GENETIC
recombination happens to be a vigorous tonic for chromosomes, good
for organisms, and the main point of sexual reproduction. When recombination
is suppressed, genetic integrity comes tumbling down. The X chromosome
can recombine along its whole length with a sister X whenever it passes
through a female. The Y chromosome, however, never recombines along
most of its length. Without recombination, genetic disintegration
follows. Exactly why recombination is so useful remains to be plumbed.
Though we may not know exactly how recombination exerts its cleansing
powers, we know that without it DNA rearrangements accumulate, genes
decay, and useless bits of DNA amplify.
Mostly, cells scrupulously guard genetic integrity,
but when DNA becomes useless, repetitive and devoid of genes, cells
can toss it out without suffering any damage. Over time, the Y apparently
lost nearly all the genes it once shared with the X. The mammalian
Y is so degenerate that until recently many researchers believed that
it did nothing except determine sex.
When the Y degenerates, a male keeps only one copy
of each of the thousands of genes it once shared with the X. A female
still has two copies of these genes, intact on her X chromosomes.
To balance dosage of gene products, it benefits the male to amplify
expression of X-linked genes. Fruit flies stop at that to achieve
dosage compensation. Mammals have gone one step further. Expression
from the X was probably amplified in both males and females. Then
it became in the females' interest to inactivate one of the Xs.
Ultimately, these two things evolve in tandem:
the Y loses its similarity with the X, and X inactivation spreads.
But by what steps did (and does) this process occur during evolution?
Such a question would probably be impossible to answer for mammals
if the X and Y were fully differentiated as previously supposed. The
genes that escape X inactivation were not considered evolutionary
intermediates. They seemed to be flukes; perhaps their dosage doesn't
particularly matter.
But their dosage does matter. Why else would most
of the genes that escape X inactivation in humans also have conserved
Y cousins (or homologs)? The homologs on the Y--and this has been
proven in the case of one gene--appear functionally interchangeable
with their cousins on the X.
Genes that escape X inactivation and have Y homologs
are caught in intermediate stages of evolution. Finding trapped intermediates
lets us reconstruct the pathway by which mammalian sex chromosomes
have evolved, just as trapped chemical intermediates can permit the
reconstruction of a biochemical pathway.
Several Y genes that have decayed, or whose function
has become limited, still have X homologs that escape X inactivation.
But no cases are known in which a gene is subject to X inactivation
yet has a Y homolog that remains conserved in structure and widely
expressed. In other words, Y degeneration or divergence appears invariably
to precede the expansion of X inactivation. Decay or divergence of
genes on the Y drives the acquisition of X inactivation, not the other
way around.
Yet, the story of the Y is not solely a story of
decay. Genes shared with the X have tended to be lost, but about half
of the genes on the human Y arrived there relatively recently. Chromosomes
are labile enough that genes can be transferred piecemeal from different
places. Once something lands on the Y, genes tend to be amplified
in copy number and rearranged. The new residents of the Y may be most
likely to survive on the rogue chromosome if they confer male-specific
advantages. Indeed, some of the newcomers to the human Y appear to
help in sperm formation. This is yet another example of similar, independent
evolutionary trajectories. The collection of genes on the human Y
is not related to the collection on the fruitfly Y, but in each case,
the Y appears to be a bastion of male-fertility factors.
CHROMOSOMES
may hardly seem like a personal subject, but the X and the Y show
a dramatic range of character: the two sister X chromosomes, one vivacious
and airy, the other silent and bundled into itself; the deadbeat-father
Y shirking its responsibilities and leaving more and more responsibility
to the X; the single-mother X making do by evolving complex adaptations.
Think of it as a poignant tale of fallibility--and of compensation.
