Dynamics of an enzyme polymorphism in the isopod, Sphaeroma rugicauda (Leach). II. Sexual, gametic and fecundity selection.

The estuarine isopod Sphaeroma rugicauda (Leach) exhibits a diallelic polymorphism at the locus coding for the enzyme Pgi. An investigation carried out during the animal's breeding season (June-July) in 1979 and 1980 indicated that the animals were mating at random. It also showed the s/s-homozygote females to be the earliest to release offspring and to be the most fecund of the three genotypes (f/f, f/s and s/s). Further, data from the wild population in 1980 and laboratory experiments indicated that male s-gametes were selected against in s/s females, resulting in offspring ratio distortions in f/s (male) X s/s (female) crosses, in favour of heterozygotes. The net effect of these selective pressures did not, however, significantly alter the genotype frequencies (found in the offspring) from expected values.