Counteradaptations of chrysomelid beetles to host plant defences over the ontogeny from larvae to adults
Final Report Abstract
Herbivorous insects have evolved various strategies to deal with the defence machineries 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 glucosionolate substrates come into contact with the myrosinase enzymes. Thus, insects feeding on Brassicaceae need to metabolise the glucosinolates in a way that they avoid the formation of toxic breakdown products. A fascinating diversity of insect counter-adaptations to the plant defence system has been revealed within the Lepidoptera, and in species of Hemiptera, Hymenoptera and one Coleoptera. In contrast, almost nothing was known about the detoxification of the glucosinolate-myrosinase system in different developmental stages of species such as other Coleoptera species, in which both larvae and adults feed on leaves of Brassicaceae or are feeding generalists. To explore the metabolism of glucosinolates in such species, feeding experiments were performed with larvae and adults of a leaf beetle specialist on Brassicaceae, Phaedon cochleariae, and a feeding generalist, Galeruca tanaceti (both Coleoptera: Chrysomelidae). Leaves of either a Brassicaceae (containing the glucosinolatemyrosinase system) or a non-Brassicaceae (lacking this dual defence), on which different glucosinolates or a respective solvent control were applied, were offered to the insects for up to 72 h. Subsequent analysis using ultra-high performance liquid chromatography coupled to quadrupole time of flight mass spectrometry (UHPLC-QTOF-MS/MS) revealed that larvae of the specialist finally convert the benzenic glucosinolates 2-phenylethyl glucosinolate, benzyl glucosinolate and 4-hydroxybenzyl glucosinolate to N-(phenylacetyl) aspartic acid, N-(benzoyl) aspartic acid and N-(4-hydroxybenzoyl) aspartic acid, respectively. These compounds could be identified in samples of larvae and their faeces and the structure of N-(benzoyl) aspartic acid was confirmed by nuclear magnetic resonance spectroscopy (NMR) after purification of this metabolite using semi-preparative HPLC. In addition, further intermediates of the detoxification pathway could be found in insects feeding on leaves treated with 4-hydroxybenzyl glucosinolate. Our findings indicate that larvae of P. cochleariae metabolise isothiocyanates or nitriles, formed from the hydrolysis of benzenic glucosinolates, to aspartic acid conjugates of aromatic acids. Fascinatingly, the same final metabolites were also found in larvae feeding on non-Brassicaceae treated with different glucosinolates, although these plants lack myrosinases. Thus, the predicted pathway can occur independently of the presence of plant myrosinases. The conjugation of compounds derived from benzenic glucosinolates with aspartic acid is a novel metabolic pathway that has not been described for other herbivores. In the generalist, more and other conjugation products were found. The identities of the proposed products still need to be confirmed. Moreover, glucosinolates with different side-chains are metabolised in distinct ways by both the specialist as well as the generalist. Thus, insects feeding on Brassicaceae as specialists or generalists use highly distinct ways of counter-adaptations and a surprising variation is found in glucosinolate-specific metabolism pathways.
Publications
-
(2020) Novel glucosinolate metabolism in larvae of the leaf beetle Phaedon cochleariae. Insect Biochemistry and Molecular Biology 124: 103431
Friedrichs J, Schweiger R, Geisler S, Mix A, Wittstock U, Müller C
