Is cold tolerance plasticity in Epiphyas postvittana (Lepidoptera: Tortricidae) related to variation in phosphoglucose isomerase?

Phosphoglucose isomerase (PGI), an enzyme involved in the second step of glycolysis, has been shown to exhibit a high degree of polymorphism in many taxa, including insects. The variation seen among PGI “allozymes” in insects has often been correlated with individual fitness traits, such as locomotion, dispersal, flight metabolism, and fecundity. Additionally, PGI variation has also been shown to co-vary with cold-stress tolerance in some species. Knowledge of thermal tolerance limits is essential to accurately predict the potential geographic spread and establishment of invasive insects. In temperate areas, cold temperature is often a major selective agent dictating when and where species successfully occur. Given this, the potential application of PGI characterization to invasion biology is increasingly being recognized, though relatively few studies have investigated this link. Here, we present preliminary work to characterize PGI polymorphisms in Epiphyas postvittana (Walker), a moth recently invasive to the U.S., and analyze the extent to which allozyme variation is correlated with phenotypic variation in larval cold tolerance. We used E. postvittana specimens preserved from a long-term experimental evolution study that attempted to select for increased tolerance to partial freezing in late instars. Allozyme genotypes of specimen samples were characterized by differences in protein electrophoretic mobility. We hypothesized that any observed variation in PGI allozyme composition would correlate with phenotypic variation in cold tolerance seen in the selection study.