Accentuated polymorphism of heterochromatin and nucleolar organizer regions in Astyanax scabripinnis (Pisces, Characidae): tools for understanding karyotypic evolution

Astyanax scabripinnis has been considered a species complex because it pres ents high karyotypic and morphological variability among its populations. I n this work, individuals of two A. scabripinnis populations from different streams in the same hydrographic basin were analyzed through C-banding and AgNOR. Although they present distinct diploid numbers, they show meta and s ubmetacentric chromosome groups highly conserved (numerically and morpholog ically). Other chromosomal characteristics are also shared by both populati ons, as the pattern of constitutive heterochromatin distribution (large blo cks in the telomeric regions of subtelocentric and acrocentric chromosomes) and some nucleolar chromosomes. Inter-individual variations both in the nu mber and size of heterochromatic blocks, and in the number and localization of NORs were verified in the studied populations, characterizing them as p olymorphics for these regions. The mechanisms involved in the dispersion of heterochromatin and NORs through the karyotypes, as well as the possible e vents related to the generation of polymorphism of those regions are discus sed. Furthermore, relationships between these populations and within the co ntext of the scabripinnis complex are also approached.