Environmental transition zone and rivers shape intraspecific population structure and genetic diversity of an Amazonian rain forest tree frog

Diversification processes acting across geographically continuous populations have been rarely documented in Amazonia, because of the lack of fine-scale sampling over extensive areas. We aimed to determine the geographic effects of an environmental transition zone and large rivers on the intraspecific population structure of the Manaus slender-legged tree frog (Osteocephalus taurinus) along a ~ 900 km transect of tropical rain forest. Using one mitochondrial (16S), two nuclear genes (TYR, POMC) and three microsatellites, we estimated the population structure, phylogenetic relationships and geographic variation of 262 O. taurinus and 5 O. oophagus (a close relative) along the Purus–Madeira interfluve (PMI) and opposite banks of the central Amazon and upper Madeira rivers, at central-southern Amazonia, Brazil. Six genetic clusters were identified: two corresponding to sympatric populations of O. taurinus and O. oophagus from their type locality, north of Amazon river. Within PMI, there were three distinct O. taurinus genetic clusters distributed along the geographic gradient with one main phylogeographic break found (concordant between 16S and TYR), that corresponds to a transition zone (ecotone) between dense and open rain forest ecotypes. The sixth cluster was an O. taurinus population isolated at the east bank of the upper Madeira river. In addition, restricted haplotype sharing was identified from the west to east banks at upper Madeira river. Within PMI, parapatric genetic structure is explained by a potential association of the genetic clusters to the different forest ecotypes they inhabit coupled with isolation by distance, thus supporting the gradient hypothesis for diversification. Differentiation of populations that are external to the PMI is most likely explained by the barrier effect of the Madeira and Amazon rivers. Our findings provide new evidence on diversification processes across continuous Amazonian landscapes; however, the specific mechanisms underlying the origin and maintenance of the identified phylogeographic break need to be further studied.