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Y chromosomes evolving much faster in humans

The Y chromosome, one of the two sex determining chromosomes in mammals, including humans, is evolving much faster through continuous renovation contrary to the popular theory that it is slowly decaying, says new evidence.

By conducting the first comprehensive interspecies comparison of Y chromosomes, Whitehead Institute researchers have found that these chromosomes have evolved more quickly than the rest of their respective genomes over the six million years since they emerged from a common ancestor.
Most mammals have one pair of sex chromosomes in each cell. Males have one Y chromosome and one X chromosome, while females have two X chromosomes.
”The region of the Y that is evolving the fastest is the part that plays a role in sperm production,” say Jennifer Hughes, study co-author and a postdoctoral researcher at 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,” said Hughes.
The study also found that there is considerable difference in the genetic sequences of the human and chimp (chimpanzee) Ys. 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 Centre at Washington University, the Page lab painstakingly sequenced the chimp Y chromosome, allowing comparison with the human Y, which the Page lab and the Genome Center had sequenced 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, says a Whitehead release.
”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.”
These findings were published online this week in ‘Nature’.