Department of Biology, Norwegian University of Science and Technology, Trondheim 2005, NORWAY.
The sense of smell is crucial for most animal species. It is critical for food-finding, reproductive behaviour, predator-prey relationship, kin and mother-infant recognition, homing behaviour and nest finding. The importance of the olfactory systems is reflected in the proportion of the genome that is devoted to the olfactory receptor proteins, e.g. comprising 3-5% in human and mouse (Young and Trask, 2002; Zhang and Firestein, 2002). For a long time the human sense of smell was considered as the most enigmatic of our senses. An intriguing question was; what mechanism could explain our ability of recognizing and remembering more than 10 000 distinct odorants (Buck, 2004). Buck and Axel (1991) performed a breakthrough by the discovery of the large family of olfactory genes in the rat. Buck and Axel were in 2004 honoured with the Nobel Prize in physiology and medicine for this study and the following series of pioneering work on the subject. The knowledge about the olfactory genes is obviously important for studies of the function of the olfactory receptor neurons (RNs), both in solving the transduction mechanisms and the specificity of the RNs. In the search for which odorants the olfactory receptors are evolved, insects are suitable model organisms. Herbivore species are particularly interesting, since many of them share the same plant species and their survival depends on olfactory cues in locating their host for feeding and reproduction (mating and oviposition).
Many studies have been performed on tritrophic interactions, i.e. between plants, herbivores and herbivorous predators or parasitoids. Particularly interesting are the findings showing increased production and release of volatiles during caterpillar attack (Turlings and Benrey, 1998; Paré and Tumlinson, 1999; Dicke and Van Loon, 2000; Schmelz et al., 2003). These toxins have a negative effect on the development and survival of several cotton pest insects (Sharma and Agarwal, 1982; Stipanovic et al., 1990; Hedin et al., 1991). In Nicotiana species, the content of nicotine increases after herbivory or mechanical damage (Euler and Baldwin, 1996). These toxic plant metabolites are deterrents (inhibit feeding) to several pest insects and protect plants against predation (Bernays and Chapman, 1994).
The subfamily Heliothinae (Insecta; Lepidoptera; Noctuidae) constitute a large group of herbivore insects, of which the three important agricultural pest species Heliothis virescens, Helicoverpa armigera and Helicoverpa assulta were chosen for the present studies. H. armigera and H. virescens are both generalist feeders (polyphagous) exploiting a wide range of plant species across different families (e.g. Leguminosae, Solanaceae, Malvacea and Compositae) (Fitt 1989; Matthews 1991).
When the study of this thesis was initiated, hardly any work had been carried out on how plant odour information was encoded by the olfactory RNs in heliothine moths. The method of gas chromatography linked to single cell recordings (GC-SCR) was employed and improved for identifying naturally occurring plant odorants that are detected by single RNs and can be considered as biologically relevant. Three species of the subfamily Heliothinae were included in this work, the two polyphagous H. virescens and H. armigera and the oligophagous H. assulta. The American H. virescens is geographically separated from the other two species. H. armigera and H. assulta are partly sympatric in Asia and Australia.