In this study, we used a portion of mitochondrial cytochrome b to evaluate the current genetic structure of the Hypophthalmus marginatus Valenciennes 1840, from the Tocantins River in the eastern Amazonia. Genetic diversity was measured in two downstream (Abaetetuba and Cametá), and two upstream (Tucuruí and Itupiranga) stocks under influence of the Tucuruí Hydroelectric Dam. Additionally, one stock from the Araguaia River system was included in the analysis. Our findings provide evidence for the existence of reduced levels of genetic diversity in H. marginatus of the Tocantis basin. The pattern of distribution of haplotypes observed in the present study appears to reflect the migratory characteristics of H. marginatus (short distance migratory), suggesting gene flow mainly in the upstream to downstream. Alternatively, presence of rare sequences only in Cametá and Abaetetuba may result from recent mutations, since they differ from connecting haplotypes by only one nucleotide substitution.
Key words: Hydroelectric dam impact, Hypophthalmus marginatus, population genetic structure.
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