Selection component analysis of the PGI polymorphism in Sphaeroma rugicauda

Abstract

Genotype frequencies in samples of males, sterile females, fertile females and random offspring were used to determine whether natural selection was acting on a diallelic phosphoglucose isomerase polymorphism. There was some evidence that female participation in reproduction depended upon genotype. The most consistent and significant effect was that the frequency of alleles transmitted from males to offspring varied with the genotype of the mother. The direction of this effect was such as to maintain both alleles in the population.

Multiple mating in a natural population of the isopod Sphaeroma rugicauda; evidence from distorted ratios in offspring

“…To this would have to be added those multiple matings which will yield undistorted ratios (where males are FF+SS, FS+FS). This suggests that a considerable proportion of females had been fertilised by two males which comes as something of a surprise, given the life history details already described and the general observation that males are less common than females in the population (Riddoch, 1988). It compares quite closely with the figure of 30 per cent multiple matings reported by Sassman (1978) for the terrestrial isopod Porcellio dilatatus, although it is lower than the figure of 80 per cent given by the same author for P. scaber.…”

“…It does not invalidate the results of the selection component analysis (Heath et a!., 1988) which used mother/single offspring combinations, an analysis developed for organisms where multiple mating may occur (Christiansen and Frydenberg, 1973). Multiple mating per se would not be expected to alter the dynamics of a polymorphism unless either the efficiency of multiple mating depends on genotype (e.g., Prout and Bundgaard, 1977) or multiple mating leads to other types of selective interaction between gametes or developing zygotes.…”

“…A population of S. rugicauda has been the subject of an intensive investigation of a diallelic phosphoglucose isomerase (PGI) polymorphism (Heath et a!., 1988). This involved (amongst other things) the collection of fertilised females from the population and the examination of the genotypic ratios in their broods.…”

Selection component analysis of the Mpi locus in the amphipod Platorchestia platensis

“…The homozygote of the rarer allele had higher fitness for two components of selection of Pgi in S. rugicauda; however, gametic selection favoured the common allele in a way that could yield a balanced polymorphism (Heath et a!., 1988). The common homozygote had higher fecundity than heterozygotes for Pgi in Asellus aquaticus (Shihab and Heath, 1987 that zygotic selection may favour heterozygotes at some times of the year (Shihab and Heath, 1987b).…”

“…Fewer still have measured more than one component of selection; among the only loci for which gametic, fecundity, sexual, and zygotic selection have been measured in natural populations of animals are the loci coding for esterase in Zoarces viviparus (Christiansen et al, 1973(Christiansen et al, , 1977, lactate dehydrogenase in Porcellio scaber (Sassaman, 1978), phosphoglucose isomerase in Sphaeroma rugicauda (Heath et a!., 1988), and three loci in Peromyscus maniculatus Baccus, 1981, 1983;Snyder, 1983). All components but fecundity have been measured for two loci in Gammarus oceanicus (Siegismund, 1985a).…”