Us double-X-ers can't feel quite so smug any more: apparently the Y chromosome isn't stagnating or decaying, as was previously thought, but is in fact evolving quite rapidly.
Whitehead Institute researchers have found some major differences in the genetic sequences of the human and chimpanzee Y chromosomes, indicating that they've actually evolved more quickly than the rest of the genome over the six million years since chimps and humans diverged.
"The region of the Y that is evolving the fastest is the part that plays a role in sperm production," says lead author Jennifer Hughes. "The rest of the Y is evolving more like the rest of the genome, only a little bit faster."
The researchers were surprised to find that the chimp and human Y chromosomes had big differences in their structure and gene content. The chimp Y, for example, has lost over one third of the human Y chromosome genes in a relatively short period of time.
"This work clearly shows that the Y is pretty ingenious at using different tools than the rest of the genome to maintain diversity of genes," says Wes Warren, Assistant Director of the Washington University Genome Center. "These findings demonstrate that our knowledge of the Y chromosome is still advancing."
The researchers reckon that the divergent evolution of the chimp and human Y chromosomes may be due to traits specific to Y chromosomes and differences in mating behaviors.
Female chimps put it about a bit more than their human counterparts, and multiple males may mate with a single female in rapid succession. If one male produces more sperm, he would be more likely to impregnate the female and pass on his superior sperm production genes, some of which may be residing on the Y chromosome.
But these genes may also drag along detrimental genetic traits. There's no evolutionary need to prevent this because, unlike other chromosomes, the Y has no partner with which to swap genes during cell division. Either the entire chromosome is advantageous, or it's not.
Disadvantages from a less-than-ideal gene version or even the loss of a whole section of the chromosome may have been outweighed by the advantage of improved sperm production, leaving the chimp Y chromosome with far fewer genes than its human counterpart.
The team is now sequencing and examining the Y chromosomes of several other mammals to see whether their Y chromosomes are evolving as quickly.
The findings are published this week in Nature.