Phenotypic plasticity is common in traits essential to fitness, including growth, reproduction, and defense. Although plasticity should be adaptive in variable environments, observations of natural populations indicate that trait plasticity may often not reach predicted optima. This may result from genetic constraints, such as costs of plasticity, lack of genetic variation, or strong genetic correlations among traits. The defense response of plants to herbivores, which includes tolerance and chemical/physical induced defenses, is a well-studied suite of plastic traits. While some defense traits are known to be correlated, little is known about costs of plasticity in plant induced defenses or potential genetic correlations among trait plasticities. In a field study, we examined the adaptive value and potential genetic constraints on the evolution of the defense response of the perennial plant Solanum carolinense. We grew genotypes of S. carolinense known to vary in induced resistance in environments with ambient and reduced levels of herbivores, then measured each genotype’s fitness and trait plasticities. We were then able to ask: a) is plasticity in defenses adaptive under ambient levels of herbivory? b) are there costs of plasticity for the defense response of S. carolinense? c) are there negative genetic correlations among plastic defense traits?
Results/Conclusions
Results from the field experiment reveal significant costs of tolerance and costs of plasticity in both trichome and stem spine production. Costs of plasticity were only present when herbivores were absent (plants remained undamaged). Several components of the plastic defense response were significantly genetically correlated, but the only strong negative correlation was between tolerance and plasticity in trichome density. Plasticity was not adaptive for any of the induced defenses measured under these experimental conditions (i.e., opposing selection among herbivore environments was not detected). Tolerance was also not found to be adaptive, but is predicted to be adaptive under higher average levels of herbivory than plants experienced in this experiment. Overall, these results indicate that the evolution of induced defenses in S. carolinense may be constrained by costs of plasticity rather than by strong genetic correlations, but that plasticity in defense is not adaptive. On the other hand, the evolution of tolerance, which is likely the most important response to herbivore damage for S. carolinense, is predicted to be adaptive under high levels of herbivory, may incur significant fitness costs in terms of both sexual and asexual reproduction, and may be strongly correlated with other plastic defense traits.