Selective advantage of heterozygotes at the liver isocitrate dehydrogenase locus in the plaice

A cultured-released population of the plaice, Palaichthys olivaeus, was analyzed by starch gel electrophesis for liver isocitrate dehydrogenase isozyme (IDH). The universal occurrence of two alleles (A and B) at the Idh-1 locus was observed in 23 different lots from nine different hatcheries. An excess of heterozygotes at the Idh-1 locus was observed in 19 of 23 different lots, suggesting the selective advantage of heterozygotes. On the assumption that the heterozygotes rate 0% in mortality, the survival rates of homozygotes were calculated for the 19 lots showed heterozygote excess. The survival rate of A/A and B/B homozygotes was calculated at an average of 69.2 and 75.2%, respectively. The genotypes were associated with marked differences in survival rate. The standard body length of A/B heterozygotes was larger than that of B/B homozygotes in 15 of the 19 lots which showed heterozygotes excess. These results suggest a case of balance polymorphism. The maintenance of stable polymorphism could be explained by the differences of survival and growth rates among the genotypes.