Salinity related physiological and genetic differences between populations of Mytilus edulis

Abstract

Aminopeptidase-I, the enzymatic product of the Lap locus, liberates N-terminal neutral and aromatic amino acids from oligopeptides. The enzyme is associated with the brush border of the intestine and the extensive lysosomal system in the digestive tubule cells; the enzyme functions in oligopeptide degradation and possibly amino acid transport. The Lap locus is genetically polymorphic and allele frequencies differ between populations according to environmental salinities. Using cell-free lysosomes, we show salinity related differences in both lysosomal membrane latency and lysosomal and cytosolic free-amino acid concentrations. Differences in the response of the lysosomal aminopeptidase-I enzyme to differences or changes in salinity are discussed. Antibodies against aminopeptidase-I enzyme were used to demonstrate that alleles of the Lap locus exhibit different specific activities per unit enzyme concentration. In oceanic populations, the concentrations of aminopeptidase-I enzyme of individual phenorypes are equal, suggesting that variation of specific activity among phenorypes is due to allele specific differences in catalytic efficiency. In estuarine populations, total enzyme activity is lower, which is partially due to a significantly lower concentration of aminopeptidase-I enzyme of one homozygote genotype. The consequences of a change in environmental salinity can be measured on the biochemical, physiological, and population genetic levels. We discuss the possible mechanisms by which salinity variations are responsible for the genetic polymorphism of aminopeptidase-I.