RESUMO
Podocnemis erythrocephala, the Red-headed Amazon River Turtle, is distributed in the Amazon and Orinoco basins where it predominantly inhabits blackwater and has never been found in whitewater. The only permanent river connection between the habitats in the different river basins features considerable whitewater proportions and, therefore, is hypothesised to be a dispersal barrier for this turtle species. By using variable neutral nuclear and mitochondrial markers (microsatellite loci and control region sequences), the present study assessed the genetic structure, genetic diversity, gene flow and historical biogeography of P. erythrocephala populations in Venezuela and Colombia in the Brazo Casiquiare (Amazon basin) and in the Ríos Atabapo and Inírida (Orinoco basin). The results clearly indicate a pronounced genetic differentiation between the Amazon basin populations and the Orinoco basin populations suggesting that a vicariant event separated the ancestors of the Orinoco basin populations. Several identified private mitochondrial DNA haplotypes and microsatellite alleles as well as a high genetic diversity observed in the Orinoco basin populations suggest that the Orinoco basin represents a historical blackwater refugium of P. erythrocephala. Analyses of past demographic processes revealed old bottlenecks and recent expansion. Historical and contemporary non-directional gene flow between the populations in the different river basins were identified and are supposed to have taken place via ephemeral inter-basin blackwater connections that might have formed at various locations, one of them being located on the famous travelling route of Alexander von Humboldt. The P. erythrocephala populations in the Ríos Atabapo and Inírida should together be considered as an evolutionarily significant unit, and each of them should be classified as an independent management unit for conservation.