Research paper Ecology of the hybridogenetic Rana esculenta complex: dierential oxygen requirements of tadpoles SANDRINE PLENET*, FREDERIC HERVANT and PIERRE JOLY Laboratoire HydrosysteÁmes ¯uviaux. U.M.R. CNRS. Universite  Lyon 1, Villeurbanne, France (*author for correspondence, fax: 33 04 72 43 11 41; e-mails: plenet@univ-lyon1.fr; hervant@univ.lyon1.fr; pjoly@univ-lyon1.fr) Received 4 June 1999; accepted 23 September 2000 Co-ordinating editor: N. Chr. Stenseth Abstract. Because of intrinsic demographic load induced by hybridogenesis (infertility of homo- typic hybrid matings), the maintenance of hybrid lineages supposes that they present better per- formances (heterosis) than their host species which allows them to coexist on a long-term basis. However, this necessity of high ®tness can be relaxed if a relative niche partitioning occurs between the taxa, each of them diering 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 in¯uence (that is related to a gradient of oxygen levels), and intermediate performances of their tadpoles with regard to oxygen availability (hyp- oxia). In investigating oxygen consumption rates, survival time in anoxia, and metabolite contents in the three forms of the complex, the present study con®rms 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 con®rm the hypothesis of the intermediate niche hypothesis to explain the coexistence of R. lessonae and R. esculenta and the success of the hybridogens. Key words: habitat selection, heterosis, hybridogenetic Rana esculenta complex, intermediate niche, oxygen consumption, oxygen tolerance, tadpoles Introduction Because recombinations are their only source of genetical variation, unisexual lineages are commonly considered as an evolutionary dead end for vertebrates (Muller, 1964; Vrijenhoek, 1989; Milinski, 1994). However, the long-term persistence of unisexual lineages and their potential role in speciation (Vri- jenhoek, 1989) sparked interest in the mechanisms of unisexuality that lend insight into evolutionary processes. Among the dierent mechanisms that sustain unisexuality, the origin of hybridogenesis involves hybrid phenotypes. In these hybridogens, one of the parental genomes is excluded from the ger- minal line prior to meiosis. The other genome is endoduplicated and is the only one that contributes to the production of gametes. Because of the absence of Evolutionary Ecology 14: 13±23, 2000. Ó 2001 Kluwer Academic Publishers. Printed in the Netherlands.