The Ancistrus genus differs from other Ancistrini due to its wide karyotypic diversity, varied diploid numbers, differences in sex chromosomes, and large number of species, as well as its tendency to form small populations with low vagility. This study investigated the role of 5S and 18S rDNA and telomeric repetitive sequences in the evolution of the karyotypic macrostructure of seven species of the genus Ancistrus from the Central Amazon. The results indicate a strong correlation between the location of ribosomal sites and fragile sites in the genome, particularly of 5S rDNA sequences, which are associated, in some species, with telomeric sequences at the sites of chromosomal healing. Moreover, the occurrence of two lineages was observed with regard to the synteny of ribosomal genes. The species of the genus Ancistrus showed high chromosomal lability associated with breakpoints, which was characterized by the presence of repetitive DNA sequences and this process is suggested to be an evolutionary model for the rapid fixation of structural rearrangements.

Accessing the taxonomy of fish is a basic condition for the conservation of species, and the understanding of their evolutionary relationships. Many Neotropical species of fishes of the order Characiformes are considered incertae sedis because of a lack of consistent evidence of monophyly, with the genus Astyanax among them. With a cytotaxonomic and evolutionary purpose, we compared the karyotype of two species of Astyanax: Astyanax paranae and Astyanax elachylepis. Conventional cytogenetic analyses revealed 2n = 50 chromosomes for the two species, with interspecific and interpopulation karyotype variations. Heterochromatin was primarily localized in the pericentromeric regions, and it was associated with organized regions of the nucleolus (NORs) and/or telomeric regions in some acrocentric chromosomes. The in situ localization of 18S ribosomal DNA (rDNA) and 5S probes varied among A. paranae populations. In A. elachylepis, only one chromosome pair was stained by the 18S probe and another by the 5S probe, confirming a simple NORs pattern. These results support the utility of chromosome markers in Astyanax taxonomy and reveal the conserved characteristics of the karyotype of A. elachylepis, consisting exclusively of two chromosome arms, and the occurrence of simple homologous and non-syntenic sites for 18S and 5S rDNA in this species.

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