Genetically based variation in the expression of plant secondary compounds may have important consequences for insect herbivores, potentially affecting their population dynamics, evolutionary dynamics, and community structure. Variation in induced chemical defenses, which are plastic responses to herbivory, may have particularly interesting consequences because they are a function of herbivore density. Furthermore, genetic variation in inducibility, or the magnitude of plasticity in defensive expression, is required for plants to evolve an optimal response to variable herbivore environments. In this study, we examined the effect of genetic variation in the plastic expression of plant defensive compounds on the performance of an insect herbivore. We screened clonal lines of the perennial plant Solanum carolinense (Carolina horsenettle) for genetic variation in the constitutive expression, induced expression, and inducibility of three known defensive compounds: trypsin proteinase inhibitors (tryPI), cysteine proteinase inhibitors (cysPI), and polyphenol oxidase (PPO). We determined how variation in these traits affects the performance of Leptinotarsa juncta (false potato beetle), a specialist herbivore of S. carolinense, by comparing the relative growth rates of L. juncta larvae reared on undamaged versus damaged replicates of clonal lines.
Results/Conclusions
Analyses indicate that Solanum carolinense exhibits genetic variation in the expression of chemical defenses after damage, and this variation affects the performance of L. juncta. For example, we found genetic variation in both the induced expression and inducibility of trypsin proteinase inhibitors, but no variation in the constitutive expression of this compound. This genetic variation in tryPI expression translates to genetic variation in the induced resistance of S. carolinense to Leptinotarsa juncta: larvae had lower relative growth rates on S. carolinense lines with greater tryPI induction. Furthermore, L. juncta growth rates on individual plants were negatively correlated with typPI expression of leaves on that same plant. Thus, genetic variation in the induced defenses of S. carolinense may have significant effects on the performance, and potentially the population dynamics, of its insect herbivores. Our results support the findings of previous studies showing that the evolution of induced chemical defenses in plants is probably not constrained by a lack of genetic variation.
