Dynamics of an enzyme polymorphism in the isopod, Sphaeroma rugicauda (Leach). I. Temporal variation in genotype frequencies

A 27 month study (November, 1978 through January, 1981) of the diallelic enzyme locus, phosphoglucose isomerase (Pgi), in the estuarine isopod Sphaeroma rugicauda (Leach) revealed seasonal changes in the frequencies of the three genotypes (fast/fast, fast/slow and slow/slow). A comparison of genotype frequencies in the newly-released offspring (August) to the frequencies in the same animals ten months later during the breeding season (June), revealed a decline in heterozygote frequency and a corresponding increase in the frequency of the s/s homozygote both of which appeared to be initiated at the onset of winter, a period characterised by low temperatures and salinity. Survivorship experiments in the laboratory suggested that the increase in frequency of the s/s homozygote, at least, could be explained by this genotype being favoured at low temperatures.