Cyanogenesis in Arthropods: From Chemical Warfare to Nuptial Gifts

Chemical defences are key components in insect–plant interactions, as insects continuously learn to overcome plant defence systems by, e.g., detoxification, excretion or sequestration. Cyanogenic glucosides are natural products widespread in the plant kingdom, and also known to be present in arthropods. They are stabilised by a glucoside linkage, which is hydrolysed by the action of β-glucosidase enzymes, resulting in the release of toxic hydrogen cyanide and deterrent aldehydes or ketones. Such a binary system of components that are chemically inert when spatially separated provides an immediate defence against predators that cause tissue damage. Further roles in nitrogen metabolism and inter- and intraspecific communication has also been suggested for cyanogenic glucosides. In arthropods, cyanogenic glucosides are found in millipedes, centipedes, mites, beetles and bugs, and particularly within butterflies and moths. Cyanogenic glucosides may be even more widespread since many arthropod taxa have not yet been analysed for the presence of this class of natural products. In many instances, arthropods sequester cyanogenic glucosides or their precursors from food plants, thereby avoiding the demand for de novo biosynthesis and minimising the energy spent for defence. Nevertheless, several species of butterflies, moths and millipedes have been shown to biosynthesise cyanogenic glucosides de novo, and even more species have been hypothesised to do so. As for higher plant species, the specific steps in the pathway is catalysed by three enzymes, two cytochromes P450, a glycosyl transferase, and a general P450 oxidoreductase providing electrons to the P450s. The pathway for biosynthesis of cyanogenic glucosides in arthropods has most likely been assembled by recruitment of enzymes, which could most easily be adapted to acquire the required catalytic properties for manufacturing these compounds. The scattered phylogenetic distribution of cyanogenic glucosides in arthropods indicates that the ability to biosynthesise this class of natural products has evolved independently several times. This is corroborated by the characterised enzymes from the pathway in moths and millipedes. Since the biosynthetic pathway is hypothesised to have evolved convergently in plants as well, this would suggest that there is only one universal series of unique intermediates by which amino acids are efficiently converted into CNglcs in different Kingdoms of Life. For arthropods to handle ingestion of cyanogenic glucosides, an effective detoxification system is required. In butterflies and moths, hydrogen cyanide released from hydrolysis of cyanogenic glucosides is mainly detoxified by β-cyanoalanine synthase, while other arthropods use the enzyme rhodanese. The storage of cyanogenic glucosides and spatially separated hydrolytic enzymes (β-glucosidases and α-hydroxynitrile lyases) are important for an effective hydrogen cyanide release for defensive purposes. Accordingly, such hydrolytic enzymes are also present in many cyanogenic arthropods, and spatial separation has been shown in a few species. Although much knowledge regarding presence, biosynthesis, hydrolysis and detoxification of cyanogenic glucosides in arthropods has emerged in recent years, many exciting unanswered questions remain regarding the distribution, roles apart from defence, and convergent evolution of the metabolic pathways involved.