Reduced nucleotide variability at an androgen-binding protein locus (Abpa)in house mice: Evidence for positive natural selection

Previous work has shown that the gene for the alpha subunit of androgen-bin ding protein, Abpa, may be involved in premating isolation between differen t subspecies of the house mouse, Mus musculus. We investigated patterns of DNA sequence variation at Abpa within and between species of mice to test s everal predictions of a model of neutral molecular evolution. Intraspecific variation among 10 Mus musculus domesticus alleles was compared with diver gence between M. m. domesticus and M. caroli for Abpa and two X-linked gene s, Glra2 and Amg. No variation was observed at Abpa within M. in. domesticu s. The ratio of polymorphism to divergence was significantly lower at Abpa than at Glra2 and Amg, despite the fact that all three genes experience sim ilar rates of recombination. Interspecific comparisons among M. rn. domesti cus, Mus musculus musculus, Mus musculus castaneus, Mus spretus, Mus spicil egus, and Mus caroli revealed that the ratio of nonsynonymous substitutions to synonymous substitutions on a per-site basis (K-a/K-s) was generally gr eater than one. The combined observations of no variation at Abpa within M. m. domesticus and uniformly high K-a/K-s values between species suggest th at positive directional selection has acted recently at this locus.