Genotype and environment as determinants of intraspecific variation in quaking aspen phytochemistry and consequences for an insect herbivore (Populus tremuloides)

A primary focus in the study of plant-insect interactions is to understand the proximate and ultimate causes of intraspecific variation in phytochemistry that determines the distribution and abundance of insects on plants. Chapter One suggests that the striking pattern of variable defoliation among quaking aspen clones by insect herbivores under outbreak conditions is likely driven by quantitative variation in phenolic glycosides. Plant genotype, environment and gene by environment interactions affect phytochemistry and insects feeding on those plants. The relative importance of plant genotype and environment on plant-insect interactions, however, is poorly understood. The experiments applied strong levels of nutrients, light and defoliation, treatments previously shown to be important in determining phytochemistry in many systems. None of these environmental treatments had a strong effect on phenolic glycoside concentrations. Although the responses of phenolic glycosides to the environmental treatments was generally weak, Chapters Two, Three and Four established that aspen phytochemistry generally responded in the direction predicted by substrate-driven allocation models. Based on the results of Chapters Two, Three and Four, aspen genotypes varied widely in phenolic glycoside content and genotype was much more important than environment in establishing phenolic glycoside concentrations. Given the efficacy of phenolic glycosides in suppressing gypsy moth growth, the polymorphism in phenolic glycoside concentrations is surprising (i.e., why aren't all genotypes well defended if phenolic glycosides are effective resistance compounds?). Chapter Four suggests an evolutionary mechanism for the resistance polymorphism of aspen. A trade-off between allocation to phenolic glycosides and growth was evident under resource limiting conditions, offering support for the idea that production of phenolic glycosides in quaking aspen is costly. The apparent cost of phenolic glycoside production and conflicting selection pressure for both resistance and growth may have driven the evolution of allocation strategies that results in the polymorphism in allocation seen today.