July 16, 2001 -- The poet Alexander Pope famously declared that "the proper study of mankind is man." But just what a man is depends on the definition you choose. Warrior? Not exclusively a male role anymore. Leader? Yes, but women are also leaders. Hunter-gatherer? Sorry, fellas, like the old TV commercial said, women can bring home the bacon and fry it up in a pan.
So that pretty much leaves biology. Thank goodness we can still count on the old sex chromosomes, Ms. X and Mr. Y, can't we? Sure, the X chromosome has nearly 3,000 genes on it, compared with a measly two or three dozen on the Y chromosome. But you still need a Y to make a guy, right? Technically, yes, but it seems that even here there's some disagreement.
The Mysteries of Maleness
The Human Genome Project is beginning to reveal some rather surprising clues into the role the Y chromosome has played over time, and now there's evidence to suggest that the X chromosome may also play a key role in the development of sperm. In fact, says Jeremy Wang, PhD, with the Whitehead Institute for Biomedical Research in Cambridge, Mass., some cases of male infertility may turn out to be X-chromosome-linked disorders transmitted from mothers to sons.
"This is like color-blindness, hemophilia -- those are X-linked disorders; the defect is passed on by the mother. So it's possible that male infertility could be passed on by the mother. The mother has one defective gene, the other is on the Y type [contributed by the father's sperm], and it could be passed on to her son, and her sons are infertile," says Wang, a postdoctoral fellow in the laboratory of David Page, MD, professor of Biology at the Massachusetts Institute of Technology and investigator at the Howard Hughes Medical Institute at the Whitehead, which is affiliated with MIT.
Page and colleagues have been playing gene detective, tracing through the 300-million-year history of the Y chromosome in search of clues to the mysteries of maleness, reproduction, and infertility, and what they found is enough to turn inside out the conventional thinking about gender and the contributions of mother and father.
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.
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 the testicles.
"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 role."
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 that way."