Ecological Genetics of Mpi and Gpi Polymorphisms in the Acorn Barnacle and the Spatial Scale of Neutral and Non-neutral Variation

Different allozyme genotypes at the mannose phosphate isomerase(Mpi) locus in the northern acorn barnacle (Semibalanus balanoides)show a strong association with distinct intertidal microhabitats.In estuaries along the Maine Coast, the FF homozygote has higherfitness in exposed, high-tide level microhabitats while theSS homozygote has higher fitness under algal cover or at low-tidemicrohabitats. These patterns are consistent with a Levene (1953)model of balancing selection. In these same samples, polymorphismsat the glucose phosphate isomerase locus (Gpi) and mitochondrialDNA (mtDNA) show no fitness differences among microhabitats,providing intra-genomic controls supporting selection at ornear Mpi. Here we report a similar analysis of genotype-by-microhabitatassociations at sites in Narragansett Bay, Rhode Island, closeto the southern range limit of S. balanoides. Genotype zonationat Mpi between high- and low-tide microhabitats is significantlydifferent between Maine and Narragansett Bay due to oppositezonation patterns for the SF and FF genotypes. Enzyme activitydata are consistent with this "reverse" zonation. At Gpi, thereis significant microhabitat zonation in Narragansett Bay, whilethis locus behaves as a neutral marker in Maine. Mt DNA showsno significant microhabitat zonation in either Rhode Islandor Maine. The Mpi data suggest that Levene-type selection foralternative genotypes in alternative habitats may operate atscales of both 10's of meters and 100's of kilometers. The Gpidata show how an apparently neutral locus can exhibit non-neutralvariation under different environmental conditions. We arguethat both Mpi and Gpi provide important genetic variation foradaptation to environmental heterogeneity that is recruitedunder distinct conditions of stress and carbohydrate substrateavailability.