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The Pseudopimelodidae family comprises 35 species however, cytogenetic studies have been performed in only six species. This study uncovered karyotypic data on Pseudopimelodus pulcher and Microglanis cottoides. Both species possessed 2n = 54, with 20m + 16sm + 10st + 8a and FN = 100 for P. pulcher and 30m + 14sm + 6st + 4a and FN = 104 for M. cottoides. A female of M. cottoides with 45m + 21sm + 9st + 6a (2n = 81) plus two extra small chromosomes was found, indicating a natural triploidy with supernumerary chromosomes. The formation of the polyploid individual seems to have come from a diploid female gamete, due to the presence of a marker chromosome pair partially heterochromatic presents only in females and common to that exemplar. This triploid female showed three chromosomes with nitrate staining (AgNOR), 18S rDNA probe and chromomycin A3 (CMA3) staining. AgNORs were observed on pairs 12 and 23 in P. pulcher and pair 24 in M. cottoides, results that were confirmed with an 18S rDNA probe and CMA3 fluorochrome. These are the first chromosomal data for P. pulcher and provide the first description of natural triploidy with the presence of supernumerary chromosomes in this family and emphasizing well the chromosomal rearrangements diversification between this species.

ABSTRACT. Five specimens of Rhamdia quelen collected from the Lindóia Stream, PR, Brazil, were cytogenetically analyzed. The diploid chromosome number found was 58, including 30 metacentric, 16 submetacentric, 10 subtelocentric, and 2 acrocentric chromosomes. Supernumerary or B chromosomes, frequently observed in this fish group, were not detected. One of the individuals was triploid, with 3n = 87. A silver-stained nucleolar organizer region was found on a pair of submetacentric chromosomes of the diploid specimens, and on three chromosomes of the triploid individual, confirming triploidy. Treatment with fluorochrome chromomycin A 3 revealed fluorescent bands coincident with those of the silver-stained nucleolar organizer region, in both diploid and triploid individuals, showing that this is a GC-rich region. Heterochromatin distribution was visualized by the C-banding technique, mainly in the terminal chromosome regions of the individuals and was also observed in the pericentromeric regions of some chromosomes and at both telomeres.

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