Hydrocarbons as pheromones

Several families of moths utilize hydrocarbons or epoxides of hydrocarbons as their sex pheromone. Oenocyte cells produce hydrocarbons that are transported through the hemolymph by lipophorin [71]. In a study using arctiid moths it was shown that sex pheromone hydrocarbons are transported on the same lipophorin particle as the hydrocarbons destined for the cuticular surface [ 17]. Therefore, specific uptake of the sex pheromone hydrocarbon occurred in pheromone glands [17]. Similar findings have been found with other moths [72-74]. The mechanism behind this specific uptake of one hydrocarbon from a potential pool of other hydrocarbons is unknown. 

Drosophila melanogaster is another dipteran where pheromone biosynthesis has been studied [92]. Adult sexually mature female D. melanogaster utilizes primarily Z7,Z11-27 H as a contact sex pheromone. The biosynthesis of this compound follows the biosynthesis of other hydrocarbon-derived pheromones (Fig. 3). It is biosynthesized in oenocytes [93], transported through the hemo-lymph by lipophorin [94], and deposited on the cuticle surface. Biosynthesis in the oenocytes follows a similar pathway [95] as that described for the house fly... 

As with the other insects studied that utilize hydrocarbon sex pheromones, once Z9-23 H is produced by oenocyte cells it is released into the hemolymph. Lipophorin is the transport protein that will move the hydrocarbon to cuticu-lar tissue [21]. It was found that about 24 h were required once Z9-23 H was induced to actual deposition on the cuticular surface [237]. As is the case with other insects selective partitioning of the sex pheromone was observed with relatively larger proportions of Z9-23 H being found on the cuticular surface than in other tissues [21]. 

Another dictyotalean genus, Dictyopteris, has been reported to produce an array of Cu cyclic or acyclic acetogenins derived from higher fatty acids (Stratmann et al. 1992). Examples include the hydrocarbons dictyopterene A (Fig. 1.6e) (Moore et al. 1968) and dictyopterene D [B1] (Fig. 1.6f) (Moore and Pettus 1971), which act as pheromones in sexual reproduction (Stratmann et al. 1992). The compounds are short lived and undergo facile degradative oxidation to yield compounds such as dictyoprolene (Fig. 1.6g) (Yamada et al. 1979) and dihydrotropone (Fig. 1.6h) (Moore and Yost 1973). In a tme exhibition of efficiency, these degradative products have also been shown to act as a chemical defense (Hay et al. 1998). 

The female produced sex pheromone of Aleochara curtula has been described to consist of a mixture of (Z)-7-henicosene and (Z)-7-tricosene [114]. The same compounds are reported to be used by young males as a kind of camouflage to avoid aggression from older males. Similarly, chemical camouflage by using hydrocarbons plays a role in the relations between the myrme-cophilous staphylinid beetle Zyras cones and the ant Lasius fuliginosus. The host worker ants never attack these beetles which show the same profiles of cuticular hydrocarbons as the ants [115]. 

Recently, two new facets have been added to scarab chemistry. A suite of unusual A9 10-allenic hydrocarbons like 86 has been identified among the cutic-ular hydrocarbons from several Australian melolonthine scarab beetles [184]. Though very low-level components in the related cane beetle Antitrogus parvu-luSy the major cuticular hydrocarbons in this species proved to be oligomethyl-docosanes like 87. Only the relative configurations of these compounds could be determined [185]. Whether these interesting hydrocarbons have a function as pheromones needs to be established. 

Schiestl, F. R, Ayasse, M., Paulus, H. D. et al. (2000). Sex pheromone mimicry in the early spider orchid (Ophrys sphegodes) patterns of hydrocarbons as the key mechanism for pollination by sexual deception. Journal of Comparative Physiology A 186 567-574. 

Moths in the families Geometridae, Arctiidae, and some Noctuidae utilize hydrocarbons or epoxides of hydrocarbons as their sex pheromones. Hydrocarbon biosynthesis occurs in oenocyte cells that are associated with either epidermal cells or fat body cells (Wigglesworth, 1970). Once the hydrocarbons are biosynthesized, they are transported to the sex pheromone gland by lipophorin (Schal et al., 1998). The hydrocarbons can be released directly in the case of some moths or they are transformed into epoxides by addition of oxygen across one of the double bonds. 

The //-alkanes usually range in chain length from 21 to 31 or 33 carbons. Hydrocarbons with fewer than 20 carbons commonly occur as pheromones, defensive compounds and intermediates to pheromones and defensive compounds, but their volatility makes them unsuited to function as cuticular components, n-Alkanes have been found on almost every insect species analyzed, and can range from less than one percent of the total hydrocarbons, as in tsetse flies (Nelson and Carlson, 1986 Nelson et al., 1988) to almost all of the hydrocarbon fraction, as in the adult tenebrionid beetle, Eurychora sp. (Lockey, 1985). Depending upon the species, they can consist of essentially only one major component, such as n-pentacosane in the American cockroach, Periplaneta americana (Jackson, 1972) to a series of //-alkanes, such as the series from C23 to C33 in the housefly, Musca domes-tica (Nelson et al., 1981), with trace amounts to C37 (Mpuru et al., 2001). In all cases, the odd-numbered alkanes predominate, due to their formation from mostly two carbon units followed by a decarboxylation (Blomquist, Chapter 3, this book). Small amounts of even-numbered carbon chain //-alkanes often occur, and presumably arise from chain initiation with a propionyl-CoA rather than an acetyl-CoA. Occasionally, gas chromatographic analyses reveal similar amounts of even-numbered chain //-alkanes and odd-numbered chain components. This is a red flag that the samples must be checked for contamination. 

Hydrocarbons (HC) act as pheromones in a variety of orders including the Dictyoptera (Jurenka et al., 1989 Schal et al., 1994 Lihoreau and Rivault, 2009), Coleoptera (Ginzel et al., 2003, 2006), Hymenoptera (Howard, 1993 Le Conte and Hefetz, 2008), Diptera (Carlson et al., 1971 Antony and Jallon, 1982 Blomquist et al., 1987) and several lepi-dopteran species (Roelofs and Carde, 1971 Millar, 2000). In most insects, they are present on the cuticle and are synthesized in large cells called oenocytes located within or under the abdominal integument (Diehl, 1975 Ferveur et al., 1997 Schal et al., 1998 Fan et al., 2003). In Lepidoptera, their synthesis occurs in tissues associated with the abdominal tegument (possibly oenocytes), and they are then released into a sex pheromone gland (Schal et al., 1998). 

A great deal of information on CHCs as chemotaxonomic characters and sex pheromones is available for Drosophilidae. Several species and populations within species have been identified on the basis of hydrocarbon patterns. Studies have shown that CHCs differing between close species or populations often act as pheromones and may participate in prezy-gotic isolation. This section presents examples in which sex-pheromone polymorphism has been used as a basis for quick determination of strain/species (see Table 7.1). In section four of this chapter we deal with the possible role of pheromones in speciation. 

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