RANK, N. E.; BRUCE, D. A.; MC MILLAN, D. M.; DAHLHOFF, E. P.; Sonoma State Univ.; Sonoma State Univ.; Santa Clara Univ.; Santa Clara Univ.: Effects of temperature and PGI genotype on performance in a Sierra willow beetle

We examined effects of recent thermal history on locomotor performance in Eastern Sierra, Nevada populations of the leaf beetle Chrysomela aeneicollis. In this species, allele frequency variation at the glycolytic enzyme locus phosphoglucose isomerase (PGI) correlates with environmental temperature gradients. We tested the prediction that variation at PGI (relative to two other enzyme loci) relates to variation in locomotor performance. Running speed was measured for field-acclimatized beetles, after laboratory exposure to elevated or subzero temperatures beetles experience in nature, and after a second exposure to the same (heat-heat; cold-cold) or opposite (heat-cold; cold-heat) temperature. Field-collected adult male PGI 1-1 and 1-4 (but not 4-4) genotypes ran faster than their female counterparts. All adults ran slower after exposure to extreme temperatures. However, PGI 1-1 and 1-4 genotypes ran significantly faster than 4-4 homozygotes after the first exposure, while 4-4 homozygotes ran faster after the second exposure. Large field-collected larvae had higher running speeds than small ones, and the magnitude of this effect varied among PGI genotypes (4-4>1-4>1-1). After the first temperature exposure, larval PGI 4-4 homozygotes ran almost 50% faster than 1-1 and 1-4 genotypes, whereas running speeds declined in most cases after second treatment. Results suggest that selection at PGI may operate in opposing directions in adults and larvae. In addition, individuals possessing the PGI 4 allele may better tolerate exposure to thermal extremes, while individuals possessing the 1 allele may perform better over a broader range of moderate conditions.