Large-Scale Hybridisation as an Extinction Threat to the Suweon Treefrog (Hylidae: Dryophytes suweonensis)

SIMPLE SUMMARY: A large number of amphibian species are now endangered, mostly because of human activities. An example is land modification, which may bring species that were previously isolated in contact, and allows them to hybridise. Here, we assessed the presence of hybrid individuals between the endangered Suweon treefrog (Dryophytes suweonensis) and the widespread Japanese treefrog (Dryophytes japonicus). We found hybrids to be relatively widespread and present at all populations where the Suweon treefrog occurred. This is important, as it results in an additional threat to the Suweon treefrog. ABSTRACT: Amphibians are in the midst of a sixth mass extinction, and human activities play a major role in pushing species towards extinction. Landscape anthropisation has impacts that indirectly threaten species, in addition to the obvious destruction of natural habitats. For instance, land modification may bring human-commensal species in contact with sister-clades from which they were previously isolated. The species in these new contact zones are then able to hybridise to the point of reaching lineage fusion, through which the gene pool of the two species merges and one of the parental lineages becomes extirpated. Here, we documented the patterns of hybridisation between the spatially restricted D. suweonensis and the widespread D. japonicus. On the basis of the analysis of Cytochrome c oxidase subunit I mitochondrial DNA sequences (404 individuals from 35 sites) and six polymorphic microsatellites (381 individuals from 34 sites), we revealed a generalised, bi-directional, and geographically widespread hybridisation between the two species. Evidence of fertile back-crosses is provided by relatively high numbers of individuals in cyto-nuclear disequilibrium, as well as the presence of hybrid individuals further south than the species distribution limit, determined on the basis of call properties. Hybridisation is an additional threat to the endangered D. suweonensis.