LAUSR

Abstract Oligosarcus is a small genus of fish of the Characidae family and currently has 24 species widely distributed in Brazilian hydrographic systems. Cytogenetic studies were carried out in a few species of the genus, most of them from the Paraná river basin, as is the case of O. paranensis that exhibited interpopulational variations. In the present study, two populations of O. paranensis were analyzed cytogenetically in order to verify if the geographic distance between them generated some karyotypic alteration. Both populations exhibited 50 chromosomes and only two 5S rDNA sites. A marked variability in karyotype formulas, distribution of heterochromatic regions and 18S rDNA patterns were observed and their origin reflects the participation of different chromosomal rearrangements, which are considered the major events involved in the chromosome evolution of this group of fish.