The nature of gene evolution on the mammalian Y chromosome: lessons from Sry

With the exception of a small region, heteromorphic sex chromosomes of mammals do not undergo recombination in male meiosis. As a result, the majority of the Y chromosome is clonally transmitted through paternal lineages. Numerous phenomena, including the Hill-Robertson effect, Muller's ratchet, genetic hitch-hiking, and male-driven molecular evolution, are associated with the special transmission properties of the Y chromosome, and can potentially explain the tempo and pattern of gene evolution on the mammalian Y. We explore these phenomena in light of comparative data from the Y-linked sex-determining locus, Sry. Sry exhibits rapid amino acid divergence between species and little to no variation within species. We find no evidence for directional selection acting on this locus. The pattern of evolution between species is consistent with the Hill-Robertson effect and Muller's ratchet. Lack of variation in Sry within species may reflect genetic hitch-hiking, however, we cannot exclude the confounding effects of small effective population size of Y chromosomes. We find no support for male-driven molecular evolution for Sry in Old World mice and rats. However, a more appropriate test of this hypothesis would be to compare the evolution of Sry to the X-linked Sox3 gene in these same species. Clearly, more comparative studies of Sry and other Y-linked loci are needed to characterize the effects of Y chromosome transmission on the evolution of Y-linked sequences.