X Marks the Spot for Male Fertility
Infertile? Blame Mom
X and Y: From Then to Now
For those who are a little rusty on Genetics 101, a brief
review may be in order. Each normal cell in the human body has 46 chromosomes:
22 pairs of autosomes -- "ordinary" chromosomes that are identical in
men and women --and two sex chromosomes. Women have two Xs, and men have one X
and one Y, and that's usually enough to make all the difference.
But 300 million years ago, when our ancestors were still
crawling around swamps on their bellies, there were no sex chromosomes.
"It turns out that once upon a time, the X and the Y were
the same. They were the two members of a perfectly ordinary pair of
autosomes," Page said at a recent Whitehead seminar. "I will argue that
300 million years ago, when we were reptiles, we had males and females, we
existed as males and females. The males made sperm, the females made eggs, but
we didn't have sex chromosomes, we only had ordinary chromosomes, and our sex
was likely determined -- whether we as a reptilian embryo developed as a male
or female -- by the temperature at which we, as an egg, incubated."
Since that time, however, as the human race worked its way up
the evolutionary ranks, it seems that the genes that control spermatogenesis --
the creation of sperm -- have become so essential to the species that they've
been copied over and over again and shuffled from the garden-variety autosomes
into the "newly" created sex chromosomes.
"You started with an ancestral gene in flies and worms that
was required for spermatogenesis. That then got duplicated in higher animals.
And then very recently in Old World monkeys and humans, there was a duplication
onto the Y, and that Y gene itself was multiply duplicated," says Steven A.
Wassermen, PhD, professor of biology at the University of California at San
Diego. "What appears to have happened in humans in particular is that you
moved these genes onto the Y chromosome that are responsible for
spermatogenesis and therefore subject to sexual selection.