Allopatric divergence in Astyanax aff. fasciatus Cuvier, 1819 (Characidae, Incertae sedis) inferred from DNA mapping and chromosomes

The fish genus Astyanax represents a suitable model to test the role of chromosomal changes in population structure and speciation. Therefore, we performed a detailed karyotypic analysis of Astyanax aff. fasciatus from rivers in South America with incorporation of multivariate analyses as an innovate approach to infer chromosomal diversity. Ten populations of Astyanax aff. fasciatus from distinct coastal basins in northeastern Brazil were cytogenetically analyzed using Giemsa, Ag-NOR and fluorochrome staining, C-banding, mapping of rDNA and a genus-specific satellite DNA (As51). A multivariate analysis of chromosomal data was carried out to provide a consensus dendrogram of intraspecifical relationships. In spite of sharing the same diploid number, similar C-banding patterns, the populations varied in karyotype formulae and location of 18S and 5S rDNA cistrons, GC-rich segments, and As51 satDNA. The multivariate analysis showed CG-specific markers are poorly correlated to 18S rDNA. The dendrogram revealed that intraspecifical chromosomal differences are not explained by geographic distance. Instead, this variation seems to be related to the biogeographic history of northeastern Brazil and/or to the presence of unique evolutionary units under incipient speciation. Finally, we strongly recommend the incorporation of statistical analysis in cytogenetic studies of fish, particularly in groups of remarkable intraspecifical diversity as Astyanax. Such studies are useful because they provide a reliable interpretation of chromosomal variation.