Abstract
Theory predicts that herbivory should primarily determine the evolution of herbivore‐induced plasticity in plant defenses, but little is known about the influence of other interactions such as pollination. Pollinators may exert negative selection on the herbivore‐induced plasticity of chemical defenses when floral signals and rewards are indirectly affected, provoking deterrent effects on these mutualists. We investigated the influence of constant herbivory and pollination on the evolved patterns and degree of herbivore‐induced plasticity in chemical plant defenses and floral morphometry and volatiles in fast‐cycling Brassica rapa plants. To do this, we used plants from an evolution experiment that had evolved under bee/hand pollination and herbivory manipulated in a 2 × 2 factorial design during six generations, producing four selection treatments. We grew sibling plant pairs from each of the four selection treatments of the last generation and infested one group with herbivores and left the other uninfested. Herbivore‐induced plasticity was analyzed within‐ and between‐selection treatments. We found support for the hypothesis that constant herbivory favors the evolution of higher constitutive yet lower herbivore‐induced plasticity in defenses. However, this only occurred in plants that evolved under hand pollination and constant herbivory. Bee pollination had a strong influence on the evolution of herbivore‐induced plasticity of all traits studied. Plants that evolved under bee pollination, with and without constant herbivory, showed remarkably similar patterns of herbivore‐induced plasticity in their defense‐ and floral traits and had a higher number of plastic responses compared to plants with hand pollination. Such patterns support the hypothesis that bee pollination influenced the evolution of herbivore‐induced plasticity, most likely via indirect effects, such as links between defense‐ and floral traits. We conclude that interactions other than herbivory, such as pollination, may impact herbivore‐induced plasticity, through indirect effects and metabolic trade‐offs, when it contributes to trait evolution in plants.
Highlights
Theory predicts that herbivory should primarily determine the evolution of herbivore-induced plasticity in plant defenses, but little is known about the influence of other interactions such as pollination
Plants that evolved under bee pollination, with and without constant herbivory, showed remarkably similar patterns of herbivore-induced plasticity in their defense- and floral traits and had a higher number of plastic responses compared to plants with hand pollination
Plants that evolved under bee pollination and no herbivory (BH–) showed the highest number of plastic glucosinolates, whereas only one leaf glucosinolate was plastic in plants that evolved under hand pollination and constant herbivory (HH+) (Table 1)
Summary
Theory predicts that herbivory should primarily determine the evolution of herbivore-induced plasticity in plant defenses, but little is known about the influence of other interactions such as pollination. Plants that evolved under bee pollination, with and without constant herbivory, showed remarkably similar patterns of herbivore-induced plasticity in their defense- and floral traits and had a higher number of plastic responses compared to plants with hand pollination. Such patterns support the hypothesis that bee pollination influenced the evolution of herbivore-induced plasticity, most likely via indirect effects, such as links between defense- and floral traits. A prominent example of phenotypic plasticity in plants is changes that occur in chemical defenses and floral traits triggered by herbivory Such herbivore-induced plasticity has mostly been studied in terms of mechanisms and ecological consequences. Phenotypic plasticity, the change in an organism’s phenotypic characteristics in response to an environmental signal (Schlichting and Smith 2002), is thought to evolve as a mechanism to express adaptive phenotypes in variable environments and stressful conditions, allowing organisms to sustain fitness (Agrawal 1999; DeWitt et al 1999; Levis and Pfennig 2016)
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