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Cytogenetic analysis was performed in six nominal taxa of the genus Ctenomys with a diploid number of 2n=47/48. The studied species were: C. australis (2n=48, FN=76); C. mendocinus (2n=47/48, FN=68/75/76); C. porteousi (2n=47/48, FN=71/72/73); C. azarae (2n=47, FN=71); C. sp. ('chasiquensis') (2n=47/48); and C. talarum (2n=48, FN=80). The first three species shared the whole complement, C. talarum shares with them 19 arms from a total of 43 (44%). In all species analyzed constitutive heterochromatin was detected in most short arms, and in several centromeres. Polymorphisms for several pairs involving the heterochromatic short arms together with a complex polymorphism of pair A 1 were found in C. azarae, C. sp., C. mendocinus and C. porteousi. Intraindividual variation found in one specimen of C. porteousi involving heterochromatic arms is discussed.These results lead us to propose the inclusion of all species except C. talarum, within a complex called the mendocinus-group. A new case of conservatism for chromosomal number in the genus Ctenomys is found in this group.

A series of populations of spiny rats (Proechimys) of NW and N Central Venezuela were found to belong to a circle of species and subspecies which surrounds the Andes of M6rida-CordiUera de la Costa mountain axis. This 'Rassenkreis' consists of 6 successive karyomorphs which exhibit a stepped-dine distribution (2n = 42, 44, 46, 48, 50 and 62). Each of the karyomorphs occupies a discrete subarea in an almost continuous territory. Polymorphism is very rare and no intermediate karyotypes have been found in the borders of each subarea. Although contact zones have not been studied in detail, the distribution seems to be mostly parapatric. Areas are 12-100 km apart, in most cases without effective barrier between them. Only 1 Robertsonlan rearrangement accounts for each of the steps from 2n = 44 to 2n = 50. The transition from 2n = 42 to 2n = 44 involves 1 Robertsonian change and 3 pericentric inversions. The transition from 2n = 50 to 2n = 62 requires 6 Robertsonian rearrangements and 2 pericentric inversions. It is inferred that successive populations of the 2n = 44 -2n = 50 series may be at an incomplete stage of reproductive isolation by partial reproductive failure of the FI hybrids. Reproductive isolation may be almost complete between the 2n = 42 and 2n = 44 and the 2n = 50 and 2n = 62 karyomorphs. This 'Rassenkreis' is classified as a superspecies, Proechimys [guairae], comprising three allospecies: P. poliopus, P. guairae, and a new unnamed species (Barina's~ sp. n.). Within P. guairae 4 subspecies (or semispecies) are recognized, 2 of which are named (P. guairae guairae and P. guairae ochraceus). It is postulated that differentiation within this superspecies occurred during the Late Pleistocene. The tentative interpretation is advanced that rapid colonization occurred over most of the present territory by a chromosomically uniform invading stock, which subsequently became differentiated by parapatric chromosome raciation. This was followed by further chromosome differentiation in peripheral isolates which formed in unfavorable, dry climatic phases. During humid climatic phases the ranges expanded and zones of secondary contact with the original populations were established.

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Bone marrow chromosomes were studied in South American water rats of the genus Nectomys from Venezuela, Trinidad, and Argentina. Specimens of N. squamipes from western and southern Venezuela showed a 2n = 52–53 karyotype, whereas a 2n = 56–57 karyotype was found in specimens from northeastern Argentina. In both cases, odd karyotypes can be explained by the presence of a supernumerary chromosome. In contrast, water rats from northeastern Venezuela and Trinidad showed a strikingly reduced 2n = 16–17 polymorphic chromosome complement. Six different karyomorphs were found among the latter, which may have resulted from a combination of pericentric inversions in two pairs of autosomes and a cen-tromeric fusion in another autosomal pair. It is proposed that the new 2n = 16–17 cytotypes belong to a species of its own, for which the name N. palmipes is suggested.