Hybridization in leopard frogs (Rana pipiens complex): Larval fitness components in single-genotype populations and mixtures

Recognizing the predominant mode of selection in hybrid systems is importan t in predicting the evolutionary fate of recombinant genotypes. Natural sel ection is endogenous if hybrid genotypes are at a disadvantage relative to parental species independent of environment. Alternatively, relative fitnes s can vary in response to environmental variation (exogenous selection), an d hybrid genotypes can possess fitness values equal to or greater than that of parental species. I investigated the nature of natural selection in a l eopard frog hybrid system by rearing larvae of hybrid and parental genotype s between Rana blairi and R. sphenocephala in 1000-L outdoor experimental p onds. Three hybrid (F-1, backcross(1) [B-1], backcross(2) [B-2]) and two pa rental (R. blairi [BB] and R. sphenocephala [SS]) larval genotypes were pro duced by artificial fertilzations using adult frogs from a natural populati on in central Missouri. Resultant larvae were reared in single-genotype pop ulations and two-way mixtures at equal total numbers from hatching to metam orphosis. In single-genotype ponds, F-1 hybrid larvae had highest survival and BE were largest at metamorphosis. When F-1 and SS larvae were mixed tog ether, F-1 hybrids had reduced survival and both F-1 and SS larvae metamorp hosed at larger body masses than when reared separately. When mixed, both B -1 and SS larvae had shorter larval period lengths than when reared alone. Higher proportion of B-1 metamorphs were produced when larvae were mixed wi th either parental species than when reared alone. Larval fitness component s as measured by survival, body mass at metamorphosis, proportion of surviv ors metamorphosing, and larval period length for B-1 hybrid and BB larvae w ere similar in single-genotype populations and mixtures. Comparison of comp osite fitness component estimates indicated hybrid genotypes possess equiva lent or higher larval fitness relative to both parental species for the lif e-history fitness components measured. Despite reduced survival of F-1 hybr ids in mixtures, backcross-generation hybrid genotypes demonstrated high le vels of larval growth, survival, and metamorphosis in mixtures with parenta l species. Consequently, this study suggests natural hybridization and subs equent backcrossing between R. blairi and R. sphenocephala can produce nove l and relatively fit hybrid genotypes capable of successful existence with parental species larvae. Thus, the evolutionary fate of hybrid and parental genotypes in this system may be influenced by exogenous selection mediated by genotypic composition of larval assemblages.