Contrary to a widely held scientific theory that the mammalian Y chromosome is slowly decaying or stagnating, new evidence suggests that in fact the Y is actually evolving quite rapidly through continuous, wholesale renovation.
By conducting the first comprehensive interspecies comparison of Y chromosomes, Whitehead Institute researchers have found considerable differences in the genetic sequences of the human and chimpanzee Ys -- an indication that these chromosomes have evolved more quickly than the rest of their respective genomes over the 6 million years since they emerged from a common ancestor. The findings are published online this week in the journal Nature.
"The region of the Y that is evolving the fastest is the part that plays a role in sperm production," say Jennifer Hughes, first author on the Nature paper and a postdoctoral researcher in Whitehead Institute Director David Page's lab. "The rest of the Y is evolving more like the rest of the genome, only a little bit faster."
The chimp Y chromosome is only the second Y chromosome to be comprehensively sequenced. The original chimp genome sequencing completed in 2005 largely excluded the Y chromosome because its hundreds of repetitive sections typically confound standard sequencing techniques. Working closely with the Genome Center at Washington University, the Page lab managed to painstakingly sequence the chimp Y chromosome, allowing for comparison with the human Y, which the Page lab and the Genome Center at Washington University had sequenced successfully back in 2003.
The results overturned the expectation that the chimp and human Y chromosomes would be highly similar. Instead, they differ remarkably in their structure and gene content. The chimp Y, for example, has lost one third to one half of the human Y chromosome genes--a significant change in a relatively short period of time. Page points out that this is not all about gene decay or loss. He likens the Y chromosome changes to a home undergoing continual renovation.
"People are living in the house, but there's always some room that's being demolished and reconstructed," says Page, who is also a Howard Hughes Medical Institute investigator. "And this is not the norm for the genome as a whole."
Wes Warren, Assistant Director of the Washington University Genome Center, agrees. "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," he says. "These findings demonstrate that our knowledge of the Y chromosome is still advancing."
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