Heterogeneous supernumerary chromosomes (Bs) are recognized in the oryzomyines Holochilus brasiliensis, Nectomys rattus, N. squamipes, Oligoryzomys flavescens and Sooretamys angouya, representing about 10% of all known B-containing rodent species. They provide an outstanding model for understanding the origin, evolution and diversity of Bs in a phylogenetic context. Therefore, whole chromosome-specific probes were generated from flow-sorted Holochilus brasiliensis (HBR) autosomes 11 and 25+26 and chromosomes X, Y and Bs. Hybridizations were performed on male metaphases of 15 Oryzomyini species of which 3 are B-containing species. The results reveal that among the species sampled, 12 of them, belonging to a monophyletic Oryzomiyini subclade, are positive for an anonymous Oryzomyini shared heterochromatic region (OSHR) on both sex chromosomes. The OSHR is also present on Bs of Holochilus brasiliensis, Nectomys rattus and N. squamipes but not on Bs of O. flavescens and S. angouya. Two distinct additional OSHR/autosome associations are observed on S. angouya. The three species that are OSHR negative belong to an outgroup. Molecular dating suggests that the OSHR originated between 7.8 and 3 Mya on ancestral sex chromosomes. A tentative explanation for the OSHR-positive nature of B regions in three species could be that transposable elements (TEs) from this specific sex chromosome region may have invaded existing B chromosomes. The presence of the OSHR on entire Xp and Yp adjacent to interstitial telomeric sequences at pericentromeric positions, as observed in Drymoreomys albimaculatus, show a similar organization as on B chromosomes in Nectomys squamipes. The diversity of the Oryzomyini Bs in number, size, morphology and genetic content may be explained by the independent origin of B chromosomes in different subgroups of species, with Bs in Holochilus brasiliensis, Nectomys squamipes and N. rattus sharing the OSHR with sex chromosomes, and those in Oligoryzomys flavescens and Sooretamys angouya lacking OSHR in Bs. The species-specific pattern of Bs is probably a consequence of their independent evolutionary origin.

Oryzomyini represents the most diverse and speciose tribe of subfamily Sigmodontinae, with 29 genera and about 141 species. This great diversity of species is distributed from southeastern North to southern South America. Its systematics have passed through major changes in the last years due to the integration of molecular data with morphological characters in phylogenetic inferences. Unsurprisingly, cytogenetic studies on Oryzomyini reflect such diversity, with chromosome diploid number varying from 2n = 16 to 2n = 88. In addition, some species present autosomal and sex chromosome polymorphisms, besides the presence of B chromosomes. However, despite decades of cytogenetic studies, our knowledge about the karyotype variability in this group were still poorly known. Considering such deep and profound changes on the tribe, along with important new evidence that was continuously being produced associated to field work in several areas of Brazil and South America, we performed a cytogenetic review of the Oryzomyini group. We provide standardized descriptions summarizing all the knowledge associated to the known species of the tribe. We also describe seven new karyotypes for the tribe, Euryoryzomys sp., 2n = 58 and FN = 92; Neacomys sp. 1, 2n = 48 and FN = 54; Neacomys sp. 2, 2n = 54 and FN = 62; Oecomys sp. 1, 2n = 54 and FN = 84; Oecomys sp. 2, 2n = 64 and FN = 92; Oecomys sp. 3, 2n = 84 and FN = 110; and Scolomys sp., 2n = 62 and FN = 80.

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Supernumerários, ou cromossomos Bs, são elementos não-essencias do genoma. Apesar desta característica, já foram descritos em mais de 2000 espécies de eucariontes, sendo que na maioria destas sua função, composição molecular e origem permanecem desconhecidas. Dentre os mamíferos, a ordem Rodentia abrange 70% das espécies portadoras de Bs. Em Dicrostonyx torquatus, por exemplo, a quantidade de Bs pode variar de 0 a 42 Bs, enquanto em Holochilus brasiliensis varia de 0 a 2 Bs, nesta última um dos Bs representa o maior cromossomo do complemento. As duas principais hipóteses para o surgimento dos Bs em roedores apontam para os autossomos e/ou sexuais. A aplicação de diferentes técnicas de citogenética clássica e molecular foram essenciais para caracterizar a principal composição dos supernumerários, as sequências repetitivas. Os Bs de Nectomys squamipes, por exemplo, apresentam sequências teloméricas intersticiais, enquanto os Bs de Akodon montensis, Sooretamys angouya e Rattus rattus apresentam Regiões Organizadoras de Nucléolos, com genes ativos ou não. Outra etapa importante para elucidar a origem destes elementos foi o isolamento e posterior hibridação dos Bs em metáfases da mesma espécie ou de espécies relacionadas. A hibridação dos supernumerários de H. brasiliensis em metáfases de 15 espécies da tribo Oryzomiyni, mostrou sinais de hibridação nas regiões pericentroméricas e intersticiais dos cromossomos X e Y de 12 destas espécies. Apesar de cincos destas espécies apresentarem Bs, somente em três foi detectado sinal de hibridação nos Bs, indicando que mesmo em espécies relacionadas os supernumerários tiveram diferentes origens. Outro resultado similar foi encontrado na espécie Apodemus flavicollis, onde a hibridação dos Bs em metafases da mesma espécies apresentou sinais de hibridação nas regiões pericentroméricas dos sexuais e de dois pares de autossomos. Contudo, a origem e função dos Bs ainda é pouco compreendida, estudos envolvendo técnicas mais avançadas, como o sequênciamento genômico, podem vir a esclarecer melhor os enígmas envolvendo estes elementos.