Scientists at the Whitehead Institute in Cambridge, Massachusetts say it's fortunate for humans that the Y chromosome is not on the verge of extinction. That's because the Y is the chromosome that makes males males. The outlook is less favorable for the chimpanzee Y chromosome. NPR's Jay Palca has our report.
The Y chromosome holds a special fascination for David Page. Page is a geneticist and now the director of the Whitehead institute. The Y is unique among human chromosomes. All the others come in pairs but there is not counterpart to Y. Two years ago Page and his colleagues produced a detailed picture of the genetic makeup of the human Y chromosome. Since then, he has been working on a similar picture of the chimpanzee Y. Page wanted to learn if anything had changed in the 6 million years since human separated from chimps on the evolutionary tree. The results appear in today's issue of the journal Nature.
What we found, much to our surprise, was that it looked as though almost nothing had changed on the human Y chromosome during the past 6 million years. Page says the reason that it's a surprise is that some have suggested the genes on the Y are disappearing. After all, when the Y first emerged as the genetic entity 3 hundred million years ago, Page says, the Y chromosome had on the order of a thousand genes. Today it has on the order of 80. At that rate, the Y could be gone in ten million years or so.
If the Y is in the process of running into the ground within the next ten million years, then surely there should be evidence if that process had been playing out during the last six million years, and in fact, we find quite the opposite.
On the other hand, Page says some of the genes in the chimpanzee do seem to be accumulating errors and could risk rotting away in time. Page isn't sure why this is happening, but he has a theory.
Chimps are much more promiscuous resulting in what can be called sperm competition. Multiple males mate with a female chimp in a short period of time.
To be successful in this sperm competition, you want Y chromosomes with good sperm producing genes. Page says the other genes on Y may suffer as a result. In the chimpanzee lineage several of these genes may have been civilian casualties of the sperm war that's really focused on the Y chromosome sperm production genes.
Now these casualties probably don't spell big problems for male chimpanzees. Brian Charleswoods is a geneticist at the University of Edinburgh. These are probably minor effects on the fitness, not probably even easy to measure, you know, unless you count thousands and thousands of individuals. Charleswoods says many of the Y genes have counterparts on the X chromosome, and since males have both an X and a Y, the X genes can step up if the Y genes are damaged, so Charleswoods says a decaying Y probably does not mean/ the chimpanzee species is in trouble. Not because of its Y chromosome, its probably severely danger of departing the world because of human activities leading to their extinction. That's another issue.
And not one you can blame on the Y chromosome.
Joe Palca, NPR news, Washington.
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