Natural hybridization between allopatric species following secondary contact has been poorly documented for Neotropical anurans inhabiting the Amazonian lowlands. We conducted a genetic survey across a contact zone between two species of litter frogs, Allobates hodli and Allobates femoralis (family Dendrobatidae), located on the left riverbank of the upper Madeira River, State of Rondônia, Brazil. We obtained tissue samples from 11 sampling sites on both riverbanks, covering approximately a 400 km long transect. We evaluated the genetic relationships between samples using haplotype networks and a distance-based phylogenetic tree obtained from a dataset of 16S rRNA mtDNA sequences. Estimates of genetic diversity, population structure, and the identification of sites where genetic admixture occurred were carried out by means of frequency-based methods and Bayesian inference on mtDNA and a set of four microsatellite loci, including samples collected throughout the study area. A reduced dataset including only microsatellite loci genotyped from samples on the left riverbank was applied in assignment tests for detecting levels of admixture at the contact zone and adjacent sampling sites, and for detecting and quantifying hybrid individuals. Our results suggest that genetic introgression between A. hodli and A. femoralis is restricted to the core area of the contact zone, where potential hybrids are less frequent than parental genotypes. Effects on the genetic variability of adjacent populations are only detected at sites located 1. 5 km downstream and upstream of the core area, suggesting the existence of selection against hybrids, possibly mediated by postzygotic isolation mechanisms. The contact zone between A. femoralis and A. hodli is the first well delimited suture line between anuran species ever documented in the Brazilian Amazon. The settlement of two dams along the upper Madeira River poses an immediate threat to the gene flow and hybridization balance observed between the populations studied. Our results provided guidelines for a current monitoring program, aiming at the impacts of dams on this evolutionary system's dynamics. © 2012 Springer Science+Business Media B.V.
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