Pollinating bumblebees and butterflies help plants grow prettier flowers, but harmful herbivores don't, a new study shows. Instead, abundant harmful herbivores diminish local plants' attractiveness to pollinators but increase their energy spent producing offspring through autonomous self-pollination. This switch for the plants can happen in just a few generations. The results reveal the rapid evolutionary adaptations plants undergo in response to the combined selective pressures of interacting pollinators and herbivores - which has been difficult to study. Pollinators are attracted to elegant floral displays; plants that develop traits like colorful, fragrant and nectar-filled flowers, for example, benefit by increasing their reproductive potential. However, these same traits can also attract herbivores like caterpillars. Defensive traits, like the production of toxic compounds, can help ward off herbivores, but they may negatively impact floral displays that attract pollinators. While the evolutionary trade-offs imposed on plants by pollinators and herbivores are generally recognized, experimental studies have focused on the effects of both as single selective agents. As such, the combined impact of interacting organisms on plant evolution remains unknown. Sergio Ramos and Florian Schiestl address this question by investigating the evolution of floral traits, mating systems and plant defense in a population of fast-cycling Brassica rapa plants. Ramos and Schiestl split the plants into four groups and manipulated the presence or absence of bumblebee pollinators and caterpillar herbivores, allowing the plants to evolve in these selective conditions for six generations. The results show that B. rapa plants under selection by bumblebees alone evolved larger, more fragrant flowers; however, these traits became far less pronounced for plants in the presence of caterpillars. Furthermore, plants under selection by both bees and caterpillars evolved a greater ability to produce offspring through autonomous self-pollination. What's more, the evolution of these traits was rapid; different lines of the plant became visible after only six generations. In a related Perspective, Jon Agren discusses the study in more detail.
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Journal
Science