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This report is a review of 20 years of cytogenetics studies in tuco-tucos from the south of Brazil and the implications for the evolution of these species as well as the relation of these data with the geological history of the Coastal Plain of southern Brazil. Two forms of Ctenomys torquatus, 2n = 44 and 46, are separated by a geographic barrier. Ctenomys flamarioni shows a constant karyotype (2n = 48) and presents high variability in FN due to constitutive heterochromatin variation. Ctenomys minutus presents the highest chromosomic variation among the species in the south of Brazil (2n = 42-50) with three chromosomic hybrid zones. Ctenomys lami, like C. minutus, presents a high chromosomic variation due to fusions and fissions of chromosome pairs 1 and 2. Both species present a close evolutionary relationship, including a chromosomic hybrid zone.

The present work describes chromosomal polymorphisms in zones of contact between divergent populations of Ctenomys minutus parapatrically distributed in the coastal plain of southern Brazil, and inter-specific hybridization with C. lami a closely related species. A sample of 171 specimens from 32 sample sites distributed along 161 km of the coastal plain was cytogenetically analyzed. Nine polymorphic populations were found: four with specimens with 2n = 46–48 (autosomal arm number (AN) = 76); three only have specimens with 2n = 47 and 48; one population sampled presented specimens with 2n = 43–46 (AN = 74–76) and one population with 2n = 50–52 (AN = 76–80). The remainder populations were fixed for 2n = 42, 46 or 48. The variation is the result of Robertsonian mechanisms of chromosomal evolution and a fusion in tandem rearrangement. The polymorphisms have been considered the result of secondary contact of populations after divergence in allopatry. The geomorphological evolution of the coastal plain provides clues to the possible existence of past geographic barriers acting over populations of Ctenomys, during the Holocene.

The rodent genus Ctenomys (tuco-tucos) comprises more than 60 described species, and shows extraordinary inter- and intraspecific karyotypic variation. The most widely distributed species of Ctenomys in Brazil is C. torquatus. Although several cytogenetic studies have been done, the karyotypic variability of this species is still poorly known. In this paper we report two new diploid numbers for C. torquatus: 2n = 40 and 2n = 42, both showing AN = 72. The new distribution limits of C. torquatus here reported include localities in the southern, central and western parts of Rio Grande do Sul (RS) State in southern Brazil. The phylogenetic relationship between C. torquatus from Alegrete, RS, and Ctenomys sp. from Corrientes, Argentina, is described by means of mtDNA cytochrome b analysis. Although both entities share similar karyotypes and sperm morphology, these two species are not phylogenetically close.

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 São José 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 20km apart along the coastal plain of Rio Grande do Sul between the municipalities of Tavares (31 degrees 23'S 51 degrees 09'W) and São José do Norte (31 degrees 52'S 51 degrees 54'W). We were able to extend the known geographic distribution of C. minutus by 90 km, from Tavares southwards to São José 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.

Chromosomes were analyzed in 91 specimens ofNectomys squamipes, collected in three distinct geographic regions of Brazil. Chromosomal polymorphism due to supernumerary chromosomes, Robertsonian rearrangement and variation of sex chromosomes is reported. Samples collected in the northeastern region had 2n=52, FN=52; 2n=53, FN=54; 2n=54, FN=56; 2n=55, FN=56; 2n=56, FN=56; 2n=57, FN=57. The specimens from the southeastern and southern regions showed 2n=55, FN=56; 2n=56, FN=56; 2n=57, FN=58; 2n=58, FN=60; 2n=59, FN=62. Seven different types of supernumeraries were observed. These vary in size, morphology and C-banding characteristics.

The G- and C-banded karyotype ofHolochilus brasiliensis collected in central Brazil (2n=55, AN=56, acrocentric X and Y) can be regarded as the most representative of the ancestral form of the genus.H. magnus (2n=52, AN=58, acrocentric X and Y) andH. brasiliensis vulpinus (2n=40, AN=56), both living in southern Brazil, would be in different phases of chromosome number reduction due to centric fusions.H. magnus, in addition, shows an X of variable morphology and a metacentric Y. This fact, coupled with its distinct morphology and restricted distribution, suggests that it may represent a distinct trend in the genus' main evolutionary line.