Abstract
Cytogenetical studies were carried out on 13 specimens of Rhamdia sp. from Grande stream (Paraiba do Sul river basin, Mantiqueira hills, Brazil). It was evidenced a standard diploid number 2n=58 chromosomes (46M/SM and 12ST), plus 1 to 4 B chromosomes, found in 10 specimens. One individual presented natural triploidy, lacking B chromosomes. Fluorescent in situ hybridization with 5S and 18S rDNA probes was employed to improve the conventional karyotypical analyses. Inferences about the presence of supernumerary chromosomes within the genus Rhamdia, based on the karyotypical evolutionary pattern of this group, as well as the reoccurrence of natural triploidy in fish at Mantiqueira hills are discussed.
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