Hoverfly glutathione S-transferases and effect of Brassicaceae secondary metabolites

The Episyrphus balteatus hoverfly is commonly found in very diversified biotopes including Brassicaceae crop fields. Glutathione S-transferases (GST) are detoxification enzymes that are involved in the metaboliza-tion of plant allelochemicals such as glucosinolates and related volatile products (mainly isothiocyanates) from Brassica plants. Developmental expression of E. balteatus glutathione S-transferases was studied. With regard to the high level of glutathione S-transferase activity in syrphid adults, the latter stage was also used to assess the glutathione S-transferase distribution in several parts of the insect. Higher glutathione S-transferase activities were found in the thorax (47.5 and 38.5%) followed by the abdomen (32.4 and 32.9%) and the head (20.4 and 28.5%) with l-chloro-2,4-dinitrobenzene, and 2,4-dinitro-l-iodebenzene, respectively. Analysis of Brevicoryne brassicae, the cabbage aphid, by GC-MS revealed the presence of isothiocyanates. The latter are the main degradation products of the glucosinolate from the host plant at neutral pH. Feeding the predators with this aphid species and with the generalist Myzus persicae reared on Brassica napus induced glutathione S-transferase activity. Impact of volatiles from the host plant on the glutathione S-transferase activity was also determined by exposure of syrphid larvae to isothiocyanates (allyl- and benzyl-isothiocyanates). Enzyme inductions were observed only from a 1 ppm concentration. Enzymatic activity evolution was discussed as an adaptation to cope with allelochemicals from plants. Finally, the glutathione 5-transferase activities of several hoverfly species were compared in relation to their feeding behavior (coprophagous, saprophagous, and aphidophagous) at the larval stages. The results seem to indicate quantitative and qualitative differences of glutathione S-transferases according to the feeding behavior of the larvae.