Temperature-dependent kinetic variation among phosphoglucose isomerase allozymes from the wing-polymorphic water strider, Limnoporus canaliculatus

Phosphoglucose isomerase (PGI) allozymes were isolated from the wing-polymorphic water strider, Limnoporus canaliculatus, and were characterizedbiochemically with respect to temperature-dependent kinetic andthermostability properties. At higher temperatures, the allozymes exhibitedsignificant differences in Michaelis constant (Km) values for substrates ofboth the forward and reverse reaction directions. Results were consistentwith expectations of adaptive kinetic differentiation based on thelatitudinal variation of PGI allele frequencies. PGI genotypes alsodiffered with regard to maximal velocity (Vmax)/Km ratios at highertemperatures. These differences were due primarily, if not exclusively, toallozyme-dependent variation in Km values. The allozymes also exhibiteddramatic differences in thermostability. However, no thermostabilitydifferences were observed when the substrate analogue 6-phosphogluconatewas present in the incubation medium. The data from this study, togetherwith data from Mytilus edulis and Metridium senile on temperature-dependentkinetic variation among PGI allozymes, form a consistent picture of naturalselection influencing the clinal variation of alleles at this locus inthese three phylogenetically distant organisms. More definitive support ofthis hypothesis, however, must await additional studies on thephysiological effects of the allozymic variation as well as directmeasurements of fitness differences among the enzyme genotypes.