Abstract
There are two common alleles at the Amylase-2 locus in populations of Gammarus palustris, the salt marsh amphipod. Intensive sampling of individuals from two localities at Jamaica Bay revealed a consistent pattern of heterozygote deficiency.-Five possible sources of heterozygote deficiency were examined in this study. Four of them-selection against heterozygotes, null alleles at the locus, assortative mating for amylase genotype and inbreeding-are inconsistent with the evidence and are rejected. The fifth possibility, Wahlund effects due to genetic differentiation of the population, is tentatively accepted. Although there is no direct evidence for differentiation within this population, separate populations along the Eastern seaboard are highly differentiated in a nonclinal pattern. Furthermore, the Wahlund hypothesis is consistent with observations on differences in degree of deficiency exhibited among collections at Jamaica Bay.-Animals from this population exhibit feeding preferences correlated with genotype. Given the choice of two green algae, Enteromorpha or Ulva, the frequency of the slow allele among individuals choosing Enteromorpha was higher than among those choosing Ulva. This suggests that the animals assort themselves in the field into subpopulations with different allelic frequencies. This assortment could contribute to the maintenance of the polymorphism and to the observed heterozygote deficiency. We hypothesize that genotype influences behavior in this system through the action of enzyme on substrate, which determines the nature of the oligosaccharide pool liberated early in amylolysis.