Adaptation at specific loci .6. Divergence vs parallelism of polymorphic allozymes in molecular function and fitness-component effects among Colias species (Lepidoptera, Pieridae)

In lowland Colias butterflies, genotypes of the enzyme phosphoglucose isomerase (PGI) show major differences in molecular function, from which genotypic differences in organismal performance and fitness components in the wild are accurately predictable. The alpine species Colias meadii seems to share electromorph alleles with lowland congeners at PGI and phosphoglucomutase (PGM). However, high-resolution electrophoresis finds differences between PGI electromorphs of meadii and those of lowland taxa. Common C. meadii genotypes differ in thermal stability and are less thermally stable than similar electromorph genotypes in lowland Colias eurytheme. These meadii genotypes show heterozygote advantage in the kinetic parameters K-m and V-max/K-m (and differ sharply from genotypes of C. eurytheme). The thermally more stable homozygote is the kinetically less effective one, extending the tradeoff of kinetics vs. stability in PGI homozygotes, seen in lowland taxa, to C. meadii. Positive evidence is given for the absence of assortative mating and segregation distortion at both PGI and PGM. The functional differences among PGI genotypes explain previously observed heterozygote advantage in flight capacity and survivorship and correctly predict heterozygote advantage in male mating success, of C. meadii's PGI genotypes. Though functional information is not yet available on C. meadii PGM variants, these also show heterozygote advantage in male mating success but do not interact with PGI. Thus, differences in molecular function result in fitness component differences among PGI allozymes in alpine Colias, as well as in lowland ones. This is the more remarkable because the parallelism is not based on allelic identity. These results support expectations of evolutionary bioenergetics and emphasize the hazards of using ordinary electrophoresis to infer allozymes' identity among taxa. In alternative scenarios for Colias' PGI evolution-divergence from ancestral polymorphism, or independent origin-these results suggest major constraint, based in protein structure, on ability of PGI alleles to maximize fitness-related biochemical performance when homozygous.