1982). Studies of the regulation of the variation in dark respiration suggest that control is not in mitochondria, but in glycolysis ( Day et al. 1985). The 6PGD locus segregates two common alleles, and the Kms are heterogeneous among the three common genotypes ( Rainey-Foreman and Mitton 1995). Rates of dark respiration differ among 6PGD genotypes, with the dark respiration rate of 6PGD-11 exceeding the rate of 6PGD-22 by more than 50% (P <.01; Rainey et al. 1990). The Q10 of dark respiration, or the increase in dark respiration with a 10°C increase in temperature, is highest in 6PGD-11, intermediate in 6PGD-12, and lowest in 6PGD-22. The different values of Q10 may provide an advantage of 6PGD-12 over 6PGD-11 during heat stress ( Rainey, Mitton, and Monson 1987). Studies with double-labeled carbon revealed that the flux through glycolysis is heterogeneous among the genotypes ( Rainey-Foreman and Mitton 1995). Although 6PGD is not in glycolysis, the reaction catalyzed by 6PGD produces ribulose 5-phosphate, which inhibits phosphoglucose isomerase and consequently reduces flux through glycolysis ( Noltman 1972).
A locus with strong overdominance for viability in rapid cycling brassica, Arabidopsis thalliana, was revealed in an interval-mapping, study using molecular markers ( Mitchell-Olds 1995). The locus was mapped to a short segment on chromosome I, but this type of study does not indicate anything about the function or product of the gene. Homozygotes at this locus have viabilities 50% lower than those of heterozygotes.
Kinetic studies of both hemoglobin and enzyme polymorphisms typically reveal biochemical differences among the gene products of alternative genotypes at a locus. Biochemical differences among genotypes can have measurable effects on the physiology of whole individuals. Although the time at which an egg hatches, the swimming endurance, the ability to adjust cell volume when salinity changes, or the ability to take flight on a chilly morning may at times be trivial variations among individuals, at other times they may play a direct and major role in the determination of fitness. The kinetic properties and physiological effects of LDH in killifish, Lap in blue mussels, and PGI in Colias butterflies clearly contribute to fitness and, in combination with environmental variation, contribute to the formation and maintenance of geographic patterns of allelic frequencies.