The compositional evolution of the murid genome

Murid rodents show much less variation in isochore base composition than do most other mammals, a difference which has been referred to as the murid shift. We have investigated the murid shift by asking (1) whether the murid shift is ongoing and (2) whether there is any evidence of selection or biased gene conversion affecting base composition in the present-day mouse genome. By estimating the ancestral base composition of protein-coding genes in murids we can confirm that the murid shift is ongoing. Tests using nongenic polymorphism data fail to reject the hypothesis that base composition is due to mutation bias alone. However, the patterns of compositional change suggested by the polymorphism and divergence data differ, suggesting the possibility of two murid shifts.