Abstract
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 Pimelodusblochii 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.
Highlights
The genus Pimelodus LaCépède, 1803 (Siluriformes, Pimelodidae) comprises 35 valid species exclusively distributed in neotropical freshwater drainages
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
We investigate the karyotype of Pimelodus blochii from the lower portions of the Tapajós, Amazonas and Trombetas Rivers in order to evaluate their chromosomal features and contribute the debate on the species taxonomy
Summary
The genus Pimelodus LaCépède, 1803 (Siluriformes, Pimelodidae) comprises 35 valid species exclusively distributed in neotropical freshwater drainages. It is commonly recorded in the Amazonas, Orinoco, Araguaia-Tocantins, São Francisco, and Paraná-Paraguay River basins. Diploid chromosome number variations were reported in Pimelodus fur Lütken, 1874 and Megalonema platanum Günther, 1880 species samples with 2n=54 and in Pimelodus blochii (Della-Rosa et al 1980) species samples with 2n=58 (Sánchez et al 2000; Garcia and Moreira-Filho 2008; Carvalho et al 2011). Diploid chromosome numbers of eleven previously investigated Pimelodus species showed variation from 54 to 58 with eleven distinct karyotype formula (see Table 2 in the Discussion section). Karyotype variation with B chromosomes had been reported in P. ortmanni and Pimelodus sp. (Borin and Santos 2004)
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