Cytogenetic studies were performed in Serrasalmus spilopleura, a piranha species, from three sampling sites in the Central Amazon basin: 1. Catalão; 2. Manacapuru, 3. Itacoatiara. All specimens displayed 2n = 60 chromosomes and no sexual chromosomal heteromorphism was found. However, by considering the karyotypic formula, three cytotypes were identifyed: cytotype A consisted of 24M+20SM+4ST+12A, and was the most common being found in all three sampling sites; cytotype C consisted of 23M+21SM+4ST+12A, was found only in Catalão and differ from cytotype A by the presence of a heteroziguous inversion in the first chromosome pair and a distal heterocromatinization in the long arm of this same pair, probably caused by the rearrangement or due to a chromosomal adjustment; cytotype D consisted of 24M+20SM+4ST+12A, was found only in Manacapuru and differ from cytoytpe A by a heteroziguous size polymorphism on the second chromosome pair, probably due to a translocation. C-banding revealed the same constitutive heterochromatin pattern for the three cytotypes except for the chromosome pair that underwent pericentric inversion (cytotype C) which possessed different heterochromatic blocks. Analysis in the number of active nucleolar organizing regions (NORs) revealed no differentiationamong cytotypes and NOR-bearing chromosomes varied from 10–12 acrocentric chromosomes. © 2002 Taylor & Francis Group, LLC.
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