The Physiological Basis of Natural Selection at the Lap Locus

An extensive research program was undertaken to evaluate the contribution of genetic variation at the Lap locus to variation in physiological traits under natural conditions. Rates of carbon and nitrogen metabolism were monitored in a population of the mussel Mytilus edulis near the center of the Lap allele frequency cline on the north shore of Long Island. The goal of this research was to establish whether the previously described genotype-dependent differences in physiological phenotype are meaningful in ecologically relevant circumstances. It was predicted from laboratory studies that, in nature, genotype-dependent differences will exist for rates of nitrogen excretion and that other aspects of the animal's physiology, particularly rates of carbon metabolism, will be unaffected by Lap genotype. Rates of amino acid and ammonia excretion were significantly dependent upon Lap genotype; individuals with the Lap94 allele exhibited greater rates of nitrogen loss. These differences among genotypes were most evident in the fall, between September and December. The genotype-dependent component of rates of nitrogen loss were also largest relative to the total rate of excretion during the fall period. As predicted, other aspects of the nitrogen metabolism (acquisition) and rates of carbon metabolism were independent of Lap genotype. There was a striking congruity among a variety of observations that all indicate that phenotypic differences in nitrogen metabolism are the basis of natural selection at the Lap locus in Long Island Sound. Rates of growth were minimal during the fall months (Hilbish, 1985) and mussels are known to lose weight in a genotype-specific manner during this period (Koehn et al., 1980). Rates of elemental gain and loss were summed to produce carbon and nitrogen budgets; these data show the fall to be a period of extended deficit in carbon and nitrogen balance. Genotype-dependent losses of ammonia and amino acids were greatest during the fall months. Finally, selection against the Lap94 allele occurs predominantly in the fall (Hilbish, 1985). The data indicate that the depletion of nitrogen resources provides the basis for selection against Lap94 genotypes during the fall months.