Allele frequency variation at the phosphoglucose isomerase (PGI) locus in Californian populations of the beetle Chrysomela aeneicollis suggests that PGI may be undergoing natural selection. We quantified (i) apparent Michaelis-Menten constant (K-m) of fructose 6-phosphate at different temperatures and (ii) thermal stability for three common PGI genotypes (1-1, 1-4, and 4-4). We also measured air temperature (T-a) and beetle body temperature (T-b) in three montane drainages in the Sierra Nevada, California. Finally, we measured 70-kDa heat shock protein (Hsp70) expression in field-collected and laboratory-acclimated beetles. We found that PGI allele 1 predominated in the northernmost drainage, Rock Creek (RC), which was also significantly cooler than the southernmost drainage, Big Pine Creek (BPC), where PGI allele 4 predominated. Allele frequencies and air temperatures were intermediate in the middle drainage, Bishop Creek (BC), Differences among genotypes in K-m(1-1 > 1-4 > 4-4) and thermal stability (4-4 > 1-4 > 1-1) followed a pattern consistent with temperature adaptation. In nature, T-b was closely related to T-a. Hsp70 expression in adult beetles decreased with elevation and differed among drainages (BPC > BC > RC). After laboratory acclimation (8 days, 20 degrees C day, 4 degrees C night) and heat shock (4 h, 28-36 degrees C), Hsp70 expression was greater for RC than BPC beetles. In RC, field-collected beetles homozygous for PGI 1-1 had higher Hsp70 levels than heterozygotes or a 4-4 homozygote, These results reveal functional and physiological differences among PCI genotypes, which suggest that montane populations of this beetle are locally adapted to temperature.
