Study Suggests 'Y' the Male Chromosome Will Endure

They looked at the few genes on the Y that are not directly involved in sperm production, all of which have counterparts on the X. They reasoned that if the Y was destined to decay further, some of the non-sperm genes that humans and chimps once shared on their Y chromosomes would have disappeared in the 6 million years since the two species diverged. What they found, however, was that all 16 of the non-sperm genes on the human Y had counterparts on the chimp chromosome. There were no genes on the chimp Y that were not also represented on the human Y. The researchers concluded no genes have been lost on the path to humanness.

Why not?

Page believes it is because natural selection -- the driving force of evolution -- is simply powerful enough to preserve even the Y's highly vulnerable non-sperm genes.

He theorizes that when a mutation occurs in one of those genes, it may somehow make a man slightly less fertile or "fit" even if it isn't directly involved in sperm production. If that is true, then those men will tend to have fewer offspring over time than men with intact Y genes. The mutant-Y lineages, although they might survive for a while, would be out-competed by the intact-Y lineages and die out.

However, this Y-preserving scenario is clearly not absolute, because most of the Y has in fact disappeared over the last 300 million years. And, curiously, in the chimpanzee -- but not in the human being -- it has continued to disappear in the last 6 million.

Page and his collaborators found that five of the 16 non-sperm genes in the chimp were no longer active. Mutations had killed them.

What could possibly have made that happen?

Page doesn't know, but he has a guess.

Chimpanzees are sexually promiscuous. Females mate with multiple males. Just as males compete with each other for territory on the ground, billions of their sperm compete inside the female's genital tract as they race to fertilize the egg.

If a mutation occurs in a sperm-producing gene on the Y that leads the chimp to make more or faster sperm, the mutation will tend to be preserved over evolutionary time. It will be preserved even if, by chance, it is accompanied by a damaging mutation in one of the Y's non-sperm genes. The work done by that killed gene would probably be made up by their "cousin" genes on the X.

Page speculates this may be what happened with the chimpanzee.

If he is right, it is an example of how behavior -- in this case, chimpanzee sexual promiscuity -- has shaped the species's genome. The traditional assumption has been the opposite, namely that genes drive behavior.

"I think we had a simple-minded idea that once we knew the DNA sequence of the genome, we could read out behaviors as if we were looking at a cookbook. But the connection between our genomes and our behaviors works in both directions," he said.

So is the Y out of the woods? Will it live forever despite its long history of decay?

Huntington F. Willard, director of the Institute for Genome Sciences and Policy at Duke University, said he thinks the theory of the Y's "impending demise" is still viable. It may simply be that the chimp Y is closer to its end than the human one.

Sexual reproduction is not likely to disappear, Willard believes. But the current method, which requires a Y chromosome, just might.