Elucidating the biosynthesis of the aphid sex pheromone

The female-produced sex pheromone for many aphid species comprises a mixture of the iridoids (4aS,7S,7aR)-nepetalactone (I) and (1R,4aS,7S,7aR)-nepetalactol (II). Plant-derived biosynthesis of I and II, in plant species such as Nepeta spp. (Lamiaceae) and Catharanthus spp. (Apocynaceae), respectively, has received considerable attention due to their role as precursors to many pharmacologically active compounds, including the indole alkaloids. To test the hypothesis that aphids biosynthesize I and II de novo similarly to that elucidated for plants, isotopic labelling techniques have been deployed with oviparous pea aphids, Acyrthosiphon pisum (Homoptera: Aphididae). A range of unlabelled and deuterium-labelled putative sex pheromone precursors were synthesised and applied to aphids via one of two methods; dietary incorporation or topical application. For feeding experiments, A. pisum oviparae were artificially fed using diet sachets. Under autumnal conditions, a simple aqueous 200 g/L D-sucrose solution, into which the compounds were dissolved, was found to sufficiently maintain oviparae. Topically-treated aphids were exposed to methanolic solutions of each compound by application of 0.2 µL onto their dorsal surface. Following treatment, the volatile organic compounds (VOCs) released by oviparae were collected by dynamic headspace collection and analysed by gas chromatography coupled to mass spectrometry (GC-MS) to confirm the presence and position of labelling. Using unlabelled derivatives to establish optimal VOC collection parameters, dietary incorporation of all deuterated putative precursors evaluated failed to achieve deuterium incorporation into lactol I and lactone II. Successful deuterium incorporation, as evidenced by the presence of molecular ions at m/z 172 and 170 (corresponding to 2H4-lactol I and 2H4-lactone II, respectively), was demonstrated following the topical application of [C-8,C-10]-deuterated C-8-oxidised geranyl and neryl derivatives. Conversely, deuterium incorporation was not evident using the citronellyl derivatives, highlighting the importance of 2,3-unsaturation during biosynthesis. Through the synthesis and application of [C-4,C-9-2H5]-labelled monoterpenyl derivatives, it was demonstrated that a kinetic isotope effect was not responsible for the lack of deuterium incorporation following application of deuterated monoterpenols and monoterpenals. To address unclarified aspects of lactol I and lactone II biosynthesis in aphids, such as precursor identity and cyclisation mechanism, the synthesis and purification of 8 hydroxymonoterpenol-8-β-D-glucoside and NADP2H probes was attempted but was unsuccessful. These results suggest that aphids follow a route to lactol I and lactone II that is also utilised by plants such as Catharanthus roseus (Apocynaceae). This work is expected to support future research into fully elucidating aphid sex pheromone biosynthesis, including identifying the genes, and therefore enzymes, involved. Future implications in the field of biotechnology-based iridoid synthesis and pest management are anticipated