X Marks the Spot for Male Fertility
Infertile? Blame Mom
X and Y: From Then to Now continued...
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.
According to Page, about half of the genes found on the Y
chromosome are expressed -- that is, become active -- in the making of sperm in
"It turns out there's a medical consequence of this,"
Page says. "It turns out that the most common known cause of spermatogenic
failure, of male infertility, is deletion, is the absence of a part of the Y
chromosome. There are various parts of the Y chromosome, [and] dropping out any
one of those sections will shut down sperm production. These are common causes
of spermatogenic failure in human populations."
But here's the kicker that may bruise a few fragile male egos,
says Wang: It turns out that the Y chromosome is only part of the story. He
discovered that in mice and men, the X chromosome actually seems to
carry about 10 genes that are important in determining the production of
primitive cells that in the developing embryo that will later determine sperm
production or its absence. That's about three sperm-making genes on the
"female" chromosome for every one on the male.
"This changes our way of thinking: Before, everyone in this
field thought that the Y chromosome plays a very important role in
spermatogenesis, if not a monopoly, but nobody thought about the X
chromosome," Wang tells WebMD. "These findings change that completely:
X chromosome not only plays a role, it looks like it plays the most important
Yet while all this may be somewhat counterintuitive, it's
actually good news, because it opens up new avenues for investigation, says a
researcher who specializes in male infertility.
"Genetic work is still in its early stages for whichever
disease you're dealing with, and infertility is a disease state as well,"
says Erol Olen, MD, chief of the division of male infertility and sexual
dysfunction at New England Medical Center in Boston. "Right now, it's more
for education, more to let the patients know that, 'Looking at your genes, it's
very unlikely that we'll recover sperm if we do some sort of intervention.' But
ideally, this is something where in the future we'll be able to say, 'Well,
these genes don't seem to be producing any signals, and if we can give you this
or that and somehow turn them on, we might be able to increase spermatogenesis