Gene loss on Y chromosomes: a consequence of purifying selection?

Recombination restriction between evolving sex chromosomes leads to the degeneration of the chromosome that is present only in the heterogametic sex (the Y chromosome in XY species). The evolutionary forces driving Y chromosome degeneration, however, are still under debate and include positive and negative selection models. In a recent study, we showed that the rate of accumulation of loss-of-function mutations on the neo-Y chromosome of Drosophila miranda is compatible with the process of Muller's ratchet, the stochastic loss of the best mutational class of individuals from a small asexual population. Purifying selection at amino acid sites can accelerate the ratchet, and the speed of degeneration depends on the number of genes still present on the evolving Y chromosome. Our study shows that Y chromosome degeneration does not require the action of selective sweeps at linked sites, and can take place under realistic parameters of purifying selection only.