Cytogenetics and systematic approach on a new Oryzomys species, of the nitidus group (Sigmodontinae, Rodentia) from Northeastern Brazil

The diploid number 2n=76 was found in five specimens (four males and one fe male) of a new species of Oryzomys belonging to the nitidus group (Sigmodon tinae, Rodentia) from Pacoti, state of Ceara, Northeastern Brazil. Cytogene tic data included CBG and GTG-banding, Ag-NOR staining as well as fluoresce nce in situ hybridization (FISH) with (T(2)AG(3))(n) as probe, which reveal ed exclusive signals in all telomeres of all chromosomes. The karyotype com prises two large pairs (1 and 3) and four small metacentric pairs (34 to 37 ); 31 acrocentric pairs from large to small (pairs 2, and pairs 4 to 33). T he X chromosome is a large subtelocentric and the Y is a medium size chromo some with two different morphologies: acrocentric and submetacentric. Pairs 1 and 3 are the result of Robertsonian rearrangements when compared with t he 2n=80 karyotypes observed in Oryzomys russatus, O. nitidus and O. emmons ae. Interstitial telomeric sites (ITS) after FISH were not detected in thos e specific chromosomes.