Geographic structure, gene flow, and maintenance of melanism in Ctenomys rionegrensis (Rodentia: Octodontidae)

Ctenomys rionegrensis has three coat color morphs (melanic, agouti, and dark-backed) within its total distribution of 50 x 60 km area of Uruguay. The presence of two populations fixed for the melanic form is remarkable because this coat color contrasts markedly with the surrounding substrate. Starch gel electrophoresis was used to analyze variation in 20 allozyme loci assayed in 100 individuals from seven populations of C. rionegrensis to test the hypothesis that melanism was fixed by genetic drift in small, isolated populations. Seven loci were monomorphic (95% criterion) and no alleles correlated exclusively with a particular coat color. Average heterozygosity was H = 0.038 (range 0.022-0.058). Using pairwise comparisons of all populations, the mean number of migrants ((M) over cap) was 6.342 for all pairs except those involving the population at Los Arrayanes (agouti), for which the average value was 1.532. Our results indicate that gene flow in C. rionegrensis is sufficiently high to prevent fixation of alternative alleles exclusively by drift. The absence of a pattern of genetic variation due to isolation by distance suggests that the current distribution resulted from a recent range expansion.