Variation at the phosphoglucose isomerase locus

Phosphoglucose isomerase (PGI) (see also Section 5.2.2) is involved in fuel supply to the flight muscles, catalysing an intervening step in glycolysis. Genetic variation at the PGI locus of Colias butterflies has been exploited as a model system for examining the fitness implications of different phenotypes (for review see Watt 1991). There are four major PGI alleles, and in vitro study shows that their thermal stability is inversely related to kinetic effectiveness. This trade-off between kinetics and stability of the enzyme should maintain the polymorphism in a thermally fluctuating environment. Among genotypes in the wild, thermally stable allozymes are favoured under warm conditions as predicted, while the most kinetically effective genotypes fly over a wider temperature range and survive better in low-to-moderate temperature habitats than their relatives with the thermally more stable genotypes (Watt et al. 1983). Much of the selection on the PGI locus appears to operate at more extreme temperatures. There is, thus, good correspondence between whole-organism results and biochemical properties. The major fitness components of adult survival and male mating success vary predictably with PGI genotype, and egg-laying by female Colias also depends on flight performance throughout the day, as females lay eggs singly and fly between scattered host plants. As predicted, the thermally stable genotypes have the lowest fecundity in a cool habitat (Watt 1992). Adult pierids have lifespans of a few days in the field, and their populations may be severely limited by the inability of females to lay their full complement of eggs (Kingsolver 1989).

Climate-related variation in PGI genotypes and other components of the thermal response have also been explored in chrysomelid beetles (Dahlhoff and Rank 2000; Neargarder et al. 2003; Section 5.2.2).

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