Can Intra-Y Gene Conversion Oppose the Degeneration of the Human Y Chromosome? A Simulation Study

The human Y is a genetically degenerate chromosome, which has lost about 97% of the genes originally present. Most of the remaining human Y genes are in large duplicated segments (ampliconic regions) undergoing intense Y–Y gene conversion. It has been suggested that Y–Y gene conversion may help these genes getting rid of deleterious mutations that would inactivate them otherwise. Here, we tested this idea by simulating the evolution of degenerating Y chromosomes with or without gene conversion using the most up-to-date population genetics parameters for humans. We followed the fate of a variant with Y–Y gene conversion in a population of Y chromosomes where Y–Y gene conversion is originally absent. We found that this variant gets fixed more frequently than the neutral expectation, which supports the idea that gene conversion is beneficial for a degenerating Y chromosome. Interestingly, a very high rate of gene conversion is needed for an effect of gene conversion to be observed. This suggests that high levels of Y-Y gene conversion observed in humans may have been selected to oppose the Y degeneration. We also studied with a similar approach the evolution of ampliconic regions on the Y chromosomes and found that the fixation of many copies at once is unlikely, which suggest these regions probably evolved gradually unless selection for increased dosage favored large-scale duplication events. Exploring the parameter space showed that Y–Y gene conversion may be beneficial in most mammalian species, which is consistent with recent data in chimpanzees and mice.