Model-based riverscape genetics: disentangling the roles of local and connectivity factors in shaping spatial genetic patterns of two Amazonian turtles with different dispersal abilities

In fluvial systems, the process of isolation by distance is often strong and can mask the importance of barriers, landscape resistance and local environmental factors on shaping genetic patterns. By comparing two Amazonian river turtle species with distinct dispersal abilities, we assessed how differently and which local and connectivity variables influence, respectively, their genetic diversity and differentiation. With broad sampling throughout their distribution in the Amazon basin, we estimated genetic diversity and differentiation for 14 localities totaling 273 samples of Podocnemis erythrocephala and 20 localities totaling 336 samples of P. sextuberculata. We applied model selection on models associating genetic diversity to local variables representing hypothesis of climate and productivity, instability of interannual water levels, hunting pressure and downstream increase in genetic diversity. We used General Dissimilarity Modelling to model the relationship of genetic differentiation with connectivity variables representing hypothesis of isolation by distance (IBD), isolation by resistance (IBR) and isolation by barrier (IBB). Local variables were more important in explaining genetic diversity of the high-dispersal species (P. sextuberculata) than of P. erythrocephala, with best models including productivity, distance from downstream locality, density of human villages and historical climatic suitability. Connectivity factors in general were not important in explaining genetic differentiation turnover for either species, but GDM models explained a larger amount of deviance for the low-dispersal species, P. erythrocephala. Also, IBB and IBR models explained more genetic differentiation turnover than IBD. We showed that, although local variables are often overlooked in Landscape/Riverscape Genetics studies, they can influence intrapopulacional genetic diversity of aquatic species, even those with high dispersal ability. By applying a novel resistance-model framework in Riverscape Genetics and by using riverscape factors relevant in Amazonian context, we provide an approach to study the roles of local and connectivity variables in shaping genetic patterns of aquatic vertebrates in fluvial systems.