| Summary: | The ecological factors that maintain defensive chemical variation within and between plant species have intrigued ecologists for decades. While theory posits that polymorphisms may be maintained different forms of balancing selection, relatively few experimental studies have tested whether such balancing selection can maintain defensive chemical trait polymorphisms in nature. Further, evidence demonstrating balancing selection is rare for any trait. Here, I investigated a stereochemical trait polymorphism in Xanthium strumarium. This species is polymorphic with respect to the stereochemistry of the lactone ring junction of a prominent defensive compound class ? the sesquiterpene lactones. Individual plants typically produce only cis-fused or trans-fused lactones across their entire suite of compounds. Sesquiterpene lactones are known to influence feeding behavior and growth rates of various herbivores, but nothing is known about the ecological implications of variation in this stereochemical trait. I first examined whether sesquiterpene lactone stereochemical variation can influence folivore feeding behavior in the laboratory. Using pure sesquiterpene lactones in controlled feeding experiments, I found that laboratory-reared grasshoppers were less deterred by the cis-fused compounds than the trans-fused compounds. I then found that these patterns extended to the field: in common gardens, plants producing cis-fused lactones received more damage than plants producing trans-fused lactones. Additionally, folivore damage was negatively correlated with plant fitness. Taken together, these results indicate that herbivores can impose natural selection on this stereochemical trait polymorphism in nature. Finally, I found evidence that spatially variable selection leads to fitness patterns conducive to the maintenance of this polymorphism. Further, I found that the intensity of folivore damage across spatial scales predicted in which environments each morph outperformed the other, with plants producing cis-fused lactones achieving higher fitness than plants producing trans-fused lactones when herbivore pressure was low (and the reverse being true when herbivore pressure was high). This work demonstrates that relatively minor defensive chemical variation can have far-reaching impacts on the ecology and evolution of plant populations.
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