RANK, N.E.; DAHLHOFF, E.P.: PGI genotype affects heat shock protein expression and freeze tolerance in the leaf beetle Chrysomela aeneicollis
Natural selection may act on enzyme loci through genotypic differences in physiological performance at different temperatures. Allele frequency variation at the phosphoglucose isomerase (PGI) locus in the montane leaf beetle Chrysomela aeneicollis and biochemical differences among PGI allozymes suggest that this enzyme may be under natural selection. In this study, we show directional changes in PGI allele frequency in natural populations of beetles in the Eastern Sierra Nevada of California (Bishop Creek: 2900-3200 m) during a period of increasing precipitation and decreasing air temperature (1988-1996). The PGI-1 allele increased by 11% during this period, while the PGI-4 allele decreased. No directional changes were observed for isocitrate dehydrogenase or phosphoglucomutase. We also determined the effects of temperature extremes experienced by beetles in nature on survival and on expression of HSP70-class heat shock proteins. We found that PGI 1-1 homozygotes survived 8 h exposure to -5oC better than 1-4 heterozygotes or 4-4 homozygotes. We measured the induction profile of a stress-inducible isoform of HSP70 at 3-4 o C increments from 20 to 36oC after laboratory acclimation (8d; 20oC d, 4oC n). HSP70 expression peaked for PGI 1-1 individuals at a lower temperature (30oC) than PGI 1-4 (33oC) or 4-4 (36oC) individuals. Our data suggest that C. aeneicollis adapts to climate change via links between PGI genotype, cold tolerance, and the energetically costly heat shock response.
