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Genetic diversity and structure of Pseudoplatystoma corruscans and P. reticulatum, large migratory South America catfishes, where overfishing and the construction of numerous dams in their feeding and reproducing areas are affecting their migratory processes negatively, were studied using microsatellites in samples from Paraguay (that comprises the Pantanal biome), and the upper and lower Paraná Basins. Genetic diversity was in accordance to that observed for other large migratory fishes, but the most geographically isolated populations of P. reticulatum and those P. corruscans subject to anthropogenic effects (stocking and dams) showed lower genetic diversity and evidences of bottlenecks compatible with low effective population size. Pseudoplatystoma reticulatum presented subtle genetic differentiation within the Paraguay area, especially between the edges of its distribution. Pseudoplatystoma corruscans, in this same area, presented a quite homogeneous but significant genetic break between the Paraguay and upper Paraná populations, apparently resulting from natural and historical isolation between the basins until recently. These data demonstrates that, although these Pseudoplatystoma spp. are abundant in the Pantanal area, anthropogenic events are leading to negative effects on their populations, particularly in the upper Paraná Basin. Genetic differentiation observed along each species distribution demands conservation actions to preserve each population's biodiversity. These results represent important genetic information using new microsatellite markers and the first genetic study of P. reticulatum covering this area of its native distribution. Data may also contribute to a better understanding of species migration patterns and to be used as a baseline for proper management., (© 2017 The Fisheries Society of the British Isles.)

The North American channel catfish Ictalurus punctatus Rafinesque 1818 is cultivated in the United States, Asia and Brazilian fish farms, and also utilized as a model species in aquaculture and genetic studies. In this work, cytogenetic analysis of . punctatus from Brazilian aquaculture revealed for the first time the presence of extra elements (supernumerary or B chromosomes) in this species. These elements were characterized as dot-like micro B chromosomes and were found in three individuals (varying from 0 to 1) and in one individual with higher incidence per cell (varying from 0 to 5; mean number of Bs per cell = 2.01). More specific cytogenetic techniques in this individual revealed 58 A chromosomes (standard complement) containing heterochromatic bands in the centromeric regions, a single Ag-NOR in a subtelocentric pair (also positive for 18S rDNA using the FISH technique) and multiple 5S rDNA clusters in three different subtelocentric chromosomes. Four B chromosomes were entirely Ag-NOR positive (also fully heterochromatic) and three presented 18S rDNA clusters by FISH. The occurrence of Ag-NOR and 18S ribosomal genes in both A and B chromosome complements may indicate an intraspecific origin for these extra chromosomes. Additionally, the terminal location of 18S ribosomal clusters in the Ag-NOR-bearing chromosomes and the presence of active NOR in the B chromosomes suggested that breakage events may be related to a possible recent origin of these extra elements. We suggest this data may be useful as cytogenetic information for future elucidation of the composition, origin and evolution of extra chromosomes in fishes.