Homozygous excess in sand flounder,Rhombosolea plebeia,produced by assortative mating

The polymorphic enzyme glucosephosphate isomerase (Gpi-2) was studied in muscle tissue from spawning sand flounder, Rhombosolea plebeia, in Wellington Harbour, New Zealand, between April 1984 and June 1986. There was a significant difference in Gpi-2 allele frequencies between early (March-May) and late (June-September) spawners. Pooling equal numbers of early and late spawners produced a significant excess of homozygotes in the 1984 and 1986 samples. This type of partial assortative mating may account for the frequently observed excess of homozygotes in other marine organisms with long spawning seasons.