Summary: Hughes et al. (2010)

It is thought that the human Y chromosome evolved a sex-determining function millions of years ago by gene loss. Given this, theories of decelerating decay predicts that chimpanzee and human male-specific regions on the Y-chromosome (MSY) should not show much divergence. Hughes et al. (2010) accurately and completely sequenced the MSY in the chimpanzee, using large-insert bacterial artificial chromosome clones and iterative mapping and sequencing strategies, in order to compare it to the human MSY. They found differences in sequence structure and gene content, suggesting rapid evolution within the last 6 million years, contrary to the prediction that much divergence should be unlikely. They found that both human and chimp MSY euchromatin is largely comprised of ampliconic and X-degenerate sequence classes but noticed that, while humans have less massive palindromes than chimps, chimps have lost large portions of MSY protein-coding genes and gene families. Hughes et al. (2010) suggest that this divergence in chimp and human MSYs could be driven by four synergistic factors: (1) MSY's role in sperm production; (2) "genetic hitchhiking"; (3) frequent ectopic recombination; and (4) mating behaviour differences. They favour (3): that ectopic recombination between MSY amplicons has sped up MSY structural remodelling in both species. This study is the first to fully compared the Y chromosomes from two closely related species. The importance is two-fold: (1) it provides empirical insight into the evolution of the Y-chromosome; and (2) it provides a test of decelerating decay theories.