A review on the cytogenetics of the tribe Oryzomyini (Rodentia: Cricetidae: Sigmodontinae), with the description of new karyotypes Author Moreira, Camila Do Nascimento Author Ventura, Karen Author Percequillo, Alexandre Reis 0000-0001-9490-5457 cmoreirabio@gmail.com Author Yonenaga-Yassuda, Yatiyo text Zootaxa 2020 2020-11-06 4876 1 1 111 journal article 7983 10.11646/zootaxa.4876.1.1 4ee8b608-7c73-4980-9e86-3e3719e91c7f 1175-5326 4423612 190EC586-E14B-4AEF-A5EF-3DA401656159 Oecomys bicolor Karyotype: 2n = 80 and FN = 140. Autosomal complement: 19 metacentric and submetacentric pairs medium to small decreasing in size, 12 subtelocentric pairs medium to small decreasing in size, and eight acrocentric pairs medium to small decreasing in size. Sex chromosomes: X, a large metacentric (the largest chromosome of the complement); Y, a small chromosome that appears to be acrocentric ( Patton et al . 2000 , pp. 127, Fig. 87). Another fundamental number of 134 or 136 was reported by Gardner & Patton (1976) , which was reported with three additional pairs of uniarmed and three less biarmed autosomal elements. However, according to Patton et al . (2000) distinctions between the morphological categories can be quite difficult with such small chromosomes, and not too much emphasis should be placed on the presumptive differences between these two karyotypes. G-banding karyotype was performed by Baker et al . (1983) . Another karyotype was applied for this species by Gomes-Júnior et al . (2016) . Karyotype: 2n = 80 and FN = 142. Autosomal complement: nine metacentric pairs medium to small decreasing in size, five submetacentric pairs medium to small decreasing in size, 18 subtelocentric pairs (one large and the remaining medium to small decreasing in size), and seven acrocentric pairs medium to small decreasing in size. Sex chromosomes: X, a large submetacentric (the largest chromosome of the complement); Y, a medium subtelocentric. C-banding metaphases exhibited blocks of constitutive heterochromatin on the pericentromeric region of all autosomes, in the majority of metacentric and submetacentric the C-band extends to the short arm. The X chromosome presented the short arm entirely heterochromatic. The Y chromosome presented the long arm entirely heterochromatic. G-banding was also performed. NORs were localized at the telomeric regions of the short arms of seven chromosomes of medium and small size. FISH with 18S rDNA revealed signals at the telomeric regions of the short arms of 12 medium to small autosomal pairs, while FISH with 5S rDNA revealed signals at the pericentromeric region of medium pair. FISH with telomeric sequences revealed signals exclusively at the ends of all chromosome arms and no interstitial signals were observed ( Gomes-Júnior et al . 2016 ). Two more karyotypes, with 2n = 54 and FN = 82, and 2n = 82 and FN = 116, were attributed to this species by Gomes-Júnior et al . (2016) (unpublished data from T. Lira), based on molecular data. However, these karyotypes were distinctly from those previously reported to this species. Thus, we suggested a more detailed analyses, using morphological and molecular data combined, in the specimens studied by T. Lira, in order to correctly identify these specimens. Finally, these variation in diploid and fundamental number of O. bicolor occurs in different localities of Amazon rainforest ( Table 8 , Fig. 12 ).