Comparison of substitution rates in ZFX and ZFY introns of sheep and goat related species supports the hypothesis of male-biased mutation rates.

There is a growing body of evidence that males serve as the major generators of mutations, due to the larger number of cell divisions involved in sperm compared to egg production. In mammals, this hypothesis (referred to as "male-driven evolution") has been tested by comparison of nucleotide substitution rates on the X and Y sex chromosomes in a limited number of taxa, predominantly primates and rodents. This study asks whether male-driven evolution is a more general phenomenon among mammals, by comparison of paralogous ZFX and ZFY intron sequences in sheep and goat species (the tribe Caprini). The male-to-female mutation ratio, alpha(m), was estimated to be between 2.93 (95% CI, 1.51-8.61) and 3.94 (95% CI, 1.25-32.29) when calculated using pairwise distance and branch length, respectively, suggesting that the Caprini are subject to weak, male-driven evolution. Comparison to published values for primates, felids, and rodents implies that there may be some correlation with reproductive life span. However, this is difficult to test with current data because confidence intervals are large and overlapping. Nonindependent evolution of paralogous sequences and/or the presence of selective constraints could lead to inaccurate estimates of alpha(m). No evidence for gene conversion between the ZFX and the ZFY introns was found, and this suggests that they have evolved independently during the radiation of the Caprini. Finally, there was no apparent evidence that these introns are subject to selective constraints, although low levels of intraspecific polymorphism reduce the power of neutrality tests.