Positive and Negative Selection on Noncoding DNA Close to Protein-Coding Genes in Wild House Mice

During the past two decades, evidence has accumulated of adaptive evolution within protein-coding genes in a variety of species. However, with the exception of Drosophila and humans, little is known about the extent of adaptive evolution in noncoding DNA. Here, we study regions upstream and downstream of protein-coding genes in the house mouse Mus musculus castaneus, a species that has a much larger effective population size (N-e) than humans. We analyze polymorphism data for 78 genes from 15 wild-caught M. m. castaneus individuals and divergence to a closely related species, Mus famulus. We find high levels of nucleotide diversity and moderate levels of selective constraint in upstream and downstream regions compared with nonsynonymous sites of protein-coding genes. From the polymorphism data, we estimate the distribution of fitness effects (DFE) of new mutations and infer that most new mutations in upstream and downstream regions behave as effectively neutral and that only a small fraction is strongly negatively selected. We also estimate the fraction of substitutions that have been driven to fixation by positive selection (alpha) and the ratio of adaptive to neutral divergence (omega(alpha)). We find that alpha for upstream and downstream regions (similar to 10%) is much lower than alpha for nonsynonymous sites (similar to 50%). However, omega(alpha) estimates are very similar for nonsynonymous sites (similar to 10%) and upstream and downstream regions (similar to 5%). We conclude that negative selection operating in upstream and downstream regions of M. m. castaneus is weak and that the low values of alpha for upstream and downstream regions relative to nonsynonymous sites are most likely due to the presence of a higher proportion of neutrally evolving sites and not due to lower absolute rates of adaptive substitution.