Counteradaptations of chrysomelid beetles to host plant defences over the ontogeny from larvae to adults

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Herbivorous insects have evolved various strategies to deal with the defences of their host plants. The plant family of Brassicaceae is well known for its glucosinolate-myrosinase defence system, in which toxic hydrolysis products such as isothiocyanates are formed once the substrate comes into contact with the myrosinase enzymes. Thus, insects feeding on Brassicaceae must metabolise the glucosinolates in a way that they avoid the formation of toxic breakdown products. A fascinating diversity of insect counteradaptations to the plant defence system has been revealed within the Lepidoptera, and in species of Hemiptera, Hymenoptera and one Coleoptera. In contrast, almost nothing is known about the detoxification of the glucosinolate-myrosinase system in different developmental stages of species such as chrysomelid beetles, in which both larvae and adults feed on leaves of Brassicaceae. Preliminary experiments indicate that the specialist chrysomelid Phaedon cochleariae uses a novel mechanism of dealing with the glucosinolate-myrosinase system which is distinct from all counteradaptations that have been discovered for other Brassicaceae-feeders. In this project, I aim to investigate the glucosinolate metabolism of two chrysomelid species, the mentioned specialist and a generalist, over ontogeny. Larvae versus adults will be studied to test whether the detoxification of the dual defence may differ between stages because of the major physiological reorganisation during metamorphosis. Furthermore, I aim to study the specificity of glucosinolate metabolism with regard to the different structures of glucosinolates. To answer these questions, feeding experiments will be carried out with labelled and unlabelled glucosinolates followed by chemical analysis of breakdown products. Additionally, imaging of sections of larvae and adults, taken after different feeding durations, using matrix assisted laser desorption ionisation-mass spectrometry imaging (MALDI-MSI) will reveal the localisation of intact glucosinolates and their metabolites in the insects, allowing to identify the speed with which glucosinolates are metabolised as well as the organs in which metabolism takes place. Finally, measurements of myrosinase activities in gut sections will reveal whether these enzymes are inhibited during the passage through the gut in larvae or adults. From the herbivore perspective, the results of the project will reveal novel insight in the impressive divergent evolution of insect adaptations to this dual plant defence system and will demonstrate in how far these adaptations change over ontogeny. From a plant evolutionary perspective, we may learn in how far the insect processing of different glucosinolates and dealing with the myrosinases may lead to the fact that plants synthesise such a large variety of glucosinolate structures and glucosinolate hydrolysis products.