Herbivory, inbreeding, and trichome density contribute to variation in Mimulus guttatus foliar volatile phenotype
Plants have evolved a variety of chemical and physical traits as mechanisms of defense against herbivores. The volatile compounds emitted by plant foliage play a well-established role in defense. Although plants constitutively release volatile compounds that can deter herbivores, herbivory also induces changes in volatile phenotype. Induced volatile compounds can repel herbivores by indicating reduced host plant quality, or they can function as an indirect defense by attracting herbivore enemies. Some plants also contain trichomes, leaf hairs that act as a physical obstacle that inhibits insect movement. Previous studies have found interactions between trichomes and other plant defense traits that range from synergistic to antagonistic in mediating insect behavior. Inbreeding is a common population-level process in plants that accentuates the phenotypic effects of deleterious recessive alleles and could disrupt biosynthesis of volatile compounds. We investigated the roles of herbivory, trichome density, inbreeding effects, and their interactions in foliar volatile production in the mixed-mating herb Mimulus guttatus. We collected volatile samples from M. guttatus in a 2x2x2 factorial design with herbivory (damaged or undamaged), trichome density (high or low), and pollination (inbred or outbred) as treatments. We quantified volatile compounds using a gas chromatograph coupled with a flame ionization detector.
The foliar volatile phenotype of M. guttatus comprised 21 compounds and exhibited significant quantitative and qualitative changes in response to herbivory. Total volatile mass was over 3 times greater in damaged than undamaged plants, and this effect was significant regardless of pollination regime and trichome density. This difference was primarily explained by increased emission of 6 of the most abundant compounds, suggesting that these compounds function in defense against herbivores. For the highest-mass compound, emission in undamaged plants was 2.3 times higher in low-trichome than high-trichome plants, indicating a potential tradeoff between these physical and chemical defense traits. Two other compounds exhibited a significant inbreeding effect. In particular, for both compounds emission was significantly higher in outbred than inbred damaged plants, and increased emission in response to herbivory was significantly higher for outbred than inbred plants. These findings suggest that inbreeding could inhibit increased production of these compounds as a defense response, potentially increasing vulnerability to herbivores. Future work will examine the role of these volatile phenotypic differences in resistance of M. guttatus to its specialist herbivore Junonia coenia. These results will have broader implications for the evolution and ecology of plant-herbivore interactions and the evolution of plant mating systems.