Contrasting amounts of geographical variation as evidence for direct selection: the Mpi and Pgm loci in eight crustacean species

Abstract

At the Mpi locus in the amphipods Hyale plumulosa and Orchestia grillus, one allele in each species is less common inside six small bays on Long Island, New York, when compared with nearby exposed habitats on Long Island Sound. These consistently repeated differences in allele frequency could result from selection on the Mpi locus itself, or from selection on loci in linkage disequilibrium with Mpi. When one exposed and one protected site were compared in H. plumulosa, O. grillus, and six other peracarid crustacean species, the Mpi locus exhibited greater between-site variation than the Pgm locus in all eight species. The consistently greater geographical variation in Mpi is unlikely to result from selection on linked loci, and indicates that selection results directly from the Mpi polymorphisms.