Cuticular hydrocarbons species

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. 

It has been suggested that the ethers, compounds unique to spiders, may provide reliable signals for pattern recognition and species determination. In contrast, a pattern of hydrocarbons, as used in several insect species, might be susceptible to contamination from cuticular hydrocarbons from insect prey remnants, which might alter the blends produced by the spiders and deposited on the webs (Schulz, 1997a, 1999). 

Parallel to studies in D. melanogaster, there have been searches for pheromones in other Drosophila species. In the sibling species D. simulans, no sexual dimorphism was found among cuticular hydrocarbons. 7-T is the major compound for both sexes in most strains, although 7-P is more abundant in a few strains living around the Benin Gulf (see section 9.2). Jallon (1984) established that synthetic (Z)-7-T applied on dummies could stimulate the wing vibration behavior of... 

Coyne J. A. (1996a) Genetics of a difference in male cuticular hydrocarbons between two sibling species, Drosophila simulans and D. sechellia. Genetics 143, 1689-1698. 

Jallon J. M. and David J. R. (1987) Variations in cuticular hydrocarbons among the eight species of the Drosophila melanogaster subgroup. Evolution 4, 294—302. 

Central to investigations of the biosynthetic pathway and regulation of the contact pheromone of B. germanica was the observation that the major cuticular hydrocarbon in all life stages of this species is an isomeric mixture of 3,7-, 3,9-and 3,11-dimethylnonacosane (Jurenka el al., 1989). The presence of only the 3,11-isomer in the cuticular dimethyl ketone fraction and only in adult females prompted Jurenka et al. (1989) to suggest that production of the pheromone might result from the female-specific oxidation of its hydrocarbon analog. This scheme follows the well-established conversion of hydrocarbons to methyl ketone and epoxide pheromones in the housefly (Blomquist et al., 1984 Ahmad et al., 1987). 

Carlson, D.A. and Service, M. W. (1979). Differentiation between species of the Anopheles gambiae Giles complex (Diptera Culicdae) by analysis of cuticular hydrocarbons. Ann. Trop. Med. Parasitol., 73, 589-592. 

Howard, R. W., McDaniel, C. A., Nelson, D. R Blomquist, G.J., Gelbaum, L.T. and Zalkow, L.H. (1982a). Cuticular hydrocarbons of Reticulitermes virginicus (Banks) and their role as potential species- and caste-recognition cues../. Chem. Ecol., 8, 1227-1239. 

The hundreds of different cuticular hydrocarbon components reported on insects can be divided into three major classes, n-alkanes, methyl-branched components and unsaturated hydrocarbons. There are reports of methyl-branched alkenes (see below), but these are rare. The hydrocarbon components on the surface of insects are usually complex mixtures comprised of anywhere from a few to up to hundreds of different components in some species. 

Dimethylalkanes have been identified in numerous insect species, and the major components usually have the methyl groups on odd-numbered carbons. A common motif is noted with the methyl groups separated by three methylene groups, but the number of methylene groups between methyl branches can be 5, 7, 9, 11, or 13 (Blomquist et al., 1987 Lockey, 1985). Cuticular hydrocarbons with adjacent methyl groups have been reported, but no cases have been unambiguously confirmed (Blomquist et al., 1987). Likewise, methyl-branched alkanes with one methylene group between the methyl branches are rare. 

It is generally accepted that insects synthesize a majority of their cuticular hydrocarbons (Nelson and Blomquist, 1995), although studies have shown that dietary hydrocarbons are incorporated into cuticular lipids (Blomquist and Jackson, 1973a). However, for most species it appears that dietary lipid accounts for very small amounts of insect cuticular hydrocarbon. Some inquilines, which use cuticular hydrocarbons in chemical mimicry, synthesize hydrocarbons with a composition very similar to those of their host termites (Howard et al., 1980 see also Chapter 14). A number of studies with widely diverse insect species have established that the major site of hydrocarbon biosynthesis occurs in the cells... 

A number of studies with widely diverse species have established that the major site of cuticular hydrocarbon synthesis is within the cells associated with the epidermal layer or the peripheral fat body, particularly the oenocytes. In Schistocerca gregaria, Diehl (1973, 1975) separated the oenocyte-rich peripheral fat body from the central fat body tissue and observed the highest rate of hydrocarbon synthesis in the oenocyte-rich peripheral fat body. In Tenebrio molitor, Romer (1980) demonstrated that isolated oenocytes efficiently and specifically incorporated [14C]acetate into hydrocarbon. Similar studies in Periplaneta americana (Nelson, 1969), Sarcophaga bullata (Arnold and Regnier, 1975), and Musca domestica (Dillwith et al., 1981) demonstrated that hydrocarbon synthesis occurs primarily in the epidermal tissue. 

The transport of hydrocarbons by social insects can be involved in creating the hydrocarbon signature . Evidence was first obtained in the termite Zootermopsis nevaden-sis (Sevala et al., 2000). Comparison of cuticular lipids with internal and hemolymph hydrocarbons in different castes showed that, as in other species, the content was qualitatively similar. However, quantitative differences were observed between hemolymph and cuticular hydrocarbon profiles. Sevala et al. (2000) showed that hemolymph hydrocarbons were associated with a dimeric high-density lipoprotein (HDLp) lipophorin, similar to those described from other insects (see above). This lipoprotein consisted... 

Like other insects, lepidopterans bear cuticular hydrocarbons to protect them against desiccation. These hydrocarbons can be used as taxonomic markers (Table 7.6). In the Danaus genus, D. erippus and D. plexippus show reproductive isolation. Some taxonomists view them as separate species, but others consider D. erippus as a subspecies of D. plexippus. Hybridization experiments showing prezygotic and postzygotic isolation, as well as differences in CHC chromatograms, strongly support the hypothesis that the two species are separate (Hay-Roe et al., 2007). 

Coleoptera. A recent work on chrysomelidae (Peterson et al., 2007) evaluated the evolution of sexual isolation between two leaf beetles, Chrysochus cobaltinus and C. auratus, in a hybrid zone in Washington state (USA). By painting beetle cadavers with various cuticular extracts, the authors demonstrated a strong male preference for conspecific females according to species and sexual chemical specificity of their respective cuticular hydrocarbon profiles. This male mate choice reinforced sexual isolation. 

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