Cytonuclear Interactions Among Metabolic Enzyme Loci Mediate Reproductive Success and Larval Performance Along Environmental Gradients in Natural Populations of a Montane Leaf Beetle

Organisms in natural populations are confronted with significant challenges posed by environmental change. In order for populations to persist, individuals must overcome these challenges; however, the physiological mechanisms that enable survival and reproduction are not well understood. Here, I report the results of a field study that contributes to our understanding of how organisms respond to changing environments. I examined how environmental conditions and an interaction between two metabolic genes influences individual survival and reproduction. I focused on a nuclear gene coding for the glycolytic enzyme phosphoglucose isomerase (PGI), and a mitochondrial gene, cytochrome oxidase II (COII), important for energy production during aerobic metabolism. To investigate this relationship, I studied the Sierra willow leaf beetle Chrysomela aeneicollis. In California, beetle populations live at the edge of their species' range in the Eastern Sierra Nevada mountains. Populations are polymorphic at PGI and COII, and these genes vary along a latitudinal temperature gradient to a greater extent than other genetic markers. I evaluated how local microenvironments where beetles occur differ along elevation gradients, and I also investigated how abiotic stress influences the dynamics of predator-prey interactions. I found that localities varied greatly in abiotic features such as air temperature, sun exposure and soil moisture, and that these site characteristics were associated with elevation. Fecundity of females who laid eggs was greatest for females that possessed the PGI genotype and COII haplotype most commonly found in the south, but was lowest for those with the southern PGI genotype and northern COII haplotype. Larval development depended on the interaction between maternal PGI genotype and COII haplotype, and exhibited the same genotypic pattern observed for fecundity. These results suggest that an interaction is occurring between PGI and COII. The effect of natural enemies was greatest at low and mid elevation sites, but was lowest at high elevation sites, where .beetle egg and larval survival was low regardless of whether they were exposed to enemies. This study may help us predict how individuals with distinct genetic backgrounds adapt to environmental change, and advance our understanding of how organisms persist in challenging environments.