Pheromone olfactory

The peripheral pheromone olfactory system in insects targets for species-selective insect control agents... 

Pheromones are powerful modulators of insect behavior. Since the isolation and identification of the first pheromone, (10E, 12Z)-hexadec-10,12-dien-l-ol, the sex attractant of the silk moth Bombyx mori, thousands of other insect pheromones have been identified. Our understanding of the sensory apparatus required for pheromone detection has also increased significantly. Coincidentally, B. mori was instrumental in many of these advances (see below). Volatile pheromones are detected by a specialized olfactory system localized on the antennae. The precise recognition of species-specific nuances in the structure and composition of pheromone components is essential for effective pheromone-based communication. The pheromone olfactory system of species studied so far exhibits remarkable selectivity towards the species-specific pheromone blend. Pheromones are emitted in low (fg-pg) quantities and are dispersed and greatly diluted in air plumes. Thus, pheromone olfaction systems are among the most sensitive chemosensory systems known. (Schneider et al., 1968). This chapter summarizes efforts (particularly over the past 10 years) to understand the molecular basis for the remarkable selectivity and sensitivity of the pheromone olfactory system in insects. The chapter will also outline efforts to design compounds that interfere with one or more of the early events in olfaction. 

Diversity in the structure and proportion of pheromone components is mirrored in the diversity of the proteins from the olfactory system. A specialized olfactory system is responsible for distinguishing the pheromone from other odorants in the environment. The high precision of the pheromone olfactory system becomes apparent when we compare closely related species whose pheromones differ in subtle ways. For example, Heliothis species have the same unsaturated aldehyde as the major pheromone component, but their pheromone signals differ in the structure and proportion of minor components (Table 16.1). Another example is seen with the gypsy moth (Lymantria dispar) and nun moth (Lymantria monacha), both of which respond to la. The blend produced by the nun moth consists mostly of lb, which is a powerful behavioral antagonist in the gypsy moth and is behaviorally inactive in the nun moth (Hansen, 1984). Stereochemical features play an important role in the molecular recognition of pheromone components. 

Table 16.1 Studies of pheromone olfactory systems. This table does not include data on general olfaction... 

Species Phero- Major two pheromone Emit- Res- Components of the pheromone olfactory... 

In particular, the importance of chirality in diverse pheromone systems has been reviewed recently (Mori, 1998). For example, olive fruit flies (Bactrocera oleae) emit racemic 2 the males detect the R enantiomer, while females detect the S (Haniotakis et al., 1986). The sex pheromone of the Osaka beetle (Anomala osakana) is 4a, while the closely related Japanese beetle (Popilia japonica) uses 4b (Table 16.1). Interestingly, 4a is a powerful behavioral antagonist in P. japonica (Tumlinson et al., 1977). The hemlock looper, Lambdina fiscellaria, responds only to (5R, 1 S)-5,I I -dimethylheptadecane. The enantiomer or the R/R or S/S diastereomers do not elicit electrophysiological or behavioral responses (Li et al., 1993). Table 16.1 gives an overview of those species where proteins from the pheromone olfactory system have been identified. 

Figure 16.1 The three levels of molecular recognition in the pheromone olfactory system of insects. Pheromone adsorbs on the cuticle, where it enters the sensillum lymph through pores (1). The first level of molecular recognition occurs when the PBP binds and desorbs the pheromone from the cuticle (2). PBP transports the pheromone through the lymph to the receptor, where the second level of recognition occurs (3). The third level of recognition involves the pheromone-degrading enzymes, which rapidly inactivate pheromone that has dissociated from the PBP (4). PBP-pheromone and/or pheromone alone may also be removed by an endocytotic process, possibly mediated by SNMP (5). Finally, intracellular enzymes may be involved in further removal of pheromone (6).

Figure 16.3 Recognition of conformation by the pheromone olfactory system of insects. A Rotamers around the single C-C bond once removed from the Z-double bond fall into two groups. Some rotamers are mimicked by the R-cyclopentene analog and others by the S-analogue. B Steric interactions between the adjacent methyl groups impose a subtle twist on 9, the pheromone of the dried fruit beetle. The insects have adapted their olfaction precisely to the shape of 9 an increase or decrease of the twist results in loss of activity.

Research suggests there are roles for pheromones in the lives of humans as well. For example, studies have shown that the phenomenon of menstrual synchronization among women who live or work with each other is likely caused by pheromones. Olfactory sensitivity to musk, which includes steroids such as androsterone, large cyclic ketones, and lactones (cyclic esters), also varies cyclically in women, differs between the sexes, and may influence our behavior. Some of these compounds are used in perfumes, including cive-tone, a natural product isolated from glands of the civet cat, and pentalide, a synthetic musk. 

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