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The genus    Pimelodus    LaCépède, 1803 comprises 35 formally recognized species distributed along the major neotropical river basins. Despite conservatism in diploid number with 2n=56, an intense variation of chromosomal morphology (karyotypic formula) has been documented in    Pimelodus    species. In the present study, we analyzed karyotypes of 20 specimens, identified as    Pimelodus   blochii    Valenciennes, 1840 and collected from the lower courses of the Tapajós, Amazonas and Trombetas Rivers. The karyotypes were characterized by Giemsa conventional staining, C-banding, silver staining (Ag-NOR) and fluorescent in situ hybridization (FISH) with 5S and 18S rDNA probes. The karyotypes showed 2n=56 chromosomes in fish from the Tapajós River. In contrast, fish from the Amazonas and Trombetas Rivers had 2n=58. The nucleolus organizing regions were labeled on the short arm of an acrocentric chromosome as demonstrated by silver staining and FISH. Signals for 18S and 5S rDNA were co-localized on one chromosome pair. Our results demonstrate karyotypic divergence between Tapajós and Amazonas-Trombetas populations of    P.   blochii   , interpreted as supporting the existence of a species complex in this taxon.

comparative cytogenetics

Karyotypic variation in the long-whiskered catfish    Pimelodus   blochii    Valenciennes, 1840 (  Siluriformes  ,   Pimelodidae  ) from the lower Tapajós, Amazonas and Trombetas Rivers