Karyotypic characterization of two species of the genus Serrapinnus (Characiformes, Characidae), with the description of a structural polymorphism in S. heterodon

The family Characidae, one of the largest and more diversified fish families in the Neotropical region, is certainly not a monophyletic group, which is supported by the cytogenetic data showing its extensive chromosome variability. In this work, two species of the genus Serrapinnus, S. heterodon and S. piaba, from the São Francisco river basin were analyzed. Both species presented the same diploid number, 2n=52 chromosomes, and the same karyotypic formula, 16m+20sm+14st+2a. The heterochromatin distribution, evidenced by C-banding, was also similar between both species. On the other hand, two NORs were evidenced in S. heterodon, while a multiple NOR system was characterized in S. piaba, both confirmed by silver nitrate staining as well as by fluorescent in situ hybridization with 18S rDNA probe. Furthermore, two additional karyotypes (cytotypes) were observed in S. heterodon, with 15m+20sm+14st+3a and 17m+20sm+14st+1a chromosomes, respectively, characterizing a structural chromosome polymorphism in the population. The differences between the three S. heterodon karyotypic forms are probably due to pericentric inversions related to the 8th and 26th chromosome pairs.