Plant chemical defences play a central role in mediating interactions between plants and their enemies. Phytochemical diversity may be advantageous to reduce herbivore pressure, and plants commonly produce vast numbers of chemicals. However, the diversity of functional classes of defensive chemicals is often more limited and subject to strong phylogenetic constraints. Such functional conservatism may accelerate the evolution of tolerance in specialized herbivores, resulting in plant-herbivore systems dominated by specialists resistant to host plant defences. This presents major challenges for the study of phytochemically-mediated coevolution, as most systems lack the early stages of coevolutionary interactions that are crucially important to predict evolutionary trajectories. Occasionally however, the gain of functionally novel traits allows plants to escape their coevolved herbivores. The genus Erysimum (Brassicaceae) has gained functionally novel cardenolides in addition to ancestral glucosinolate defences, allowing it to escape several glucosinolate-adapted specialists. Making use of the unique natural and emerging molecular resources in this system, CARDEVOL will comprehensively evaluate the ecological, physiological, and evolutionary consequences of novel defences for the plant and its herbivores. CARDEVOL has four main objectives: 1) to characterize the full extent of natural variation in defence of a widespread Erysimum species and to identify environmental drivers; 2) to manipulate both defences and evaluate their contributions to plant fitness in the field; 3) to evaluate tolerance and resistance mechanisms of a community of non-adapted specialist herbivores towards the new defence; and 4), to evolve herbivores under artificial selection for increased resistance. CARDEVOL thus aims at pushing the boundaries of chemical ecology and transforming the field by elucidating the causes and consequences of phytochemical diversification involving gains of function.
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