Karyoevolutionary Inferences in Enigmatic Taxon Astyanax (Teleostei: Characidae).

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.

Cytogenetic analysis in the neotropical fish Astyanax goyacensis Eigenmann, 1908 (Characidae, incertae sedis): karyotype description and occurrence of B microchromosomes.

BACKGROUND: B chromosomes, also known as supernumerary or accessory chromosomes, are additional chromosomes over the standard complement found in various groups of plants and animals. We investigated the presence of, and characterized, supernumerary microchromosomes in Astyanax goyacensis using classical and molecular cytogenetic methods. FINDINGS: Three specimens possessed 2n = 50 chromosomes (8m + 26sm + 8st + 8a), and two specimens contained 1 to 9 additional B microchromosomes varying intra- and inter-individually. Chromosome painting with a B chromosome-specific probe yielded signals for several B microchromosomes, with one exhibiting no markings. Acrocentric chromosomes of the standard complement were also painted. Fluorescence in situ hybridization (FISH) using ribosomal probes located two chromosome pairs carrying 18S rDNA marked on the short arm, and one pair carrying 5S rDNA with pericentromeric markings. One chromosome was observed in synteny with 18S cistrons. CONCLUSION: These data contribute to knowledge of the karyotype evolution, the origin of B chromosomes, and to an understanding of the functionality of rDNA.