Mechanisms of Olfaction in Insects - ResearchSpace@Auckland ...

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General Introduction 6 Cactoblastic cactorum (Pophof et al., 2005) in electrophysiological studies. Sensilla auricillica is a shoehorn shaped, porous sensilla that is innervated by the branched dendrites of three neurons. They are involved in detecting plant volatiles (Anderson et al., 2000) and sex pheromone components in Cydia pomonella (Ebbinghaus et al., 1998; Ansebo et al., 2005). Sensilla chaetica is similar to the trichoid sensilla except that it has thicker walls and the base has a flexible circular membrane (Schneider, 1964). This sensilla type are either devoid of cuticular pores with a function in mechano-reception, or have only one pore at their tip and function in gustatory reception (Keil, 1999). Sensilla styloconica are short, devoid of pores with a peg-like shape and function as thermo– or hygro– receptors (Shields and Hildebrand, 2001). The last sensilla type, sensilla coeloconica are very short, double walled, shaped like peg and are located in pits below the antennae cuticle. The sensilla are aporous but the double wall is not fused hence odours can move through it. This sensilla type is innervated by dendrites of five neurons (Altner et al., 1977) and are involved in detection of aliphatic acids and aldehydes in B. mori and C. cactorum (Pophof, 1997; Pophof et al., 2005). The cascade of events that leads from the detection of an odorant present in the environment and its conversion into a neuronal signal in the antennal lobe of the moth is still unclear. A proposed mechanism of the perireceptor events that follow detection of an odorant is as follows (Kaissling, 2009). The hydrophobic odorant molecule enters the sensillum lymph via waxed pores in the cuticular wall of the sensilla. This hydrophobic molecule has to travel through the aqueous layer to the membrane bound receptors in order for signal transduction to occur. Soluble proteins present in abundance in the lymph may function in transporting the odorants by forming complexes. The binding of the odorant to the membrane receptor activates a series of signalling cascades in the ORN causing action potentials to be generated. This results in the conveyance of the signal to the antennal lobe and processing in higher brain centres. Odorants need to be cleared from the olfactory system once the signal has been relayed to the ORNs. This ensures the sensitivity and specificity of the system and the detection of new odorants or different concentrations of odorant plumes that direct the moths‟ flight (towards or away from attractants and repellents) (Kaissling, 1974; Vogt, 1987; Prestwich et al., 1989; Kaissling, 2009).
General Introduction 7 1.5 Components of the olfactory system The identification of several protein components of the olfactory system in insects such as fruit flies and moths has paved the way for further detailed study of these proteins and the peripheral events involved in signal transduction. The identification, isolation and functional analysis of these proteins are described below. 1.5.1 Proteins found in the sensillum lymph Abundant in the sensillum lymph are small, soluble proteins, ranging in size from 14– 16 kDa called odorant binding proteins (OBP). These proteins are produced in the sensillum support cells and secreted into the lymph (Vogt and Riddiford, 1981). The first OBP to be identified was a 15 kDa male antennal specific protein, unique to the pheromone sensitive sensilla, that bound sex pheromones in the wild silkmoth Antheraea polyphemus (Vogt and Riddiford, 1981). Due to its close association with sex pheromones, it was called a pheromone binding protein (PBP). PBPs are generally found at high concentrations (10 mM) in male insect antennae (Klein, 1987), but some PBPs have also been identified in female moth antennas (Callahan et al., 2000). Low levels of PBP have been identified in some Lepidoptera species while high expression levels of PBPs have been shown in members of the Noctuoidea (Klein, 1987; Callahan et al., 2000). It was not until 1991 when the first evidence of multigene families of OBPs emerged with the identification of two groups of general odorant binding proteins (GOBP), namely GOBP1 and GOBP2; proteins that are not sex-biased and have highly conserved regions within members of the groups (Vogt et al., 1991; Krieger et al., 1993; Krieger et al., 1996). Most PBPs were identified and characterised by their binding to radio-labelled versions of known sex pheromone components. GOBPs on the other hand have been less studied in terms of their binding partners. In 1997, Feng and Prestwich deduced the ligand of GOBP2 from Manduca sexta by competitive displacement of tritium-labelled diazoacetate pheromone analog, [ 3 H]-6E,11Z-16:Dza, by the plant odorants (Z)-3-hexen-1-ol, geraniol, geranyl acetate and limonene (Feng and Prestwich, 1997). In a recent study of OBPs from B. mori, GOBP2 was shown to bind the sex pheromone of B. mori, bombykol and also to be able to discriminate it from its antagonist, bombykal (Zhou et al., 2009). The specific function of OBPs is still unclear but several roles of OBPs
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Page 3 and 4: Abstract Abstract Olfaction or the
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3.1 Introduction 3 The roles of Epi
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4.1 Introduction 4 Identification o
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Identification of putative odorant
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Microarray OR 454 Transcriptome OR
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5.1 Introduction 5 Concluding Discu
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Concluding Discussion 129 redundant
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Concluding Discussion 131 against t
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Concluding Discussion 133 sequencin
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Appendix B 135 Buffered TB (Sambroo
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Appendix C 137 C. Appendix C - Clus
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Appendix D 139 D. Appendix D - Solu
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Appendix E 141 I II
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Appendix E 143 V
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Appendix E 145 VII continued Figure
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References 147 Ansebo, L., Ignell,
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References 149 Butenandt, A., Beckm
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References 151 Fedurco, M., Romieu,
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References 153 Große-Wilde, E., St
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References 155 Hrdy, I., F. Kuldova
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References 157 Kaissling, K. E. (19
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References 159 Krieger, J., Große-
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References 161 Lopez-Gomez, R. and
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References 163 Miura, N., Nakagawa,
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References 165 Pell, J. K., Macaula
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References 167 Røstelien, T., Stra
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References 169 Stowers, L. and Loga
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References 171 Turner, C. T., Davy,
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References 173 Vogt, R. G., Riddifo
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References 175 Widmayer, P., Heifet
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