Two Tandem Fusions and Supernumerary Chromosomes inNectomys Squamipes(Cricetidae, Rodentia)

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Abstract
In Nectomys squamipes collected in different regions of Brazil, two basic chromosomal entities with 2n = 52 and 56 chromosomes occurred. Both entities are characterized by variations in the diploid numbers due to the presence of supernumerary chromosomes (2n = 52, 53, 54, 55 and 2n = 56, 57, 58, 59). Two tandem fusions have been recognized as the mechanism involved in the differentiation of the two basic karyotypes (2n = 52 and 2n = 56). G- and C-band patterns, localization of the nucleolus organizer regions (NOR's) and R-banding obtained after 5-BrdU incorporation are presented. The supernumerary chromosomes, the Y and the short arm of the X chromosome are late replicating.