A study of genetic distances and variability in several species of the genus Ctenomys (Rodentia: Octodontidae) with special reference to a probable causal role of chromosomes in speciation

Abstract

Genetic variability measured by allozymic electrophoresis has been studied in several species of the subterranean rodent genus Ctenomys (Octodontidae). The study was carried out with the main purpose of analysing a special group known as the 'Corrientes group' which inhabits the province of Corrientes in Argentina. The members of the group are, with high probability, isolated reproductively due to their karyotypic differences (diploid numbers are between 49 and 70). To evaluate whether the chromosomal differences were the causes of speciation or if they arose a posteriori, we compared the level of the genetic distance among taxa within the Corrientes group with all those measured between intraspecific populations in Ctenomys. The results indicate that the distances among the populations of the Corrientes group are at or below the level of those measured among intraspecific populations in Ctenomys. Thus, it is unlikely that genetic differentiation triggered speciation. It is shown that this low level of genetic differentiation is not in contradiction with the high levels expected for species originating from one of the possible mechanisms of chromosomal speciation named, chromosomal transilience. Although for geographical reasons it seems obvious that gene flow is precluded among the members of the Corrientes group, the Slatkin method for estimating Nm values was also used. Because high values of Nm exist but no isolation by distance could be detected, it is suggested that reproductive and geographical isolation are very recent. Finally, analysis of population variability suggests that the high levels of heterozygosity observed (a) can be explained by the population structure, and (b) are within the range of expected values if bottlenecks have occurred in the recent history of the Corrientes group. As a general conclusion, the results indicate that in the Corrientes group the genetic data support a putative causal role for chromosomes in speciation.