Adaptive maintenance of genetic polymorphism in an intertidal barnacle: Habitat- and life-stage-specific survivorship of Mpi genotypes

In the northern acorn barnacle, Semibalanus balanoides, genotype frequencie s of three genetic markers were tracked over time in four types of intertid al habitats. These habitats were selected to represent natural variation in several environmental parameters, specifically the degree of physical stre ss experienced by barnacles. Frequencies for one allozyme locus (Gpi) and a presumably neutral mtDNA marker were homogeneous among habitats in each te mporal sample. Similarly, no temporal stratification in genotype frequencie s was evident across the five sampling intervals: from planktonic larvae sa mpled in March to juveniles collected at the end of June. In contrast to th e Gpi and mtDNA loci, Mpi genotypes significantly changed in frequency in t wo habitats in the high intertidal zone. On exposed substrate, the Mpi-FF h omozygote increased in frequency, whereas the alternative homozygote, Mpi-S S, significantly decreased in frequency. Barnacles that were protected from environmental stress at high intertidal heights by the Ascophyllum nodosum algal canopy demonstrated the opposite pattern. In both habitats, the chan ge in frequency of the heterozygote was intermediate to that of the homozyg ous genotypes. Furthermore, these patterns of genotype-by-environment assoc iation reflected a pulse of genotype-specific mortality that occurred over a two-week interval subsequent to metamorphosis from the larval to the adul t form. These data indicate that each Mpi homozygote is the highest fitness genotype in some portion of the intertidal environment. Using the Levene ( 1953) model to evaluate the spatial variation in genotypic fitness, the sta ble maintenance of the Mpi polymorphism is predicted under certain subsets of conditions. Environmental heterogeneity in the intertidal zone translate s to spatial variation in selection pressures, which may result in the acti ve maintenance of the Mpi polymorphism in this species.