Tests for positive selection on immune and reproductive genes in closely related species of the murine genus Mus

We examine variation among species of Mus in four genes involved in reproduction and the immune response for evidence of positive selection: the sperm recognition gene Zp-3, the testis-determining locus Sry, the testicular cell surface matrix protein Tcp-1, and the immune system protein β₂m. We use likelihood ratio tests in the context of a well-supported phylogeny to determine whether models that allow for positively selected sites fit the sequences better than models that assume purifying selection. We then apply a Bayesian approach to identify particular sites in each gene that have a high posterior probability of being under positive selection. We find no evidence of positive selection on the Tcp-1 gene, but for Zp-3, Sry, and β₂m, models that allow for positively selected sites fit the sequences better than alternatives. For each of these genes, we identify sites that have a high (>95%) posterior probability of being positively selected. For Zp-3, two of these sites occur near the sperm-binding region, while one occurs in a region whose functional role remains unstudied but where the pattern of change predicts functional importance. A single site in Sry shows an elevated rate of replacement substitution but occurs in a region of apparently little functional importance; therefore, relaxation of functional constraints may better explain the rapid evolution of this site. Three sites in β₂m have a posterior probability >50% of being under positive selection. While the functional role for two of these sites is unknown, the third is known to influence the ability of MHC class I molecules to present antigens to the immune system; therefore, the elevated rate of replacement substitutions at this site is consistent with selection acting to promote variability in immune system proteins.