Evidence for extensive non-allelic gene conversion between sex chromosomes in humans.

Human sex chromosomes co-evolved from a pair of autosomes through the suppression of recombination over progressively larger regions. It has long been accepted that meiotic recombination between human sex chromosomes is limited to short telomeric portions, known as pseudoautosomal regions, which mark the boundaries of a male specific region (MSY). However, in the last years, the idea that the human sex chromosomes did not have an independent evolutionary history has begun to emerge with the discovery of some MSY regions interested by X-to-Y gene conversion. In this study we explored how pervasive XY gene conversion has been during the evolution of human sex chromosomes. Using human- chimpanzee interspecies X-Y sequence comparisons we identified 19 regions showing signatures of historical gene conversion. Further, to explore the dynamics of XY non-allelic recombination in recent human evolution, we resequenced these gene conversion hotspots in 68 widely divergent Y haplogroups, and found that at least five of them are still active in humans. Our results show evidence of extensive XY non-allelic recombination between gametologous regions of sex chromosomes suggesting that the sequence landscape of MSY could be modulated by the transfer of genetic information from the X chromosome, and providing an additional explanation for the ability of the Y chromosome to retard degradation during evolution.