INTERDEME AND INTRADEME GENETIC DIFFERENT IATION IN GASTROPOD MOLLUSKLITTORINA-SITKANA

Two settlements of the periwinkle Littorina sitkana from the Vostok Gu lf of the Sea of Japan were compared for their genotypic composition a t five polymorphic enzyme loci: esterase D, glucosephosphate isomerase , inorganic pyrophosphatase, and nonspecific esterase (two loci). The settlements were located four km apart on the opposite coasts of a sma ll gulf. As Littorina sitkana is a sedentary animal without a pelagic developmental stage, this distance prevents genetic exchange between t he settlements, which can thus be regarded as separate demes. In each settlement, snails of two different age classes (juveniles and older a nimals) and those differing in shell morphology (with and without shel l sculpturing) were compared. Genetic differences were found between t he settlements at all five loci and between the age groups in each set tlement at three loci. The age changes were shown to have the same tre nd for both settlements and both groups differing in shell sculpturing . The latter groups did not exhibit genetic differences within a settl ement. The greatest difference between settlements was demonstrated at the loci that displayed age differences. The most plausible explanati on for the observed intrademe age differences is natural selection. Th e interdeme genetic variation appears to result from the combined acti on of selection and genetic drift.