Variation of plastic responses to oxygen availability within the hybridogenetic Rana esculenta complex

The hypothesis of superiority of hybrid phenotype to explain the maintenanc e of hybridogenesis (exclusion of one parental genome in the germline befor e meiosis) was investigated in the Rana esculenta complex. Survival and gro wth of progenies from each parental and hybrid lineages were compared acros s contrasted conditions of oxygen availability (this factor is suspected to influence habitat utilization of each form of the complex). Whereas growth and development were strongly affected by constant hypoxic conditions in t he R. ridibunda lineage, they were not altered in the R. lessonae lineage. Because the performance of the hybrid (R. esculenta) lineage proved to be i ntermediate between the parental species, no heterosis effect was detected. These results supported the intermediate niche hypothesis as an alternativ e to the heterotic hybrid superiority hypothesis. The ecological and evolut ionary implications are then discussed.