Evolution__3rd_Edition

566 PART 5 / Macroevolution
Figure 19.4
Evolution of the human X
chromosome, according to
Lahn & Page (1999). In the
modern X chromosome, shown
on the right, 19 genes (labelled
a–s) fall into four age categories,
according to the amount of
divergence between equivalent
genes on the X and Y
chromosomes. Recombination
between the sex chromosomes
may have been shut down in
four stages by four inversion
events. A molecular clock
inference gives the dates shown
for the four inversions. The
hatched regions show where
recombination between the X
and Y chromosomes occurred
at each stage. Recombination
still takes place between the tips
of the two chromosomes. Free
recombination occurs between
the two X chromosomes in
females. The Y chromosome is
not illustrated.
. . . allowing a richly predictive
theory of the history of the sex
chromosomes
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Centromere
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300–350
Myr
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130–170
Myr
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80–130
Myr
30–50
Myr
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where gene exchange shut down first. A molecular clock inference suggests that gene
exchange ceased for the genes 300–350 million years ago a about the time when the
mammalian sex chromosomal system may have originated. Gene exchange was then
shut off successively for the next three chunks of the chromosome. The most recent of
the four bands has genes that have been diverging for 30–50 million years (Figure 19.4).
What events caused the shut down in gene exchange? Lahn and Page suggest there
were four chromosomal inversions. Chromosomal inversions prevent recombination
within the inverted region (Figure 19.5). We might predict, therefore, that the Y chromosome
should have the same four bands as the X, but with the genes in an inverted order
within each band. However, the genes are not arranged in the same four bands on the Y
chromosome. This may be because the genes have moved about since the inversion
events, or because some other mechanism than the inversions hypothesized by Lahn
and Page was at work. At present, human beings are the only species for which enough
sequence information exists to allow this kind of analysis. But Lahn and Page’s hypothesis
of a four-stage shut down in gene exchange between the X and Y chromosomes
is rich in predictions about the sex chromosomes of other mammals. As genomic
sequences accumulate, a stronger test should become possible.
The cessation of gene exchange between the X and Y chromosomes, whether it
occurred in four stages or not, may explain another fact about genome evolution: the
evolutionary shrinkage of the Y chromosome. The reason why the Y chromosome has
become smaller over time is likely because recombination is advantageous (Sections
12.1–12.3, pp. 314–27). The Y chromosome now almost entirely lacks the advantages
of sex, and the genetic information on it has decayed. The X chromosome has not
shrunk in the same way, because recombination persists as normal (in females)
between the X chromosomes.
In summary, Lahn & Page (1999) have used the genomic sequence information for
human sex chromosomes to infer the evolutionary history of gene exchange. This led
them to hypothesize four stages of gene rearrangement by inversion. They were also
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