Asexual organisms are a special case in evolutionary ecology interesting to study adaptation. Sexual reproduction is advantageous since it maintains an important polymorphism and genetic diversity, but it is less taking into account its costs. The persistence of many hybrid asexual lineages among independent taxa, some old, raise questions about this reproductive mode: benefits, extinction risks of hybridization complexes. We have studied the case of the European water frogs present in France, where we find the complex P. esculentus, very common, and the complex P. graf, located in the south. These hybrid lineages are special because having a unisexual mode of reproduction called hybridogenesis. These different taxa of water frogs are so alike that their distinction is difficult and requires genetic analysis. Thus, a new PCR-RFLP based on ITS2 has been developed, allowing reliable taxonomic affiliation of the complex P. esculentus. Then, the microsatellite markers were tested as tools for taxonomic identification, without success, due to a lack of specificity, and the complex situation of French frog assemblages. The genetic load carried by clonal genomes was evaluated in an experimental breeding, where tadpoles of P. rididundus bearing either 50% clonal genome or 0% showed no differences in larval performances. Regarding hybrid fitness of P.kl. grafi tadpoles compared to that of parental species, hybrids showed intermediate performance, better than P. perezi, but worse than P. ridibundus. In the habitat use study, we were able to evidence a differential distribution of water frogs of P. esculentus complex according to habitat type, and observe new factors influencing their abundance as altitude, distance to the river, the nitrate level, alkalinity, hardness. We observed the relationship between habitat type and acclimation abilities in P. esculentus hybrids at larval stage, and performances show that forest tadpoles seem to be generalists and bear environmental modifications, while those from meadows appear locally adapted to their environment. These results bring us to discuss the future evolution of water frogs and offer opportunities, to refine our knowledge of these complexes and assist in making decisions regarding habitat to prioritize in terms of conservation management.
Hybridation complexes in European water frogs : taxonomic identification, ecological contraints, and adaptation abilities