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

Genetic variability measured by allozymic electrophoresis has been stu died in several species of the subterranean rodent genus Ctenomys (Oct odontidae). The study was carried out with the main purpose of analysi ng a special group known as the 'Corrientes group' which inhabits the province of Corrientes in Argentina. The members of the group are, wit h high probability, isolated reproductively due to their karyotypic di fferences (diploid numbers are between 42 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 amon g taxa within the Corrientes group with all those measured between int raspecific populations in Clenomys. The results indicate that the dist ances among the populations of the Corrientes group are at or below th e level of those measured among intraspecific populations in Ctenomys. Thus, it is unlikely that genetic differentiation triggered speciatio n. It is shown that this low level of genetic differentiation is not i n contradiction with the high levels expected for species originating from one of the possible mechanisms of chromosomal speciation named, c hromosomal transilience. Although for geographical reasons it seems ob vious that gene flow is precluded among the members of the Corrientes group, the Slatkin method for estimating Nm values was also used. Beca use high values of Nm exist but no isolation by distance could be dete cted, it is suggested that reproductive and geographical isolation are very recent. Finally, analysis of population variability suggests tha t the high levels of heterozygosity observed (a) can be explained by t he population structure, and (b) are within the range of expected valu es if bottlenecks have occurred in the recent history of the Corriente s group. As a general conclusion, the results indicate that in the Cor rientes group the genetic data support a putative causal role for chro mosomes in speciation..