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Abstract
Specimens of Apareiodon affinis collected from four sites of the Paraná River (called Lower Paraná, Argentina) were analyzed. This species presents a diploid number of 54 chromosomes, without any apparent sex chromosomal heteromorphism. However, it was possible to verify the occurrence of 10 cytotypes, some of them common to all collected sites, characterizing an structural chromosomal polymorphism. Data obtained from chromosomal banding methods, results further suggest an inter-individual NOR polymorphism. Such variation is possibly caused by pericentric inversions and other chromosomal rearrangements. The 5S rDNA was separated from 18S rDNA clusters, these cistrons seems to present variation, suggesting that other chromosomes could also be involved in the polymorphism. The results obtained confirmed the structural polymorphism in A. affinis.
