Human Y chromosome stays intact while chimp Y loses genes 1/9/05. By the Whitehead Institute for Biomedical Research The human and the chimpanzee Y chromosomes went their separate ways approximately 6 million years ago. But ever since this evolutionary parting, these two chromosomes have experienced different fates, new research indicates.

While the human Y has maintained its count of roughly 27 genes and gene families over the last 6 million years, some of these same genes on the chimp Y have mutated and gradually become inactive. The authors speculate that one likely reason for such disparity is due to chimpanzee mating habits.

"Contrary to the dire predictions that have become popular over the last decade, the sky is not falling on the Y," says David Page (Whitehead Institute for Biomedical Research), senior author on the study published in the 1 September 2005 issue of the journal Nature. "This research clearly demonstrates that natural selection has effectively preserved regions of the Y chromosome that have no mechanisms with which to repair damaged genes."

For many years, it had been assumed that the Y chromosome is headed for extinction because, unlike other chromosomes, it has no genetic 'mate' with which to swap genes. Having sequenced the Y, however, the Page lab and collaborators discovered that many of its genes were located in palindromes – long stretches of DNA letters that read the same forwards and backwards. By folding into a hairpin, the authors suggested in their 2003 paper, a gene might then swap the appropriate genetic material with itself. This demonstrated a process for the Y chromosome to maintain its integrity despite lacking a mate.

However, there is another region of the Y, called the 'X-degenerate' region, where the genes are not situated in palindromes.

"The genes in the palindrome region are primarily sperm-producing genes, and most other genes unique to the Y aren't located there," says Jennifer Hughes, a postdoctoral scientist in Page's lab and first author on the paper. These other genes have no obvious means for self-repair.

When this X-degenerate region of the chimpanzee Y chromosome was sequenced and compared to the human Y, Page and colleagues found that five genes on the chimp Y had suffered mutations that rendered them inactive. On the human Y, those same genes continue to function perfectly. "So then," says Hughes, "even though the Y has lost many genes since its origin about 300 million years ago, it's been holding steady in humans for the last 6 million years."

Page and his team speculate that the loss of genes on the chimpanzee Y may be due to the chimp's mating habits. Both male and female chimps engage with multiple partners when they mate. This gives a strong selective pressure on those genes that produce sperm. Conversely, it puts less pressure on evolution to preserve those genes on the Y whose functions have nothing to do with reproduction. Because humans historically have been largely monogamous, our Y chromosomes have been spared such selective-pressure imbalance.

"Of course," acknowledges Page, "this is a hypothesis that we have no way to scientifically prove or disprove. However, we believe it's currently the best explanation."

Adapted from a news release by the Whitehead Institute for Biomedical Research .

Further reading

Hughes JF, et al.: Conservation of Y-linked genes during human evolution revealed by comparative sequencing in chimpanzee. Nature 2005 437:100-3. Abstract