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A chromosomal study that included the use of uniform stain, G-, C-banding and digestion with restriction enzymes Alu I and Sau 3A was completed in 13 male and two female Ctenomys perrensi collected six localities in Corrientes (NE Argentina). The specimens studied had diploid numbers of 50–57 and FN ranging from 84 to 88. These differences resulted from the presence in heterozygosis or homozygosis of one or two fission products involving chromosomes 1, 2, 4, 7 and 8. In addition, in two localities karyotypes with 2 n=51, 53, 54, 55 and 56 were found, due to the presence of 1–4 C-negative supernumerary chromosomes. All specimens, except for one male and two females, showed more than one different karyotype. Our results suggest that in the localities studied, individuals heterozygous for one or two fission products and/or with a variable number of supernumerary chromosomes are frequently found.

A cytogenetic chromosome study was carried out on specimens of four species of Ctenomys—C. talarum, C. rionegrensis, C. pearsoni and C. dorbignyi—from 10 different populations. The analysis of chromosomes was performed through sequential uniform stain, G and C-banding, and with restriction enzymes. The results obtained are discussed in relation to phylogeny. Chromosome evolution in the species studied suggests that the chromosome number has increased due to fissions, and that a reduction of the amount of constitutive heterochromatin has occurred. Different types of heterochromatin, with different patterns, have been added in parallel during evolution. Some taxonomic suggestions can be deduced from this cytogenetic study.

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.

Chromosomes obtained from bone-marrow, spleen and testes were studied in samples from five populations of Spalacopus cyanus of Central Chile. Animals of four low-land coastal localities and animals of a high Andean valley showed a karyotype of 2n = 58 chromosomes, with almost no variation, either in number or in structure, both within each population or among the various populations. This karyotype was found to be very similar to that of Octodon degus, but very different to those in species of Ctenomys. The evolutionary and systematic significance of these results is discussed. The lack of karyotypic variation found in Spalacopus is correlated with its taxonomic uniformity, and both could be the result of the high vagility of the populations of this rodent, suggested by previous studies. This high capacity of dispersal is supposed to have enhanced gene flow, and therewith chromosome uniformity and low taxonomic diversification. The occurrence of a case of dimorphism for the presence of a secondary constriction in one of the autosome pairs is reported and discussed.