Study Suggests 'Y' the Male Chromosome Will Endure
Monday, September 5, 2005
To paraphrase Mark Twain, reports of the impending death of the human Y chromosome have been greatly exaggerated.
Research published last week revealed that the chromosome of maleness -- which under the microscope looks as worn down and misshapen as a stubbed-out cheroot -- is actually healthy and holding up fine against the ravages of evolutionary time.
It turns out the human Y has barely changed in the last 6 million years. All its important parts still work. Predictions that it will cease to exist in another 10 million years -- and with it, men as we know them -- may be wrong.
This news comes from a comparison of Y chromosomes in human beings and chimpanzees. It is just one of many insights arising from the side-by-side inspection of the genes of man and his closest animal relative.
Chromosomes are long strands of DNA that contain the genes instructing cells how to make RNA and proteins, the stuff of life. Human beings have 23 pairs of chromosomes, one inherited from each parent. One of these pairs is the so-called sex chromosomes, one of whose members is designated X and the other Y. Women have two X's; men have an X and a Y.
The X-Y chromosome pair is unique in that its two halves aren't remotely similar in appearance and content, as is the case in the other 22 pairs. The X is big and contains about 1,000 genes. The Y is squat and contains only 78. Curiously, though, they are descended from the same "ancestor chromosome," which existed 300 million years ago at the dawn of this form of sexual reproduction.
Since then, the Y has thrown out most of the genes it once had -- and that the X still has -- like a shelf of unwanted cosmetics. It has concentrated on what it does best, providing the recipe of how to make sperm.
This evolution of the Y has brought with it unique risk. When chromosomes copy themselves, as they do when cells divide, errors occasionally creep in. When this occurs in the 22 matched pairs of chromosomes, or in the X-X pair that women possess, a damaged gene in one member of the pair can be cut out and replaced with a good one in the other member. But men are X-Y. Their Y chromosome never has a "brother" to recombine with, so this kind of repair can't happen.
Two years ago, research by David C. Page's laboratory revealed that the genes devoted to sperm production on the Y have a different way of preserving themselves. The sperm genes exist in duplicate form such that the chromosome can fold over on itself and swap and splice pieces from different sections in much the same way the two members of a chromosome pair do.
Despite this mechanism, some scientists believe the huge shrinkage of the Y over evolutionary time means it is inherently unstable and will eventually wither away.
Now, it appears that this isn't happening -- at least in the case of the human Y.
This evidence, too, comes from the laboratory headed by Page, a molecular geneticist at the Massachusetts Institute of Technology and the Howard Hughes Medical Institute. He, an MIT postdoctoral researcher named Jennifer F. Hughes, and collaborators at Washington University in St. Louis described their findings last week in the journal Nature.