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 <<>> Member and <<>> investigator David Page, senior author on the study that will appear in the September 1 issue of the journal <<>>. "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's been assumed that the Y chromosome is headed for extinction because, unlike other chromosomes, it has no genetic "mate" with which to swap genes. In 2003, Page published a landmark paper in Nature challenging that claim by demonstrating how a certain region of the Y chromosome possessed a unique mechanism for repairing mutated genes.
Through sequencing the Y, the Page lab and collaborators at <<>> 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, 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. Because of this, many proponents of the "Y's demise" theory remained undaunted.
Once again collaborating with Richard Wilson from the Washington University School of Medicine in St. Louis Missouri, Page and his research team sequenced this X-degenerate region of the chimpanzee Y chromosome and compared it to the human Y.
"We were looking for any evidence that the human Y has lost genes since parting ways with the chimp," says Hughes. "Had we found active genes on the chimp Y that had become inactive on the human, that would be the smoking gun. But we didn't find any such evidence. In fact, we found the opposite."