Variation in thermal tolerance is linked to phosphoglucose isomerase genotype in a montane leaf beetle

1. Sierra Nevada populations of the Willow Beetle Chrysomela aeneicollis (Schaeffer) experience extreme elevated and subzero temperatures in nature. In these populations, frequencies of phosphoglucose isomerase (PGI) alleles vary with latitude and altitude and respond to climate change. PGI genotypes differ in expression of a stress-inducible heat shock protein (Hsp70). 2. Here, differences in tolerance of elevated and subzero extreme temperatures were compared for field-acclimatized and laboratory-acclimated larvae and adults possessing three common PGI genotypes (PGI 1-1, 1-4 and 4-4). Three indices of thermal tolerance were measured - CTmax , LT50 and Hsp70 expression level. 3. Thermal tolerance depended on life stage, prior exposure to sublethal stress and PGI genotype. Larvae were generally less tolerant of thermal extremes than adults. For both adults and larvae, prior exposure to sublethal temperatures increased survival after exposure to subsequent stress. Survival after exposure to thermal extremes was consistently related to PGI genotype (1-1 > 1-4 > 4-4), as were expression levels of Hsp70 (1-1 > 1-4 > 4-4). 4. These results suggest that PGI genotypes differ in tolerance of thermal extremes routinely experienced by beetles in nature. A trade-off between thermal tolerance and energy allocation may explain the persistence of the PGI polymorphism.