RAND, D.M; SACKTON, T.B.; SPAETH, P.S.; SCHMIDT, P.S.: Ecological Genetics of the Mpi polymorphism in the Acorn Barnacle, Semibabalnus balanoides

The mannose phosphate isomerase (Mpi) locus is one of the most consistently polymorphic enzymes of central metabolism in marine invertebrates. This protein exhibits a strong two-allele polymorphism in the common acorn barnacle, Semibalanus balanoides. Our earlier work has demonstrated that genotypes of Mpi show a clear zonation between the high- and low-intertidal zones at thermally stressed localities in Maine. Cohort analyses show that no zonation is present at larval settlement in early spring, but the zonation emerges as the season progresses. Transplant experiments show that the alternative homozygotes are favored at opposite ends of an environmental stress gradient across the intertidal (the SS homozygote is favored in low-stress, and the FF homozygote is favored in high-stress microhabitats). Growth experiments with supplemental sugars show that the allozymes have distinct genotype-by-thermal stress interactions on growth rates in a substrate-dependent manner. Here we present analyses of enzyme activities for the three genotypes of Mpi (FF, FS, and SS) under factorial designs of heat denaturation stress and substrate concentration. There are clear genotype-by-substrate interactions, and genotype-by-locality interactions on enzyme activity. Together these analyses indicate that the Mpi polymorphism is maintained by balancing selection among the heterogeneous thermal and dietary microenvironments of the rocky intertidal. The Mpi polymorphism may be a key adaptation to the environmental unpredictability inherent in the life history of pelagically dispersed organisms. Recent progress towards the identification of the nucleotide basis of the Mpi allozyme polymorphism, and neutrality tests of this adaptation hypothesis, will be presented.