Physiological stress and the fitness effects of Mpi genotypes in the acorn barnacle Semibalanus balanoides

Balancing selection has been invoked to explain the apparent maintenance of polymorphisms at the mannose-6-phosphate isomerase (Mpi) and glucose-6-phosphate isomerase (Gpi) loci in a diverse array of organisms. However, in many cases the agents of selective stress and the gram size of environmental heterogeneity necessary to preserve genetic variation remain uncertain. In the barnacle Sennbalanus balanoides, differing selection regimes have been proposed across the species' range. To test whether or not fitness differences segregate with Mpi and Gpi polymorphisms near the southern range limit, barnacles from Narragansett Bay, Rhode Island, were subjected to physiological challenges in the laboratory. Survivorship was measured in barnacles exposed to an orthogonal combination of salinity, thermal and dietary stresses. Cirral length and genotype frequency shifts at Mpi and Gpi were assessed in low salinity treatments. Mortality from thermal stress was greatly enhanced under low salinity. Across all low salinity treatments there was a nonsignificant trend of decreased fitness associated with the Mpi-SS genotype. Cirral length variation was significantly correlated with Mpi genotypes in a pattern consistent with survivorship data.