Intraspecific phytochemical variation: The ecological and evolutionary dynamics of chemical interactions
Host-consumer relationships and studies of host shifts are important for understanding the role of diversification and ecological specialization in herbivorous insects. Chemically-mediated interactions between plants and herbivores are a critical aspect of diet breadth and have been a focal point of empirical research. Conversely, little research has focused on the mechanisms underlying diversification of consumers on a single host plant and the influence of individual chemical variability. Chemical variation between host populations may create isolated microhabits via shifts in herbivore diet due to changes in the concentration of individual secondary metabolites. Piper kelleyi, a tropical plant endemic to the Eastern Andes, hosts several morpho-species of Eois caterpillars and provides an ideal system to explore specific questions focused on chemical ecology: 1) Does phytochemistry vary among and between sub-populations of the host plant P. kelleyi? 2) Is phytochemical variation within P. kelleyi correlated with genetic structure in local sub-populations of Eois? 3) Is phytochemical variation predicted by elevation or geographic site? We sampled ten sites that were separated by water drainages, mountain ranges and/or elevation. Each site was replicated four times, with randomly selected plots. Within plots, each plant was sampled for Eois caterpillars.
Population genomics was utilized to examine the landscape genetic structure of specialist Eois caterpillars in relation to the chemically variable microhabitats of Piper kelleyi. We prepped 288 individual caterpillars and processed them using next-generation Illumina sequencing. Preliminary results suggest that the two most common morpho-species are genetically indistinguishable and that gene flow may be restricted between the high elevation populations and all others. Additionally, we have identified and isolated three novel secondary metabolites in the leaves of P. kelleyi using modern spectroscopy techniques, including NMR and HPLC: a prenylated benzoic acid, a chromene and a dimeric chromane. Together these compounds compose ~10% of the dry weight of the leaf material and make-up >98% of the compounds present in the crude extract. The absolute concentration of the acid, chromene and dimer in each individual P. kelleyi plant that was sampled was quantified using NMR and HPLC-DAD with an internal standard. Preliminary results suggest that chemical variation among individual plants and locations may explain the isolation of high elevation populations and appears to affect the community assemblage of caterpillar morpho-species.