Tasting in plant-feeding insects: from single compounds to complex natural stimuli
Tasting plant compounds is of crucial importance for correct decision-making by plant-feeding insects in the fi nal phase of the behavioural sequence that leads to acceptance or rejection of a plant for feeding or oviposition (Schoonhoven et al., 2005). To understand the physiological and molecular bases of insect-plant interactions, investigations of insect taste are indispensable. Since the 1960s this notion has laid the foundation of approaches to identify the chemosensory mechanisms that underlie the high degree of dietary specialisation that is so characteristic for insectplant interactions. Major issues were the characterisation of taste neurones in terms of their specifi city ranges or ‘tuning breadth’ (Schoonhoven, 1987) and coding principles operating in taste systems of plant-feeding insects (Dethier, 1982; Van Loon, 1996). Over the last decade research activity devoted to taste in plant-feeding insects has declined whereas increased attention for olfaction can be noted (Christensen and Hildebrand, 2002 ; Hallem et al., 2006). There are several quite different reasons for this development, one of which is the general shift in attention in biology from the organismal to the molecular level. Considerable progress has since been made in understanding the molecular basis of ligand recognition by gustatory neurones. Molecular recognition of plant compounds by taste neurones resides in the expression of one or a few seven-transmembrane receptor proteins in each taste neurone (reviewed by Hallem et al., 2006). A family of 60 gustatory receptor genes which encode 68 receptor proteins was identifi ed in D. melanogaster by bioinformatic and molecular methods (Clyne et al., 2000; Robertson et al., 2003). The ligand spectrum for one of these taste protein receptors has recently been reported (Jiao et al., 2007).