Adaptation at specific loci. V. Metabolically adjacent enzyme loci may have very distinct experiences of selective pressures

Abstract

The polymorphic phosphoglucomutase (PGM) and glucose-6-phosphate dehydrogenase (G6PD) loci have been studied in parallel to experimental work on the phosphoglucose isomerase (PGI) polymorphism in Colias butterflies. PGI, PGM and G6PD are also autosomal in Colias. PGM and G6PD are loosely linked (and represent the first identified autosomal linkage group in Colias); they assort independently from PGI. Recombination occurs in both sexes. Neither PGM nor G6PD shows large, consistent differences in flight capacity through the day among its genotypes, as PGI does. PGM shows some change of allele frequencies, and match to Hardy-Weinberg expectation, with air temperature in middle and latter parts of the season, but not early in the season. G6PD may show some heterozygote excess over Hardy-Weinberg expectation early in the day, but more testing is needed. No evidence for differential survivorship was seen at PGM or G6PD, in contrast to PGI. At the PGM and G6PD loci, male heterozygotes are advantaged in mating with females, but without the evidence of female choice which occurs for PGI. These effects are not correlated among the three loci. There is no assortative mating at G6PD (nor at PGI). There is minor positive assortative mating of PGM heterozygotes, but it is too weak to account for the PGM-genotype-specific male mating advantage. No trends of multilocus genotype frequencies involving PGI are seen. Certain PGM-G6PD two-locus genotypes are over-represented, and others under-represented, in wild adult samples, particularly among males and uniformly among successfully mating males. Our results emphasize that enzyme loci sharing a substrate need not have common experience of the existence or strength of natural selection, and suggest initial food-resource processing and allocation as a possible context for fitness-related effects of the PGM and G6PD polymorphisms.