HIGH LARVAL PERFORMANCE OF LEOPARD FROG HYBRIDS: EFFECTS OF ENVIRONMENT-DEPENDENT SELECTION

Abstract

Predicting the ecological and evolutionary significance of hybridization de- pends on accurate and comprehensive measurements of the relative fitness of hybrid and parental genotypes under realistic environmental scenarios. Although matings between ge- netically distinct individuals may produce hybrid progeny of lower fitness than matings between genetically similar individuals, this is not always the case. Two species of North American leopard frogs (Rana blairi and R. sphenocephala) hybridize in central Missouri. To examine the relationship between genotypic fitness and the environment, I measured larval fitness components of the two leopard frog species and their interspecific hybrids using transplant experiments in field enclosures. This study was undertaken to compare larval fitness components of R. blairi, R. sphenocephala, and F. hybrids in three natural aquatic habitats representative of a natural environmental gradient (prairie, woodland, and river floodplain). Larvae were reared either in single-genotype stands or in mixtures of all three genotypes. There was no evidence of reduced F. hybrid larval performance in any environment. In fact, F. hybrids metamorphosed larger than R. sphenocephala, but not R. blairi, in the river floodplain habitat. All genotypes produced the highest proportion of individuals surviving and metamorphosing in the floodplain habitat. Thus, hybrid genotypes performed equivalently to or better than their parental species in three aquatic habitat types. Transplant experiments such as these are critical for directly measuring genotype-by-en- vironment interactions and for assessing the role of ecological factors in determining the adaptive potential of natural hybridization.