Molecular Evidence of Introgression Between Water Frog Species (Anura: Telmatobiidae) in the High Andes

Contact zones are classical natural laboratories to study speciation. Several evolutionary outcomes are expected from hybridisation depending on the degree of reproductive isolation, ranging from the formation of hybrid swarms in early stages of speciation to the reinforcement of species differences when speciation is complete. The genus Telmatobius is a high Andean, diverse, but poorly known group of neotropical frogs. Although hybridisation between diverged species is frequent in amphibians, it has not been observed in this group. Here, we studied hybridisation processes among three neighbouring Telmatobius species that inhabit different altitudes in the same desertic basin using nuDNA SNP and mitochondrial DNA data. The results suggest that the Chilean population of Telmatobius peruvianus has hybridised in different degrees with the species T. pefauri and T. marmoratus. We detected mitochondrial and nuclear introgression from T. peruvianus to T. pefauri and what appears to be historical introgression between T. marmoratus and T. peruvianus. No first-generation hybrids nor parental genotypes were detected in admixed localities, suggesting that the inferred hybridisation processes did not occur recently and several generations of backcrossing have passed in geographical isolation. Instead of representing stable hybrid zones, hybrid populations show a degree of genetic differentiation from parental populations. In this group of amphibians, nearly 2 million years of allopatric divergence have not been enough to develop reproductive isolation between diverged species.