Cytogenetics and karyosystematics of South American oryzomyine rodents (Cricetidae: Sigmodontinae)

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Abstract
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.

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