Nuclear and mitochondrial patterns of introgression between the parapatric European treefrogs Hyla arborea and H. intermedia

Postglacial northward expansions from distinct southern European refugia have lead to the formation of several secondary contact zones between species. These zones are providing growing insights into the relevance of introgressive hybridization in shaping patterns of genetic diversity, and into the evolutionary processes involved in the completion of reproductive isolation barriers. Using both nuclear (9 allozyme loci) and mitochondrial (PCR-RFLP of a cytochrome b gene fragment) diagnostic markers we investigated the genetic patterns of variation across a zone of parapatry between the two European treefrogs Hyla arborea and H. intermedia. Neither F-1 and F-2 hybrids nor backcrosses were identified, indicating the lack of current gene exchange between the two species. However, introgressed alleles were observed in both species and in all markers analysed, which testifies to the occurrence of past events of introgressive hybridization. A wide variation in the frequencies of introgressed alleles was observed between loci and between the two species, suggesting the action of differential selective filtering. The historical context for the occurrence of secondary contact between the two species and its evolutionary implications are discussed, together with the possible role of human disturbance and/or reinforcing selection in leading to the observed absence of ongoing gene flow between them.