New Insights into Karyotypic Relationships Among Populations of Astyanax bockmanni (Teleostei, Characiformes) of Different Watersheds

Abstract

The fish constitute about 50% of all vertebrates, including a wide morphological and biological diversity, where the genus Astyanax is the most common and diverse, as described in virtually all freshwater environments. By occupying a basal position in the phylogeny of vertebrates, fish are an extremely favorable group for cytogenetic and evolutionary studies. The karyotype found in genus Astyanax diversity may involve a number of polymorphisms, which may be related to ploidy and karyotypic macrostructure, presence of B chromosomes, heterochromatin polymorphisms, and location of ribosomal genes. Nevertheless, the relationship between populations of this species is still poorly studied. Thus, the present work aimed to investigate karyotype variation, chromosomal relationships, and the behavior of 5S and 18S ribosomal genes in six populations of Astyanax bockmanni. The results confirmed the diploid number of 50 chromosomes in all the populations sampled, with the occurrence of one supernumerary chromosome in just one of them. In addition, all populations showed divergent patterns of constitutive heterochromatin and repetitive nucleolar sites. The fluorescence in situ hybridization (FISH) technique using 5S and 18S rDNA probes revealed distinct patterns of distribution for these conserved genes, while 5S rDNA genes were found located in two chromosome pairs, the 18S genes showed multiple marks dispersed in the genome characterizing an inter and intraindividual polymorphic behavior, as previously reported to occur with the utilization of the Ag-NOR technique. Thus, besides minor modifications observed in chromosome morphology, the populations of A. bockmanni analyzed revealed a preserved macrostructural feature, especially concerning to the diploid number; on the other hand, differences in microstructural characteristics indicated by the nucleolus organizer region (NOR) location, constitutive heterochromatin patterns, and distribution of ribosomal genes along the genome were clearly evident in the populations from different river basins, even located at short distances.