SELECTIVE ADVANTAGE OF HETEROZYGOTES AT CATALASE LOCUS IN THE PACIFIC OYSTER, CRASSOSTREA-GIGAS

Wild and cultured populations of the Pacific oyster, Crassostrea gigas, were analysed by starch gel electrophoresis for catalase (CAT). The comparison of genotypic frequencies at the Cat locus revealed an apparent advantage of heterozygotes in the wild and cultured populations. The observed frequency of heterozygotes in each population was higher than the expected frequency. This result suggests a case of balanced polymorphism. The genotypes of the Cat locus were associated with marked differences in early mortality during the cultured period. The survival rate of each homozygote was lower than the mean survival rate in the population. The maintenance of stable polymorphism could be explained by differences in mortality among genotypes. Isocitrate dehydrogenase (IDH) was simultaneously examined as a control in the population structure. Homozygote excess at the Idh-1 locus was observed in some wild populations and an inbreeding structure was suggested. © 1979, The Genetics Society of Japan. All rights reserved.