Chromosomal description of the rodent genera Oecomys and Nectomys from Brazil

We analysed karyotypes of five taxa of the rodent genera Oecomys and Nectom ys, trapped in 14 localities in an area ranging from 8 degrees to 29 degree sS on Brazilian territory. Oecomys cf. concolor, collected in the Amazon an d in two localities of the Cerrado biome, showed a 2n = 60 karyotype consti tuted by a pair of large subtelocentric chromosomes, a small metacentric pa ir and 27 acrocentric pairs. The X chromosome was a large submetacentric an d a subtelo-submetacentric, the morphology of the latter showing variable C -banding patterns. In all three localities the Y chromosome was a medium si ze heterochromatic acrocentric. Two individuals from the Cerrado had a hete rochromatic acrocentric B-chromosome. Oecomys cf. bicolor presented two cyt otypes, 2n = 80 in the specimens from the Cerrado biome and 2n = 82 in indi viduals trapped in the Amazon. The 2n = 80 cytotype 1 showed a large subtel ocentric, 22 biarmed pairs (medium to small) and 16 acrocentric autosomal p airs. 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 s ubmetacentric X and median acrocentric Y) in both cytotypes. In O. cf. conc olor and in both cytotypes of O. cf. bicolor the nucleolar organizer region s were observed in 1-3 pairs, located in the short arms. Nectomys genus pre sented two cytotypes, 2n = 52-55 (N. rattus, with 0-3 biarmed heterochromat ic accessory chromosomes) and 2n = 56-59 (N. squamipes, bearing 0-3 biarmed , heterochromatic, B-chromosomes). These 2 cytotypes occupy disjunct region s of South America, with overlapping areas in the Brazilian states of Perna mbuco, Bahia, and Mato Grosso do Sul.