Sexual communication variation

Contrary to the structure similarity of the pheromones secreted by taxonomical related moths, some differences are necessary for their sexual communication systems to play an important role in their reproductive isolation. In addition to further modifications of the various structures, diversity of the lepidopteran sex pheromones is generated by blending multiple components. Innumerable pheromone blends are based not only on combinations of different components but also on variations in the mixing ratio. A pioneer study with Adoxophyes spp. (Tortricidae Tortricinae) had already proposed this concept in the early 1970s. While the smaller tea tortrix (A. honmai) and the Japanese summerfruit tortrix (A. oranafasciata) had been considered to be variant strains with different host preferences in the same species, Tamaki et al. found that females of the former pest insect in the tea garden secreted Z9-14 OAc and Zll-14 OAc in a ratio of 7 4 but females of the latter defoliator of apple trees secreted them in a ratio of 13 4 [127,128]. Furthermore, two other components (Ell-14 OAc and MelO-12 OAc) were subsequently identified from the former species [129]. 

This model has come to dominate our understanding of chemical communication in Diptera, in particular those signals involved in courtship and mating. In fact, most dipteran species do not fit the Musca model. Although some species, like houseflies, show a marked qualitative sexual dimorphism, many species express only quantitative variation for compounds shared by both sexes (Bartelt et al., 1986 Jallon and David, 1987 Toolson and Kuper-Simbron, 1989 Byrne et al., 1995), while in other species adults of both sexes appear to be virtually identical (Stoffolano et al., 1997 Howard et al., 2003). Furthermore, there are relatively few examples of the kind of rigorous demonstration of a pheromonal role for CHCs that was established in Musca (e.g. Carlson et al., 1984 Adams and Holt, 1987 Ferveur and Sureau, 1996 Lacaille et al., 2007). 

For any speciation process based on pheromonal communication to be effective, variation in CHC production should tightly co-evolve with factors regulating CHC processing. This has been observed for the desatl gene in D. melanogaster (Marcillac et al., 2005b). Such coevolution requires that pre-existing sensory structures can detect and respond to new CHC molecules. This basic assumption of the theory of sexual selection is supported by the observation that taste neurons normally used to detect bitter molecules also serve to detect an aversive sex pheromone (Lacaille et al., 2007). It is thus possible that taste neurons that were initially used by the fly to detect noxious food molecules (bitterness is often associated with alkaloids and toxic molecules) have been more recently used to detect inhibitory sex pheromones. 

A large number of studies have shown that the fragrant bouquets indicate not only the maturity, receptivity and location of the sexual partner but also its quality. Scent variation can even allow olfactory discrimination at the individual scale. It is this property that allows communication of information... 

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