Four populations of Astyanax hastatus Myers 1928 from the Guapimirim River basin (Rio de Janeiro State) were analyzed and three distinct cytotypes identified. These cytotypes presented 2n = 50 chromosomes, with 4M+8SM+10ST+28A (Cytotype A), 8M+10SM+14ST+18A (Cytotype B), 6M+8SM+4ST+32A (Cytotype C) and scanty heterochromatin, mainly located throughout pericentromeric regions of several chromosomal pairs. No homologies with the As-51 satellite DNA were observed in the three cytotypes, although all of them presented multiple 18S rDNA sites, as detected by both silver nitrate staining and FISH (fluorescent in situ hybridization). The application of the term "species complex" in Astyanax is discussed from a cytotaxonomic viewpoint.

Cytogenetic data about satellite DNA distribution in four Astyanax species (Characidae) from the Paraitinga river, Paraíba do Sul river basin, Brazil, are presented. In order to characterize the constitutive heterochromatin, C-banding, chromomycin A3 and DAPI fluorescence staining, as well as fluorescence in situ hybridization (FISH) with the satellite DNA As-51 probe were performed. A. scabripinnis and A. parahybae presented 2n = 50 and 2n = 48 chromosomes, respectively. The heterochromatin was located in the pericentromeric and terminal regions of many chromosomes, corresponding to GC-positive regions and to the As-51 satellite DNA in terminal regions. A. intermedius and A. giton, both with 2n = 50 chromosomes, showed little heterochromatin, mostly restricted to the terminal and pericentromeric regions of a few chromosomes. No GC-positive regions, neither any correspondence between the scarce heterochromatin of these species and the As-51 satellite DNA was observed. AT-positive blocks were not detected in any of the species studied. Based on these and other available data, the hypothesis that Astyanax represents a polyphyletic group is discussed.

Hoplias malabaricus (traíra) é uma espécie de peixe com ampla distribuição pela região Neotropical e alta diversidade cariotípica interpopulacional (citótipos), que podem corresponder a um complexo de espécies. Apesar da sua ampla distribuição pelas bacias hidrográficas da América do Sul, alguns autores têm questionado sua origem natural no rio Iguaçu, uma importante bacia hidrográfica brasileira que é caracterizada pela presença de várias espécies de peixes endêmicas. Assim, nós temos analisado o cariótipo de H. malabaricus de diferentes locais de coleta neste rio por métodos convencionais e bandamentos cromossômicos. Nossos resultados, em adição a dados prévios a respeito da distribuição geográfica dos citótipos contribuem para um melhor entendimento da origem de H. malabaricus no rio Iguaçu, reforçando a proposta que esta é uma espécie de peixe natural desta bacia hidrográfica.

Hoplias malabaricus (trahyra) is a widespread fish species over the Neotropical region with diversified inter-populational karyotypes (cytotypes), which may correspond to a species complex. Despite the wide distribution in the South American basins, some authors have questioned its natural origin in the Iguaçu river, an important Brazilian river basin which is characterized by several endemic fish species. We have analyzed the karyotype of H. malabaricus from different collection sites of this river, by conventional and banding methods. Our results, in addition to our previous data concerning geographic distribution of the cytotypes, contribute to better understand the origin of H. malabaricus on the Iguaçu river, reinforcing the proposition that it is a natural fish species in this river basin.

Silver nitrate chromosome staining to evidence nucleolar organizer regions (NORs) is a widely adopted methodology. The aim of the present work was to improve this technique, reducing the preparation time without decreasing the quality of the results. Microwave irradiation proved to be quite efficient and reliable for this purpose, as it allowed to identify Ag-NORs equivalent to those obtained by the conventional procedure and also to reduce the concentration of the employed reagents, as well as the precipitation of debris on the preparation.