Evolutionary genetics of Metridium senile. I. Kinetic differences in phosphoglucose isomerase allozymes

Populations of the sea anemone Metridium senile from the northeast coast of the United States exhibit a one-locus, two-allele polymorphism for phosphoglucose isomerase. No additional "hidden" variation is exposed by changes in pH, gel pore size, or heat denaturation. The allozymes are similar in pH optimum, sensitivity of Km to pH, and sensitivity of Km and Vmax to temperature. In other respects they are functionally different, with the fast allozyme having a 3.5-fold higher specific activity and a slightly higher Km of fructose-6-phosphate than the slow form. In these respects, heterozygotes produce a mixture of enzymes that appears to function roughly as the sum of its component parts. Comparisons of Vmax/Km ratios reveal significant differences among genotypes, with the fast form having higher values at all temperatures than the slow form and heterozygotes falling intermediate. In addition, there is a significant difference among genotypes in sensitivity of this parameter to temperature, with the fast homozygote and heterozygote displaying greater sensitivity than the slow homozygote. Temperature is probably an important selective agent in maintaining this polymorphism.