Variation among Metabolic Enzymes along a Thermal Gradient in a Montane Ectotherm

Many montane organisms live in fragmented populations that are especially vulnerable to climate change. The ability of populations to persist in the face of immanent environmental change depends partly on whether they possess genetic variation in their capacity to respond and adapt physiologically to altered environments. In the Sierra Nevada Mountains of California, the willow leaf beetle Chrysomela aeneicollis occurs at high elevations just below tree line (2400-3600 m). Individuals living in these high-elevation habitats experience highly variable environmental temperatures, ranging from below 0°C to above 30°C in a 24-hour period. Genetic markers reveal significant differentiation among montane drainages along a 40 km transect from Big Pine Creek to Rock Creek in this region. With the increasing accessibility of next-generation sequencing, whole-genome approaches to novel species are more readily available. I used next-generation sequencing to examine how genetic differentiation occurs in metabolic genes for three populations of willow beetles living along a latitudinal thermal gradient and three populations along an elevational (2800-3200 m) gradient in a geographically-central drainage, Bishop Creek. My results revealed that single-nucleotide polymorphisms (SNPs) in exons of metabolic genes showed higher differentiation than SNPs in introns. This was not surprising, because differentiation at introns reflects population processes such as genetic drift and migration, whereas differentiation at exons reflects the same processes but may also involve natural selection. I also found that the glycolytic enzyme locus phosphoglucose isomerase (Pgi) and the FeS subunit of succinate dehydrogenase (SdhB) showed significantly higher genetic differentiation at non-synonymous SNPs than thirteen other examined loci, which show little or no variation. Pgi shows unusually high genetic differentiation across latitude, while SdhB shows high genetic differentiation among elevations in Bishop Creek. Lactate dehydrogenase and NADP dependent mitochondrial isocitrate dehydrogenase show similar levels of genetic differentiation as those observed for Pgi and SdhB, but data analyzed thus far suggest that it is driven by synonymous SNPs. Genetic variation found across geographic and elevational gradients suggests that both temperature and hypoxia may act as important selective pressures on insect populations inhabiting montane habitats. Combined, these pressures may challenge the ability of locally adapted populations to persist in the face of climate change.