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1 Acta Theriologica

1 2001

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Chromosomal description of the rodent genera Oecomys and Nectomys from Brazil

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Andrades-Miranda J. , Oliveira L. F. B. , Zanchin N. I. T. , Mattevi M. S.

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We analysed karyotypes of five taxa of the rodent genera Oecomys and Nectomys, trapped in 14 localities in an area ranging from 8° to 29°S on Brazilian territory. Oecomys cf. concolor, collected in the Amazon and in two localities of the Cerrado biome, showed a 2n = 60 karyotype constituted by a pair of large subtelocentric chromosomes, a small metacentric pair and 27 acrocentric pairs. The X chromosome was a large submetacentric and a subtelo-submetacentric, the morphology of the latter showing variable C-banding patterns. In all three localities the Y chromosome was a medium size heterochromatic acrocentric. Two individuals from the Cerrado had a heterochromatic acrocentric B-chromosome. Oecomys cf. bicolor presented two cytotypes, 2n = 80 in the specimens from the Cerrado biome and 2n = 82 in individuals trapped in the Amazon. The 2n = 80 cytotype 1 showed a large subtelocentric, 22 biarmed pairs (medium to small) and 16 acrocentric autosomal pairs. The karyotype of the 2n = 82 cytotype 2 is constituted by 15 biarmed chromosomes (median to small) and 25 acrocentric pairs with heterochromatic blocks at pericentromeric regions. The sexual pairs were the same (large submetacentric X and median acrocentric Y) in both cytotypes. In O. cf. concolor and in both cytotypes of O. cf. bicolor the nucleolar organizer regions were observed in 1-3 pairs, located in the short arms. Nectomys genus presented two cytotypes, 2n = 52-55 (N. rattus, with 0-3 biarmed heterochromatic accessory chromosomes) and 2n = 56-59 (N. squamipes, bearing 0-3 biarmed, hetero- chromatic, B-chromosomes). These 2 cytotypes occupy disjunct regions of South America, with overlapping areas in the Brazilian states of Pernambuco, Bahia, and Mato Grosso do Sul.