GEOGRAPHIC STRUCTURE, GENE FLOW, AND MAINTENANCE OF MELANISM IN CTENOMYS RIONEGRENSIS (RODENTIA, OCTODONTIDAE)

Ctenomys rionegrensis has three coat color morphs (melanic, agouti, an d dark-backed) within its total distribution of 50 x 60 km area of Uru guay. The presence of two populations fixed for the melanic form is re markable because this coat color contrasts markedly with the surroundi ng 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 monomorp hic (95% criterion) and no alleles correlated exclusively with a parti cular coat color. Average heterozygosity was H = 0.038 (range 0.022-0. 058). Using pairwise comparisons of all populations, the mean number o f migrants ((M) over cap) was 6.342 for all pairs except those involvi ng the population at Los Arrayanes (agouti), for which the average val ue was 1.532. Our results indicate that gene flow in C. rionegrensis i s sufficiently high to prevent fixation of alternative alleles exclusi vely by drift. The absence of a pattern of genetic variation due to is olation by distance suggests that the current distribution resulted fr om a recent range expansion.