Karyotype differentiation and cytotaxonomic considerations in species of Serrasalmidae (Characiformes) from the Amazon basin

Six species of Serrasalmidae from the central Amazon, representatives of the genera Serrasalmus (S. elongatus, S. maculatus, S. cf. rhombeus, and S. rhombeus), Pygocentrus (P. nattereri), and Colossoma (C. macropomum), were analyzed regarding the distribution of the Ag-NORs, C-positive heterochromatin and 18S and 5S rRNA genes on the chromosomes. All specimens had 2n = 60 chromosomes, except S. cf. rhombeus, with 2n = 58, and C. macropomum with 2n = 54 chromosomes. The Ag-NORs were multiple and located on the short arms of subtelo-acrocentric chromosomes in all Serrasalmus species and in P. nattereri, but were found on metacentric chromosomes in C. macropomum. The 18S rDNA sites were usually coincident with Ag-NORs, although some species had a higher number and/or a distinct localization of these sites. C-positive heterochromatin was preferentially situated in centromeric regions, remarkably on metacentric pair number 7 in all Serrasalmus species and number 3 in P. nattereri, which beared a conspicuous proximal C-band on the long arms. The 5S rDNA sites were detected in a single chromosomal pair in all species. In Serrasalmus and P. nattereri, this pair was the number 7 and 3, respectively, thereby revealing its co-localization with the conspicuous heterochromatic band. However, in C. macropomum, only one homologue (probably belonging to pair number 12) exhibited 5S rDNA sites on the short arms, close to the centromere. The present data revealed reliable cytotaxonomic markers, enabling the evaluation of karyotype differentiation and interrelationships among Serrasalmidae, as well as the probable occurrence of a species complex in S. rhombeus. Seis especies de Serrasalmidae da Amazonia central, representantes dos generos Serrasalmus (S. elongatus, S. maculatus, S. cf. rhombeus e S. rhombeus ), Pygocentrus (P. nattereri) e Colossoma (C. macropomum), foram analisadas quanto a distribuicao das Ag-RONs, heterocromatina C-positiva e dos genes de RNAr 18S e 5S nos cromossomos. Todos os especimes apresentaram 2n = 60 cromossomos, exceto S. cf. rhombeus, com 2n = 58, e C. macropomum com 2n = 54 cromossomos. As Ag-RONs foram multiplas e localizadas nos bracos curtos de cromossomos subtelo-acrocentricos em todas as especies de Serrasalmus e em P. nattereri, mas foram encontrados em cromossomos metacentricos em C. macropomum. Os sitios de DNAr 18S, foram geralmente coincidentes com as Ag-RONs, embora algumas especies tenham apresentado um maior numero e/ou uma localizacao distinta desses sitios. A heterocromatina C-positiva foi preferencialmente encontrada como uma conspicua banda proximal no par metacentrico numero 7 em todas as especies de Serrasalmus e numero 3 em P. nattereri. Os sitios de DNAr 5S foram detectados em um unico par cromossomico nas seis especies sendo que nas especies de Serrasalmus, este par foi o de numero 7 e em P. nattereri o de numero 3, colocalizados com bandas heterocromaticas conspicuas. No entanto, em C. macropomum, apenas um homologo (provavelmente pertencente ao par numero 12) apresentou sitios de DNAr 5S nos bracos curtos, proximos ao centromero. Os dados apresentados revelaram confiaveis marcadores citotaxonomicos, permitindo a avaliacao da diferenciacao cariotipica, e as inter-relacoes entre Serrasalmidae, bem como a provavel ocorrencia de um complexo de especies em S. rhombeus.