Chemical ecology traits in an adaptive radiation: TPA-sensitivity and detoxification in Hyles and Hippotion (Sphingidae, Lepidoptera) larvae

Abstract

The larvae of several species in the hawk moth genus Hyles, including H. euphorbiae, feed on plants of the genus Euphorbia containing phorbol esters and are insensitive to addition of the standard phorbol ester, tetradecanoyl-phorbol-13-acetate (TPA) to their artificial diet. Specialised non-Euphorbia feeding larvae were sensitive, whereas polyphagous ones were insensitive if their natural food plant spectrum also includes Euphorbia. Larvae of Hippotion celerio, an out-group species with polyphagous larvae not using Euphorbia as food plants, were sensitive. A highly conserved sequence of the TPA binding site of the protein kinase C in H. euphorbiae and Hippotion celerio demonstrates that intoxication by phorbol esters is not avoided by preventing target binding. H. euphorbiae larvae showed no vitality loss after chemical destruction of their peritrophic matrix and subsequent TPA treatment. TPA fed larvae that had putatively piperonyl butoxide (PBO)-inhibited cytochrome P450 enzymes also showed no deficits, indicating that this is not the only detoxification pathway in H. euphorbiae. Based on these qualitative results, we postulate that proto-Hyles was polyphagous with the ability to use Euphorbia as food plants. The most ancestral Hyles species presumably remained polyphagous, with the ability to switch to the toxic food plants if necessary. Within the youngest, Palearctic radiation, the species specialised on non-Euphorbia plants subsequently lost detoxification abilities, albeit not to 100%. The species in South America, the origin of Hyles, can detoxify TPA, indicating this ability to be a plesiomorphic character state within the genus that enabled its adaptive radiation in the Palearctic, the distribution and diversity centre of Euphorbia.