Ecology of the Hybridogenetic Rana esculenta Complex: Differential Oxygen Requirements of Tadpoles

Because of intrinsic demographic load induced by hybridogenesis (infertility of homotypic hybrid matings), the maintenance of hybrid lineages supposes that they present better performances (heterosis) than their host species which allows them to coexist on a long-term basis. However, this necessity of high fitness can be relaxed if a relative niche partitioning occurs between the taxa, each of them differing in their ecological optima. In the waterfrog hybridogenetic complex (Rana esculenta complex), recent studies have revealed that hybrids show intermediate distribution between parental species across a gradient of river influence (that is related to a gradient of oxygen levels), and intermediate performances of their tadpoles with regard to oxygen availability (hypoxia). In investigating oxygen consumption rates, survival time in anoxia, and metabolite contents in the three forms of the complex, the present study confirms intermediate characteristics of hybrid tadpoles (R. esculenta) when compared to both parental lineages (R. lessonae and R. ridibunda). Whereas R. ridibunda tadpoles were the most sensitive to anoxia, R. lessonae tadpoles were the most tolerant. Because oxygen requirements of the hybrid proved to be intermediate, no heterosis was detected. These results confirm the hypothesis of the intermediate niche hypothesis to explain the coexistence of R. lessonae and R. esculenta and the success of the hybridogens.