FACTORS CONTRIBUTING TO THE MAINTENANCE OF THE GENETIC-POLYMORPHISM AT THE LOCUS LDH-B IN THE POOL FROG, RANA-LESSONAE

We tested for environmental factors that may lead to balancing selecti on and to the maintenance of a genetic polymorphism at the enzyme locu s lactate dehydrogenase B (LDH-B) in the pool frog, Rana lessonae. We raised tadpoles individually in a factorial experiment in which we man ipulated temperature, food level, and food quality. The only statistic ally significant difference among LDH-B genotypes was in growth rate, with the heterozygote performing best. Although the difference was not significant, heterozygotes also tended to perform best for size at me tamorphosis. However, heterozygotes did not perform best in terms of o ther traits (age at metamorphosis and rates of survival and metamorpho sis), where differences among LDH-B genotypes were also not significan t. The size of the effect of LDH-B genotype depended on the environmen t, which suggests that the locus may be selectively neutral in some en vironments. There were no genotype-environment interactions in the sen se that reaction norms along environmental gradients did not cross. Wh en we raised tadpoles in groups, e/e homozygotes had a significantly h igher body mass and developed at the significantly highest rate. In ad dition, there may be a trade-off between larval and adult performance: adult frogs show a different ranking in performance of LDH-B genotype s than tadpoles do. These results suggest that this genetic polymorphi sm is maintained through heterozygote advantage, possibly in conjuncti on with antagonistic pleiotropy.