New karyotypes and some considerations about the chromosomal diversification of Ctenomys minutus (Rodentia: Ctenomyidae) on the coastal plain of the Brazilian state of Rio Grande do Sul

The dominant mammals occupying the subterranean niche in South America are rodents of the genus Ctenomys, which form a large group of 56 species with chromosome numbers ranging from 2n = 10 to 70. In southern Brazil, Ctenomys minutus is the species with the widest geographic distribution, inhabiting sandy fields and dunes extending from Jaguaruna beach in the state of Santa Catarina to the town of Sao Jose do Norte in the state of Rio Grande do Sul. Eleven karyotypes (2n = 42; 2n = 46a; 2n = 46b; 2n = 47a; 2n = 47b; 2n = 48a; 2n = 48b; 2n = 49a; 2n = 49b; 2n = 50a and 2n = 50b) were described for this species and zones of hybridization are also known. A sample of 51 C. minutus specimens was collected from five sampling sites about 20 km apart along the coastal plain of Rio Grande do Sul between the municipalities of Tavares (31degrees23'S 51degrees09'W) and Sao Jose do Norte (31degrees52S 51degrees54'W). We were able to extend the known geographic distribution of C. minutus by 90 km, from Tavares southwards to Sao Jose do Norte. During our study we found five karyotypes (2n = 46b, 47b, 48b, 49b and 50b), four of which (2n = 47b, 48b, 49b and 50b) have not previously been described for this species.