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 whi ch genotypic differences in organismal performance and fitness compone nts in the wild are accurately predictable. The alpine species Colias meadii seems to share electromorph alleles with lowland congeners at P GI and phosphoglucomutase (PGM). However, high-resolution electrophore sis 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 l owland Colias eurytheme. These meadii genotypes show heterozygote adva ntage 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 kinet ics vs. stability in PGI homozygotes, seen in lowland taxa, to C. mead ii. Positive evidence is given for the absence of assortative mating a nd segregation distortion at both PGI and PGM. The functional differen ces among PGI genotypes explain previously observed heterozygote advan tage in flight capacity and survivorship and correctly predict heteroz ygote advantage in male mating success, of C. meadii's PGI genotypes. Though functional information is not yet available on C. meadii PGM va riants, 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 becaus e the parallelism is not based on allelic identity. These results supp ort expectations of evolutionary bioenergetics and emphasize the hazar ds of using ordinary electrophoresis to infer allozymes' identity amon g taxa. In alternative scenarios for Colias' PGI evolution-divergence from ancestral polymorphism, or independent origin-these results sugge st major constraint, based in protein structure, on ability of PGI all eles to maximize fitness-related biochemical performance when homozygo us.