal
region is spanned by a gene that is intact on the
X chromosome, but grossly interrupted on the Y
chromosome (12), consistent with disruption of
an ancient pseudoautosomal region by a Y-chromosomal
inversion. We speculate that this particular
event was the most recent in a series of
inversions, each of which enabled X-Y differentiation
to begin in one stratum.
This model of staged, region-by-region initiation
of X-Y differentiation also accounts for
two global features of the X chromosome’s
gene content: (i) the concentration in strata 3
and 4 of genes with detectable Y homologs
(Fig. 1) and (ii) the concentration on the short
arm (strata 2, 3, and 4) of genes that escape X
inactivation, some with and some without Y
homologs (13). Evolutionary theory predicts
that once X-Y recombination ceased within a
stratum, the genes on the affected portion of the
Y chromosome began to decay, with most of
the Y-linked genes ultimately being obliterated
(1). As an adaptive response, homologous
genes on the X chromosome were up-regulated,
and subsequently became subject to X inactivation,
processes thought to have spread during
evolution on a gene-by-gene or cluster-by-cluster
basis (14). If decay of Y-linked genes and
adaptation of X-linked homologs were gradual
evolutionary processes, then one would expect
the youngest X strata to exhibit the highest
densities of (i) genes with detectable Y homologs
and (ii) genes that escape inactivation.
Both predictions are met (Fig. 1) (13).
A comparison of the youngest (group 4)
gene pairs with the older (groups 1 through 3)
gene pairs illustrates certain temporal features of
X-Y differentiation. We measured both synonymous
and nonsynonymous substitutions for
each gene pair (Table 1). Nonsynonymous substitutions
alter the encoded protein and are constrained
by selection. Thus, their frequency (KA,
the estimated mean number of nonsynonymous
substitutions per nonsynonymous site) is a function
of both evolutionary time and selective
constraints on the encoded proteins. The degree
Fig. 1. Map of homologous
genes in nonrecombining regions
of human X and Y chromosomes.
Pseudoautosomal regions
of X and Y are black; heterochromatic
region of Y is gray.
Radiation hybrid analysis (3) was
used to map genes on the X
chromosome, which is drawn on
a centiRay scale. KS-defined strata
on the X chromosome are indicated. The
boundary between strata 2 and 1 is somewhere
between SMCX and RPS4X; here, it is arbitrarily
shown at the centromere (white oval). Genes and
pseudogenes on the Y chromosome were ordered
previously by analysis of naturally occurring deletions
(4, 5). UBE1X has a homolog on the squirrel
monkey Y chromosome but not on the human
Y chromosome (29). Brackets denote three small
gene clusters (labeled a, b, c) that are present on
both X and Y chromosomes.
Table 1. Sequence divergence between homologous X- and Y-linked genes.
Gene pair KS KA KS/KA
DNA
divergence
(%)
Protein
divergence
(%)
Sequence
compared
(nucleotides)
Group 4
GYG2/GYG2P* 0.11 0.06 1.8 7 12 525
ARSD/ARSDP* 0.09 0.07 1.3 7 13 846
ARSE/ARSEP* 0.05 0.04 1.2 4 9 615
PRKX/Y 0.07 0.03 2.3 5 8 1020
STS/STSP* 0.12 0.10 1.2 11 18 852
KAL1/KALP* 0.07 0.06 1.2 6 12 1302
AMELX/Y 0.07 0.07 1.0 7 12 576
Group 3
TB4X/Y 0.29 0.04 7.3 7 7 135
EIF1AX/Y 0.32 0.01 32 9 2 432
ZFX/Y 0.23 0.04 5.8 7 7 2394
DFFRX/Y 0.33 0.05 6.6 11 9 7671
DBX/Y 0.36 0.04 9.0 12 9 1932
CASK/CASKP* 0.24 0.22 1.1 15 32 156
UTX/Y 0.26 0.08 3.3 12 15 4068
Group 2
UBE1X/Y 0.58 0.07 8.3 16 13 693
SMCX/Y 0.52 0.08 6.5 17 15 4623
Group 1
RPS4X/Y 0.97 0.05 19 18 18 792
RBMX/Y 0.94 0.25 3.8 29 38 1188
SOX3/SRY 1.25 0.19 6.6 28 29 264
*Y copy is pseudogene. DNA and protein divergence refer to uncorrected nucleotide (coding region) and amino acid
divergence (nonidentity).
R E P O R T S
www.sciencemag.org SCIENCE VOL 286 29 OCTOBER 1999 965
of constraint can be reflected in the ratio KS/KA;
values greater than one indicate the presence of
constraints on both homologs, and values in the
vicinity of one are consistent with lack of constraint
on at least one homolog (8, 15). In groups
1 through 3, 10 of 11 gene pairs exhibit KS/KA
ratios of 3 or higher (Table 1), suggesting that
natural selection has preserved the Y copies of
these genes. Without such selection, these X-Y
homologies (especially those in groups 1 and 2)
would no longer be visible. By contrast, the
seven gene pairs in group 4 show KS/KA ratios
of 1 to 2, and in five of these pairs, the Y copy
is known to be a pseudogene. Among the group
4 pairs, X-Y homology is readily apparent even
in the absence of selective constraint, because
there has been little time for erosion of sequence
similarity. Thus, the Y-chromosomal genes of
the older groups, and especially those of groups
1 and 2, are survivors of an early winnowing
process that is still ongoing in group 4.
To determine the age of the KS-defined strata,
we used two methods. First, we considered
published information on homologs of representative
genes in diverse mammals. The maximum
age of stratum 4, for example, was suggested by
the prior observation that homologs of STS and
KAL1 are pseudoautosomal or autosomal in prosimians
(16–18). Assuming that suppression of
X-Y recombination is an irreversible evolutionary
step (14), this implies that X-Y differentiation
in stratum 4 began less than 50 million
years ago (Ma), when the simian and prosimian
lineages diverged (19). Minimum ages of the
strata could also be inferred. For example, STS
and KAL1 have been shown to have X- and
Y-specific homologs in both New and Old
World monkeys (16, 17), suggesting that X-Y
differentiation in stratum 4 began at least 30 Ma,
when the New and Old World monkey lineages
diverged (19, 20). Using similar logic, we inferred
the ages of stratum 3 (80 to 130 million
years), stratum 2 (130 to 170 million years), and
stratum 1 (130 to 350 million years) from prior
data on gene homologs in more-distantly related
species, including nonprimate mammals, marsupials,
monotremes, and birds (21).
These cross-species comparisons yielded
reasonably precise estimates of age for strata 2,
3, and 4—the younger strata—but only crude
estimates of age for stratum 1. Because this
oldest stratum might contain information about
the origins of mammalian sex chromosomes, its
age is of great interest. Here, we used a second
dating method, based on KS values for X-Y gene
pairs. Theory predicts that among human X-Y
gene pairs, KS values should be roughly proportional
to age (8). This expectation is met by the
X-Y gene pairs of strata 2, 3, and 4 (Fig. 3). By
extrapolation, we estimated that X-Y differentiation
began 240 to 320 Ma in stratum 1 (Fig. 3).
These findings suggest that X-Y divergence began
shortly after the mammalian lineage arose,
having diverged from the lineage of birds (with
Z-W sex chromosomes) between 300 and 350
Ma (19). [Because the sex chromosomes of
birds appear to be completely unrelated to the
mammalian sex chromosomes, it is thought that
they arose independently, from a different autosomal
pair (22).] Interestingly, our KS findings
indicate that SOX3 and SRY (the primary sexdetermining
gene) are among the oldest known
X-Y gene pairs in humans (Table 1). This finding
strengthens an hypothesis, by Foster and
Graves, which states that an ordinary autosomal
pair became sex chromosomes when mutations
fashioned one allele of SOX3, originally an autosomal
gene, into the male-determining factor
SRY (23). Indeed, formal cluster analysis of the
KS values we report suggests that the X-Y genes
of group 1 might actually comprise two distinct
strata, with SRY/SOX3 perhaps being older than
the two other X-Y gene pairs of group 1
(RPS4X/Y and RBMX/Y) (24). Although the difference
in KS values between SRY/SOX3 and the
two other X-Y gene pairs is not statistically
significant, the evidence is suggestive.
If future studies establish that the group 1
genes are divisible into two strata, these results
Fig. 2. Plot of KS (Table 1) versus X-chromosome
map position (Fig. 1) for 19 X-Y gene pairs.
Fig. 3. Plot of X-Y divergence time (age) versus
average KS value for X-Y gene pairs (weightaveraged)
in each stratum. The X chromosome
schematic is adapted from Fig. 1. Maximum and
minimum age estimates for strata 2, 3, and 4 are
bracketed; these are not statistical confidence
intervals. Theory predicts an approximately linear
relationship between age and KS value (8); the
shaded area is calibrated with respect to stratum
2, whose age is 130 to 170 million years (21) and
whose average KS value is 0.53. By extrapolation,
the age of stratum 1 is estimated between 240
and 320 million years.
Fig. 4. A proposed sequence of evolutionary events that generated four strata on the human X
chromosome. Four inversions on the Y chromosome are postulated. Each inversion reduced the size
of the pseudoautosomal ( X-Y recombining) region (black; for simplicity, only one pseudoautosomal
region is shown for each chromosome) and enlarged the portions of the X (yellow) and Y (blue)
chromosomes that did not recombine during male meiosis. Ongoing decay and loss of Y genes
offset these periodic expansions of the nonrecombining region of the Y chromosome. Points of
divergence from the sex chromosomes of other mammals are indicated. This model does not
preclude the occurrence of (i) additional inversions or other rearrangements within the nonrecombining
portion of the evolving Y chromosome or (ii) similar rearrangements on the evolving X
chromosome, so long as they do not disturb the fundamental order among the four strata.
R E P O R T S
29 OCTOBER 1999 VOL 286 SCIENCE www.sciencemag.org966
would also help date the emergence of X inactivation
during mammalian sex chromosome
evolution. XIST, an X-specific gene which plays
a pivotal role in X inactivation (25), is located
near RPS4X and therefore would be in the
younger of the two strata—not in the stratum
where the nascent X and Y chromosomes first
differentiated. This would controvert the hypothesis
of Chandra, who speculated that X inactivation
emerged contemporaneously with the chromosomal
sex-determining mechanism (26).
Consistent with our evolutionary map,
Graves and colleagues have postulated that the
long arm and proximal short arm of the human
X chromosome are at least 170 million years
old (27, 28). They have referred to this portion
of the X as the “XCR” (X conserved region).
Graves’s XCR corresponds approximately to
our strata 1 and 2. They have also postulated
that the distal short arm of the human X chromosome
is younger. This “XAR” (X added
region) was attributed to translocation of an
autosome to the pseudoautosomal region of
both X and Y after divergence of placental
mammals from marsupials (27, 28). Our strata
3 and 4 are found within Graves’s XAR.
In conclusion, we postulate that the evolution
of human sex chromosomes was punctuated by
at least four events, plausibly a series of inversions
on the Y chromosome (Fig. 4). Each event
suppressed X-Y recombination in one stratum
and enabled X-Y differentiation to proceed
there. The first of these events, which created
stratum 1, was roughly contemporaneous with
the birth of the mammalian sex chromosomes
and the emergence of SRY as the primary sex
determinant. This occurred about 240 to 320 Ma,
shortly after the mammalian and avian lineages
diverged. The pseudoautosomal region was expanded
by translocation of autosomal material
between the second and third events (which created
strata 2 and 3, respectively). The fourth
event occurred relatively recently, during primate
evolution, creating stratum 4, where X-Y
differentiation is still in its earliest stages.
References and Notes
1. J. J. Bull, Evolution of Sex Determining Mechanisms (Benjamin
Cummings, Menlo Park, CA, 1983); J. A. Graves,
Annu. Rev. Genet. 30, 233 (1996); B. Charlesworth,
Curr. Biol. 6, 149 (1996); W. R. Rice, Bioscience 46, 331
(1996).
2. The 19 X-Y gene pairs studied include the following:
GYG2/GYG2P [J. Mu, A. V. Skurat, P. J. Roach, J. Biol.
Chem. 272, 27589 (1997); (6)], ARSD/ARSDP, ARSE/
ARSEP [G. Meroni et al., Hum. Mol. Genet. 5, 423
(1996)], PRKX/Y [A. Klink et al., Hum. Mol. Genet. 4, 869
(1995); K. Schiebel et al., Hum. Mol. Genet. 6, 1985
(1997)], STS/STSP (16), KAL1/KALP [B. Franco et al.,
Nature 353, 529 (1991); R. Legouis et al., Cell 67, 423
(1991); (17)], AMELX/Y [Y. Nakahori, O. Takenaka, Y.
Nakagome, Genomics 9, 264 (1991)], TB4X/Y [H. Gondo
et al., J. Immunol. 139, 3840 (1987); (5)], ZFX/Y [D. C.
Page et al., Cell 51, 1091 (1987); A. Schneider-Ga¨dicke,
P. Beer-Romero, L. G. Brown, R. Nussbaum, D. C. Page,
Cell 57, 1247 (1989)], EIF1AX/Y [T. E. Dever et al., J. Biol.
Chem. 269, 3212 (1994); (5)], DFFRX/Y [M. H. Jones et
al., Hum. Mol. Genet. 5, 1695 (1996); (5)], DBX/Y (5),
CASK/CASKP [A. R. Cohen et al., J. Cell Biol. 142, 129
(1998); (6)], UTX/Y (5), SMCX/Y [J. Wu et al., Hum. Mol.
Genet. 3, 153 (1994); A. I. Agulnik et al., Hum. Mol.
Genet. 3, 879 (1994)], RPS4X/Y [E. M. Fisher et al., Cell
63, 1205 (1990)], RBMX/Y [M. Soulard et al., Nucleic
Acids Res. 21, 4210 (1993); K. Ma et al., Cell 75, 1287
(1993); M. L. Delbridge, P. A. Lingenfelter, C. M. Disteche,
J. A. Graves, Nature Genet. 22, 223 (1999); S. Mazeyrat,
N. Saut, M. G. Mattei, M. J. Mitchell, Nature Genet. 22,
224 (1999)], SOX3/SRY [M. Stevanovic, R. Lovell-Badge,
J. Collignon, P. N. Goodfellow, Hum. Mol. Genet. 2,
2013 (1993); A. H. Sinclair et al., Nature 346, 240
(1990)]. One interspecies pair was also studied: human
UBE1X [P. M. Handley, M. Mueckler, N. R. Siegel, A.
Ciechanover, A. L. Schwartz, Proc. Natl. Acad. Sci. U.S.A.
88, 258 (1991)] and squirrel monkey UBE1Y (29). In
humans, UBE1Y was deleted from the Y chromosome
(29). We used squirrel monkey UBE1Y as a substitute.
3. Using polymerase chain reaction (PCR), we tested DNAs
from the 93 hybrid cell lines of the GeneBridge 4 panel
(Research Genetics) [G. Gyapay et al., Hum. Mol. Genet.
5, 339 (1996)] for the presence of each of the X-linked
genes. PCR conditions and primer sequences have been
deposited at GenBank, where accession numbers are as
follows: GYG2, G49430; ARSD, G42687; ARSE, G42688;
PRKX, G42689; STS, G42690; KAL1, G42691; AMELX,
G42692; TB4X, G34979; EIF1AX, G34989; ZFX, G42693;
DFFRX, G34982; DBX, G34988; CASK, G49441; UTX,
G34976; UBE1X, G42694; SMCX, G42695; RPS4X,
AF041428; RBMX, G42696; and SOX3, G42697. Analysis
of the results positioned the genes with respect to
the radiation hybrid map of the X chromosome constructed
at the Whitehead/MIT Center for Genome
Research [T. J. Hudson et al., Science 270, 1945 (1995);
www-genome.wi.mit.edu/cgi-bin/contig/phys_map].
4. D. Vollrath et al., Science 258, 52 (1992).
5. B. T. Lahn and D. C. Page, Science 278, 675 (1997).
6. C. Sun et al., Nature Genet., in press.
7. Homologous X and Y DNA sequences were aligned by
means of MegAlign software (DNASTAR, Madison, WI).
For each X-Y gene pair, estimates of the mean numbers
of synonymous substitutions per synonymous site (KS),
and of nonsynonymous substitutions per nonsynonymous
site (KA)—all corrected for multiple changes—were
calculated using published algorithms (8) as implemented
in GCG software (Genetics Computer Group, Madison,
WI). Insertions and deletions were ignored in these calculations.
In the case of SOX3 and SRY, sequence similarity
is limited to, and our analysis was restricted to, the
HMG box domain. Our analyses of other X-Y gene pairs
employed all available coding sequences. Only a partial
UBE1Y (squirrel monkey) coding sequence was available
for comparison with its human X homolog. Sequences for
all pseudogenes were extracted from genomic sequences:
GYG2P, ARSDP, and ARSEP from BAC (bacterial artificial
chromosome) clone 203M13 (GenBank AC002992); STSP
from BAC clone NH0494J04 (GenBank AC006382); KALP
from BAC clone NH0292P09 (GenBank AC006370);
CASKP from BAC clone 475I1 (GenBank AC004474). Sequences
for all other genes were obtained from published
cDNAs, whose GenBank accession numbers are as follows:
GYG2, U94362; ARSD, X83572; ARSE, X83573;
PRKX, X85545; PRKY, Y15801; STS, M16505; KAL1,
M97252; AMELX, M86932; AMELY, M86933; TB4X,
M17733; TB4Y, AF000989; ZFX, X59739; ZFY, M30607;
EIF1AX, L18960; EIF1AY, AF000987; DFFRX, X98296;
DFFRY, AF000986; DBX, AF000982; DBY, AF000984;
CASK, AF032119; UTX, AF000992; UTY, AF000994;
UBE1X, M58028; UBE1Y, AJ003105; SMCX, L25270;
SMCY, U52191; RPS4X, M58458; RPS4Y, M58459; RBMX,
Z23064; RBMY, X76059; SOX3, X71135; SRY, X53772.
8. W. H. Li, J. Mol. Evol. 36, 96 (1993); Molecular
Evolution (Sinauer Associates, Sunderland, MA, 1997).
9. B. O. Bengtsson and P. N. Goodfellow, Ann. Hum.
Genet. 51, 57 (1987).
10. L. M. Silver, Trends Genet. 9, 250 (1993); R. H. Martin
et al., Hum. Genet. 93, 135 (1994); M. Jaarola, R. H.
Martin, T. Ashley, Am. J. Hum. Genet. 63, 218 (1998).
11. H. J. Blair, V. Reed, S. H. Laval, Y. Boyd, Genomics 19,
215 (1994); W. J. Murphy, S. Sun, Z.-Q. Chen, J.
Pecon-Slattery, S. J. O’Brien, Genome Res., in press.
12. P. A. Weller, R. Critcher, P. N. Goodfellow, J. German,
N. A. Ellis, Hum. Mol. Genet. 4, 859 (1995).
13. C. J. Brown, L. Carrel, H. F. Willard, Am. J. Hum. Genet.
60, 1333 (1997).
14. K. Jegalian and D. C. Page, Nature 394, 776 (1998).
15. KS/KA ratios can be depressed by positive selection,
which accelerates protein divergence (8). However,
among the X-Y pairs shown here to have relatively
low KS/KA ratios, the abundance of Y pseudogenes
( Table 1) suggests that absence of selective constraint
is the more significant factor.
16. P. H. Yen et al., Cell 55, 1123 (1988).
17. I. del Castillo, M. Cohen-Salmon, S. Blanchard, G.
Lutfalla, C. Petit, Nature Genet. 2, 305 (1992); B.
Incerti et al., Nature Genet. 2, 311 (1992).
18. R. Toder, G. A. Rappold, K. Schiebel, W. Schempp,
Hum. Genet. 95, 22 (1995).
19. S. Kumar and S. B. Hedges, Nature 392, 917 (1998);
M. J. Benton, Vertebrate Paleontology (Chapman &
Hall, New York, 1997).
20. D. Pilbeam, Sci. Am. 250 (no. 3), 84 (1984).
21. Stratum 3: Homologs of ZFX are autosomal in marsupials
(27), which diverged from placental mammals 130 Ma
(19). For ZFX/Y and UTX/Y (and for UBE1X/Y and SMCX/Y,
in stratum 2), we employed sequence-based phylogenetic
analysis to determine if differentiation into X and Y forms
had begun before or after mouse/human divergence. For
each X-Y gene pair, we used GCG software to construct
a phylogenetic tree relating human X, human (or monkey)
Y, mouse X, and mouse Y homologs. In each of the
four cases, the X homologs in human and mouse formed
a branch which was distinct from a second branch formed
by the Y homologs in human (or monkey) and mouse.
These findings suggest that X-Y differentiation of these
four gene pairs began before divergence of humans and
mice. This is consistent with X-Y divergence having initiated
before the placental mammalian radiation that occurred
80 to 100 Ma. Stratum 2: Distinct X- and Y-linked
forms of UBE1 have been found in both placental mammals
and marsupials [M. J. Mitchell, D. R. Woods, S. A.
Wilcox, J. A. Graves, C. E. Bishop, Nature 359, 528 (1992)],
but their homologs are autosomal in monotremes (29),
which diverged from placental mammals and marsupials
170 Ma (19). Stratum 1: Distinct X- and Y-linked forms of
RPS4 have been found in placental mammals and marsupials
(K. Jegalian and D. C. Page, unpublished results),
but their homologs are autosomal in birds, whose lineage
diverged from that of mammals 300 to 350 Ma (19).
Y-specific SRY sequences have been identified in both
placental mammals and marsupials [J. W. Foster et al.,
Nature 359, 531 (1992)].
22. A. K. Fridolfsson et al., Proc. Natl. Acad. Sci. U.S.A.
95, 8147 (1998).
23. J. W. Foster and J. A. Graves, Proc. Natl. Acad. Sci.
U.S.A. 91, 1927 (1994).
24. Dendrograms of the 19 KS values (Table 1) were constructed
using five clustering algorithms (average, centroid,
Ward’s, single linkage, and complete linkage) implemented
in JMP statistics software (SAS Institute,
Cary, NC). The most significant branch classification
(using any of the algorithms) had five clusters corresponding
to the four groups shown in Table 1 and Fig. 2,
but with group 1 divided into subgroups 1A (SOX3/SRY)
and 1B (RPS4X/Y and RBMX/Y). However, the difference
in KS value between SOX3/SRY (1.25 Ϯ 0.41) and
RPS4X/Y (0.97 Ϯ 0.16) or RBMX/Y (0.94 Ϯ 0.15) was
not statistically significant. At present, any distinction
between subgroups 1A and 1B is tentative.
25. H. F. Willard, Cell 86, 5 (1996).
26. H. S. Chandra, Proc. Natl. Acad. Sci. U.S.A. 82, 6947
(1985).
27. J. A. Spencer, A. H. Sinclair, J. M. Watson, J. A. Graves,
Genomics 11, 339 (1991).
28. J. A. Graves, Philos. Trans. R. Soc. London Ser. B 350,
305 (1995); R. Toder and J. A. Graves, Mamm. Genome
9, 373 (1998); J. M. Watson, J. A. Spencer, J. A.
Graves, M. L. Snead, E. C. Lau, Genomics 14, 785
(1992); J. A. Spencer, J. M. Watson, J. A. Graves,
Genomics 9, 598 (1991).
29. M. J. Mitchell et al., Hum. Mol. Genet. 7, 429 (1998).
30. We thank F. Lewitter for help with sequence comparisons,
H. Skaletsky and T. Kawaguchi for help with
database searches and analysis of mapping data, and P.
Bain, D. Bartel, A. Bortvin, B. Charlesworth, D. Charlesworth,
A. Chess, A. Clark, C. Disteche, G. Fink, S. Gilbert,
J. Graves, R. Jaenisch, K. Jegalian, E. Lander, D. Menke, W.
Rice, S. Rozen, C. Tilford, and J. Wang for discussions and
comments on the manuscript. Supported by NIH grant
HG00257.
14 June 1999; accepted 17 September 1999
R E P O R T S
www.sciencemag.org SCIENCE VOL 286 29 OCTOBER 1999 967