Characidae is one of the largest fish families of the Neotropical region, and presenting a pronounced morphological variability, certainly does not constitute a monophyletic group. The cytogenetical data also show a large chromosomal variation and can provide important information for a better understanding of the relationships between the species of this group. 18S and 5S rDNA probes were used in the present study for the chromosomal mapping in different Characidae species from the São Francisco River (Astyanax lacustris, Astyanax scabripinnis, Hasemania nana, Piabina argentea, Orthospinus franciscensis, Serrapinnus heterodon, Serrapinnus piaba and Myleus micans) and Alto Paraná (Astyanax altiparanae) basins. Species with a single pair of chromosomes bearing the nucleolar organizing regions (NORs) were identified, as well as species with multiple NORs, up to a maximum of seven 18S rDNA sites. The number of 5S rDNA site was also not constant, varying from two to eight. The mapping of the ribosomal genes was useful for the characterization and differentiation of the analyzed species.

Constitutive heterochromatin makes up a substantial portion of the genome of eukaryotes and is composed mainly of satellite DNA repeating sequences in tandem. Some satellite DNAs may have been derived from transposable elements. These repetitive sequences represent a highly dynamic component of rapid evolution in genomes. Among the genus Astyanax, the As51 satellite DNA is found in species that have large distal heterochromatic blocks, which may be considered as derived from a transposable DNA element. In the present study, As51 satellite DNA was mapped through in situ fluorescent hybridization in the chromosomes of five species of the genus. The possible roles of this type of saltatory DNA type in the genome of the species are discussed, along with its use for the phylogenetic grouping of the genus Astyanax, together with other shared chromosomal characters. However, the number of As51 clusters is presented as a homoplastic characteristic, thereby indicating evident genomic diversification of species with this type of DNA.

Specimens of Imparfinis schubarti (Gomes, 1956) collected in the Piumhi river drainage, state of Minas Gerais, Brazil, were studied cytogenetically. The river was diverted from the Rio Grande Basin into the São Francisco basin in the early 1960s. All individuals presented 2n = 58 chromosomes, including 18 metacentric, 34 submetacentric and six subtelocentric chromosomes. A secondary constriction was observed in the interstitial region of the long arm of the largest chromosome pair, coinciding with the NOR. A single conspicuous heterochromatic block located in the largest pair of metacentric chromosomes was observed, adjacent to the secondary constriction. A detectable 18S rDNA probe hybridization region occurs in only one chromosome pair and is synthenic with the marking obtained with 5S rDNA probe. These results fit the cytogenetic pattern previously described for the genus Imparfinis Eigenmann & Norris, 1900

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In the 1960s, as a part of the construction of the Furnas Hydroelectric Power Dam, Minas Gerais, Brazil, the Rio Piumhi was diverted from the Rio Grande drainage to the São Francisco River basin, with the resulting introduction of species from one basin to the other. Chromosomal characteristics of various populations of Astyanax fasciatus sensu Eigenmann from the transect region were studied using cytogenetic techniques, with the goal to identify and map the dispersal of invasive species in the Rio São Francisco. Populations of the Rio Grande and Rio Piumhi are distinct from those of the São Francisco basin based on chromosome markers associated to the heterochromatin, Ag-NORs, 18S rDNA, 5S rDNA, and As-51 satellite DNA sites. Individuals with karyomorph originally present in the Piumhi drainage are now present in the São Francisco basin, downstream from the transposition channel. This expansion of the distribution corroborates a hypothesis of progressive substitution of native populations by the invasive Piumhi form.