Summary1. Sierra Nevada populations of the beetle Chrysomela aeneicollis experience extreme high and low temperatures during summer, which pose special challenges to larvae of limited mobility. In these populations, allele frequency variation at the glycolytic enzyme locus phosphoglucose isomerase (PGI) correlates with differences in temperature between river drainages. PGI allozymes differ in functional properties, and thermal tolerance of adults and larvae depends on PGI genotype. 2. We measured effects of temperature on larval development rate and survival after reciprocal transplantation of populations between drainages. Effects of temperature on growth rate and activity were determined after laboratory acclimation of larvae from Bishop Creek (BC), where PGI alleles 1 and 4 occur in similar frequency. Hsp70 expression levels were measured for free-living larvae of known body temperature, and for laboratory-acclimated BC larvae. 3. Larval mortality was greatest in the coldest drainage and was correlated with minimum night-time air temperature. The frequency of PGI allele 1 declined for BC larvae transplanted to the warmest drainage. Development rate of BC larvae in nature was lowest for experimental groups where PGI-1 allele frequency was highest. Larval growth and activity varied with acclimation temperature and PGI genotype in the laboratory. Hsp70 expression levels in nature were higher for larvae collected later in the day, and varied consistently among PGI genotypes in nature and in the laboratory. 4. These results suggest that daytime temperatures routinely experienced by larvae cause elevated Hsp70 expression levels indicative of physiological stress. Exposure to subzero night-time temperatures appears to cause larval mortality. Up-regulation of Hsp70 may protect larvae from heat and cold stress. Variation in Hsp70 expression among PGI genotypes may result in differential mortality and developmental rates in nature.

SUMMARY Eastern Sierra Nevada populations of the willow beetle Chrysomela aeneicollis commonly experience stressfully high and low environmental temperatures that may influence survival and reproduction. Allele frequencies at the enzyme locus phosphoglucose isomerase (PGI) vary across a climatic latitudinal gradient in these populations, with PGI allele 1 being most common in cooler regions and PGI allele 4 in warmer ones. PGI genotypes differ in heat and cold tolerance and in expression of a 70 kDa heat shock protein. Here we examine genetic, behavioral and environmental factors affecting a performance character, running speed, for willow beetles, and assess effects of consecutive cold and heat exposure on running speed and expression of Hsp70 in the laboratory. In nature, running speed depends on air temperature and is higher for males than females. Mating beetles ran faster than single beetles, and differences among PGI genotypes in male running speed depended on the presence of females. In the laboratory, exposure to cold reduced subsequent running speed, but the amount of this reduction depended on PGI genotype and previous thermal history. Effects of exposure to heat also depended on life history stage and PGI genotype. Adults possessing allele 1 ran fastest after a single exposure to stressful temperature, whereas those possessing allele 4 ran faster after repeated exposure. Larvae possessing allele 4 ran fastest after a single stressful exposure, but running speed generally declined after a second exposure to stressful temperature. The ranking of PGI genotypes after the second exposure depended on whether a larva had been exposed to cold or heat. Effects of temperature on Hsp70 expression also varied among PGI genotypes and depended on type of exposure, especially for adults (single heat exposure, two cold exposures: PGI 1-1>1-4>4-4;other multiple extreme exposures: 4-4>1-4>1-1). There was no consistent association between alleles at other polymorphic enzyme loci and running speed or Hsp70 expression. These data suggest that variation at PGI is associated with considerable plasticity in running speed. Differences in Hsp70 expression among PGI genotypes suggest that the heat-shock response may buffer differences in thermal tolerance and performance among genotypes and help maintain the PGI polymorphism in a thermally variable environment.