Reaching the edge of the speciation continuum: Hybridization between three sympatric species of Hyla tree frogs

Abstract

Alloparapatric species meeting in secondary contact zones are evolutionary witnesses to how reproductive isolation progresses over time and space. Western Palearctic tree frogs of the genus Hyla are phenotypically similar and all the species pairs tested can hybridize and eventually admix at range margins, except one. The early-diverged Hyla meridionalis shows distinct phenotypic differences: a 'long' breeding call and the absence of a lateral stripe. In south-western Europe, this species co-occurs with the 'short-call' striped tree frogs H. arborea and H. molleri, two expanding lineages that admix at their parapatric margins. We estimated local gene flow between these three taxa at several syntopic breeding sites in western France. We congruently matched genotypes to phenotypes: the 'shortcall' striped individuals were a nuclear mixture of H. arborea and H. molleri; the 'long-call' stripeless individuals all featured pure H. meridionalis nuclear ancestry and mitochondrial DNA, confirming complete genetic isolation from H. arborea/molleri. Yet, we documented an F1 hybrid between a female H. arborea/molleri and a male H. meridionalis: an incompletely striped male with an intermediate breeding call. These findings suggest H. meridionalis is still able to reproduce with parapatric congeners despite 20 Myr of divergence and strong phenotypic differentiation, but that intrinsic incompatibilities (sterility) prevent genetic introgression.