Similar molecular receptive range of five plant odour receptor neurone types in three heliothine moth species

One major question in olfaction is which volatile chemicals the receptor neurones (RNs) detect. The complexity of the blends and instability of the compounds in the chemical environment of an organism, make the identification of important odorants difficult. In our laboratory we have used gas chromatography linked to single cell recordings (GC-SCR) and to mass spectrometry (GC-MS) to identify odorants in naturally produced plant volatiles that are detected by the receptor neurones in herbivorous insects. In this study we have examined the RNs in the three heliothine moth species, the polyphagous Heliothis virescens (America) and Helicoverpa armigera (south Europe, Asia, Australia, Africa, eastern Pacific) and the oligophagous Helicoverpa assulta(Asia). Olfactory RNs responding to plant odorants have been classified in 20-30 types, according to the compounds eliciting responses. For five RN types presented here, the odorants have been chemically identified by GC-MS and retested with authentic material. Interestingly, the structure-activity relationships of all five RN types were similar in the three species. On of them, a major type in all three species, is very sensitive for the sesquiterpene (-)-germacrene D, present in leaves and flowers of host and non-host plants (Røstelien et al. Chem Senses 2000, Stranden et al. Chem Senses 2002, J Comp Physiol 2003, submitted). The ranking of the secondary responses to seven other related sesquiterpenes, including (+)-germacrene D with ten times lower effect, was similar in all three species. The other four RN types were co-located in the same sensilla, of which three types responded strongest to the inducible compounds E--ocimene, E,E--farnesene and E,E-TMTT (4,8,12-trimethyl-1,3,7,11-tridecatetraene), respectively (Røstelien et al. 2000, J Comp Physiol A, Stranden et al. 2003 Chemoecology, submitted). The fourth type responded strongest to geraniol, which is a common floral volatile. All five RN types were narrowly tuned, showing responses exclusively to a few structurally similar compounds out of the hundreds of volatiles present in all plant materials tested. No overlap of the molecular receptive range appeared. These results indicate the presence of similar types of plantodour RNs in the three heliothine moths, and suggest conservation or reappearance of functional similar olfactory receptor proteins, independent of the evolution of polyphagy and oligophagy in these species.