Abstract-Fifty three species (48 species valid) belonging to the Pimelodidae, Heptapteridae and Pseudopimelodidae families have been studied cytogenetically, and in the present paper the chromosome number, the presence of B chromosomes and other relevant data were examined. The diploid number varies from 2n=42 to 2n=58 and were detected three cases of triploids with 3n=87 in Rhamdia species. The karyotypes show a high fundamental number because there are constituted predominantly by meta/submetacentric chromosomes.

Conventional karyotype and nucleolar organizer regions (NORs) of C. macropomum x P. brachypomus hybrids and parental species are reported. A modal diploid number of 54 chromosomes and a fundamental number (NF) of 108 were found for C. macropomum, P. brachypomus and their hybrids. P. brachypomus shows a pair of silver stained chromosomes, while cells with three and four Ag-NOR bearing chromosomes were observed in C. macropomum. The hybrids consistently presented cells with a single metacentric Ag-NOR bearing chromosome and cells with three Ag-NOR bearing chromosomes. The FISH technique was employed to localize 18S rDNA in the chromosomes of the parentals and the hybrids. In P. brachypomus the FITC signals appeared in the SM pair as when stained with silver salts. In C. macropomum the signals were evidenced in six chromosomes. In the hybrids, as expected, the FITC dots were observed in four chromosomes. All the techniques employed in the present work represent good tools to identify the parentals and distinguish them of the hybrids.

Only 33 species among about 300 belonging to the families Pimelodidae and Rhamdiidae have been studied cytogenetically. The diploid number varies from 2n = 46 to 2n = 63 chromosomes, with the karyotypes often being of the meta/submetacentric type. As a result, there is generally a very elevated fundamental number.

-Specimens of Pinirampus pirinampu were analyzed cytogenetically. Fifteen individuals were obtained from the Tibagi river near Sertaneja, PR, Brazil. A karyotypic struc ture consisting of 50 chromosomes distributed as 26M+12SM+2ST+10A was observed. The nucleolar organizing regions ( NORs) were identified on the short arm of a pair of subtelocentric chromosomes. A variation in size of the NOR regions was observed among the paired chromosomes. Chromomycin (CMA 3 ) staining established not only the nucleolar chromosome pair, but also fluorescent marking in the telomeric and centromeric regions of other chromosomes which seem to correspond to the distribution patterns of the constitutive heterochromatin. Restriction enzyme Alu I was employed and the reaction pattern obtained also corresponded to the heterochromatin constitutive distribution.

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