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 important in predicting the evolutionary fate of recombinant genotypes. Natural selection is endogenous if hybrid genotypes are at a disadvantage relative to parental species independent of environment. Alternatively, relative fitness can vary in response to environmental variation (exogenous selection), and hybrid genotypes can possess fitness values equal to or greater than that of parental species. I investigated the nature of natural selection in a leopard frog hybrid system by rearing larvae of hybrid and parental genotypes between Rana blairi and R. sphenocephala in 1000-L outdoor experimental ponds. Three hybrid (F , backcrossj [B ], backcross [B ]) and two parental (R. blairi [BB] and R. sphenocephala [SS]) larval genotypes were produced by artificial fertilzations using adult frogs from a natural population in central Missouri. Resultant larvae were reared in single-genotype populations and two-way mixtures at equal total numbers from hatching to metamorphosis. In single-genotype ponds, F hybrid larvae had highest survival and BB were largest at metamorphosis. When F and SS larvae were mixed together, F hybrids had reduced survival and both F and SS larvae metamorphosed at larger body masses than when reared separately. When mixed, both B and SS larvae had shorter larval period lengths than when reared alone. Higher proportion of B metamorphs were produced when larvae were mixed with either parental species than when reared alone. Larval fitness components as measured by survival, body mass at metamorphosis, proportion of survivors metamorphosing, and larval period length for B hybrid and BB larvae were similar in single-genotype populations and mixtures. Comparison of composite fitness component estimates indicated hybrid genotypes possess equivalent or higher larval fitness relative to both parental species for the life-history fitness components measured. Despite reduced survival of F hybrids in mixtures, backcross-generation hybrid genotypes demonstrated high levels of larval growth, survival, and metamorphosis in mixtures with parental species. Consequently, this study suggests natural hybridization and subsequent backcrossing between R. blairi and R. sphenocephala can produce novel 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.