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 female) of a new species of Oryzomys belonging to the nitidus group (Sigmodontinae,Rodentia) from Pacoti, state of Ceara, Northeastern Brazil. Cytogenetic data included CBG and GTG-banding, Ag-NOR staining as well as fluorescence in situ hybridization (FISH) with (T 2 AG 3 )n as probe, which revealed exclusive signals in all telomeres of all chromosomes. The karyotype comprises 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). The X chromosome is a large subtelocentric and the Y is a medium size chromosome with two different morphologies:acrocentric and submetacentric. Pairs 1 and 3 are the result of Robertsonian rearrangements when compared with the 2n=80 karyotypes observed in Oryzomys russatus, O. nitidus and O. emmomae. Interstitial telomeric sites (ITS) after FISH were not detected in those specific chromosomes.