Natural Selection Drives Altitudinal Divergence at the Albumin Locus in Deer Mice, Peromyscus Maniculatus

Abstract

In populations that are distributed across steep environmental gradients, the potential for local adaptation is largely determined by the spatial scale of fitness variation relative to dispersal distance. Since altitudinal gradients are generally characterized by dramatic ecological transitions over relatively short linear distances, adaptive divergence across such gradients will typically require especially strong selection to counterbalance the homogenizing effect of gene flow. Here we report the results of a study that was designed to test for evidence of adaptive divergence across an altitudinal gradient in a natural population of deer mice, Peromyscus maniculatus. We conducted a multilocus survey of allozyme variation across a steep altitudinal gradient in the southern Rocky Mountains that spanned several distinct biomes, from prairie grassland to alpine tundra. As a control for the effects of altitude, we also surveyed the same loci in mice sampled along a latitudinal transect through the prairie grassland that ran perpendicular to the east‐west altitudinal transect. We used a coalescent‐based simulation model to identify loci that deviated from neutral expectations, and we then assessed whether locus‐specific patterns of variation were nonrandom with respect to altitude. Results indicated that the albumin locus (Alb) reflects a history of diversifying selection across the altitudinal gradient. This conclusion is supported by two main lines of evidence: (1) Alb was characterized by levels of divergence across the altitudinal transect that exceeded neutral expectations in two consecutive years of sampling (in contrast to the spatial pattern of variation across the latitudinal transect), and (2) levels of divergence at the Alb locus exhibited a positive association with altitudinal distance in both years (in contrast to the pattern observed at unlinked loci). We conclude that clinal variation at the Alb locus reflects a balance between gene flow and diversifying selection that results from elevational changes in fitness rankings among alternative genotypes.

Combining population genomics and quantitative genetics: finding the genes underlying ecologically important traits

“…Although not at a genomic scale, Storz and Dubach (2004) showed a clear example of detecting outlier loci: the albumin (Alb) locus in the deer mouse Peromyscus maniculatus showed significant altitudinal differentiation that exceeded neutral expectations based on 18 other allozyme markers, although the precise selective agent remains unclear. Studies that implicate an environmental gradient as the selective force producing differentiation are clearly strengthened by multiple tests (e.g., multiple altitudinal or latitudinal transects), and preferably using multiple statistical approaches (Campbell and Bernatchez, 2004;.…”

Land ahead: using genome scans to identify molecular markers of adaptive relevance

“…Campbell and Bernatchez (2004) and Storz and Dubach (2004) detected outlier loci separating benthic and limnetic ecotypes of whitefish and differentiating deer mice populations from different altitudes, respectively. Several outlier locus analyses have been carried out on plant species.…”

Evidence for contrasting modes of selection at interacting globin genes in the European rabbit (Oryctolagus cuniculus)

“…In each set of iterations, sample sizes were set equal to the median of actual sample sizes in the specific data set under consideration. Coalescent simulations were used to generate a total of 50 000 paired values of F ST and H, which was then used to compute the 0.975, 0.50 and 0.025 quantiles of the conditional distribution (Beaumont and Nichols, 1996;Storz and Nachman, 2003;Storz and Dubach, 2004). Loci with F ST values that exceeded the 0.975 quantile of the null distribution were considered candidates for spatially varying selection (that is, the observed heterogeneity in allele frequencies exceeded neutral expectations).…”