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Abstract
Plants produce an outstanding diversity of metabolites and many of these metabolites are hypothesized to be involved in coevolutionary interactions. Plants in the genus Passiflora and butterflies in the tribe heliconiine are model systems in the field of coevolutionary biology and a class of defensive metabolite, cyanogenic glycoside (CNglc), is hypothesized to mediate coevolutionary interactions within this system. CNglc are abundant and diverse within the genus Passiflora, however, little is still known regarding the microevolutionary forces that dictate the evolution of CNglc within Passiflora. Here, I describe research aimed at assessing the contribution of specialist heliconiine herbivores in the evolution of CNglc in Passiflora incarnata (purple passionflower). Specifically, I investigated predictions of the geographic mosaic theory of coevolution and investigate the relationship between CNglc, herbivory, and plant fitness. To achieve this, I sampled P. incarnata populations across a large latitudinal transect in the southeastern United States across which the presence and abundance of heliconiine herbivores vary. I then took a combination of experimental approaches toassess variation in defensive metabolites among populations. First, I utilized an untargeted NMR metabolomics methodology to identify metabolites associated with lepidopteran interactions and their corresponding latitudinal distributions. I found 43 metabolites or unknown NMR features that are associated with lepidopteran interactions, however, only 15 had a clinal distribution across latitude. Additionally, I identified multiple cyclopentenoids, which are precursors involved in CNglc biosynthesis in P. incarnata. In my next experiment, I investigated if CNglc varied over latitude and the influence of this defense on herbivory. I found a positive latitudinal distribution for CNglc with greater CNglc concentrations at high-latitude. Furthermore, CNglc was positively correlated with herbivory from a primarily specialist heliconiine herbivore community. Finally, I investigated if CNglc is under natural selection from herbivores. I measured natural selection in the presence and absence of herbivores to determine the effect they have on fitness and how this relates to CNglc. I found that this defense is not under natural selection, but the presence of herbivores influences the relationship of this trait to fitness. From this body of work, I show that cyanogenic glycosides mediate coevolutionary interactions with heliconiine specialists.