Epicuticular hydrocarbons

As stated by Blomquist et al. (1998) in their chapter, the line of demarcation between glandular or cuticular release of semiochemical signals is not always clear . This statement echoes an earlier one by Blum (1985), who reported that insect exocrine glands consisting of modified epidermal cells located throughout the body could perform de novo biosynthesis and secretion of behavioral chemicals. Later, Blum (1987) put forth a unified chemoso-ciality concept proposing that epicuticular lipids carried numerous exocrine compounds and that the cuticle could be compared to a thin layer phase. Nevertheless, it is known that in various non-social insects epicuticular hydrocarbons are synthesized by modified cells often associated with the epidermis, the oenocytes (see above), and that these oenocytes can be located in several sites within insects. 

Cicindela) correlations with epicuticular hydrocarbons. J. Insect Physiol., 33, 677-682. 

Stennett, M.D. andEtges, W.J. (1997). Premating isolation is determined by larval rearing substrates in cactophilic Drosophila mojavensis. III. Epicuticular hydrocarbon variation is determined by use of different host plants in Drosophila mojavensis and Drosophila arizonae. J. Chem. Ecol., 23, 2803-2824. 

Toolson, E. C. (1984). Interindividual variation in epicuticular hydrocarbon composition and water loss rates of the cicada Tibicen dealbatus (Homoptera Cicadidae). 

Toolson, E.C. and Kuper-Simbron, R. (1989). Laboratory evolution of epicuticular hydrocarbon composition and cuticular permeability in Drosophila... 

Bagine, R.K.N., Brandi, R. and Kaib, M. (1994). Species delimitation in Macrotermes (Isoptera Macrotermitidae) Evidence from epicuticular hydrocarbons, morphology, and ecology. Am. Entomol. Soc. Am., 87, 498-506. 

Etges, W. J. and Jackson, L.L. (2001). Epicuticular hydrocarbon variation in Drosophila mojavensis cluster species../. Chem. Ecol., 27, 2125-2149. 

Hadley, N.F. and Schultz, T.D. (1987). Water loss in three species of tiger beetles (Cicindela) correlations with epicuticular hydrocarbons../. Insect. Physiol., 33, 677-682. 

Lorenzi, M.-C., Bagneres, A-G., Clement, J.-L and Turillazzi, S. (1997). Polistes biglumis bimaculatus epicuticular hydrocarbons and nestmate recognition (Hymenoptera Vespidae). Insect Soc., 44, 123-138. 

Dapporto, L., Theodora, P Spacchini, C., Pieraccini, G. and Turillazzi, S. (2004). Rank and epicuticular hydrocarbons in different populations of the paper wasp Polistes dominulus (Christ) (Hymenoptera, Vespidae). Insect. Soc., 51, 279-286. 

Markow, T. A. and Toolson, E. C. (1990). Temperature effects on epicuticular hydrocarbons and sexual isolation in Drosophila mojavensis. In Ecological and Evolutionary Genetics of Drosophila, ed. J.S.F. Barker etal. New York Plenum, pp. 315-331. 

Regulation of contact sex pheromone production in B. germanica operates at several levels, including (i) production of the 3,11-dimethylnonacosane precursor, (ii) its metabolism by female-specific oxidases to 3,ll-dimethylnonacosan-2-one, and (iii) transport and distribution of the pheromone to the epicuticular surface. Early in the life of an adult female, it appears that food intake is a major regulator of hydrocarbon production. Females produce hydrocarbons only when they feed, and experimentally starved females, or gravid females that feed less, produce significantly less hydrocarbon (Schal el al., 1994). Because a pool of precursor 3,11-dimethylnonacosane is required for pheromone to be made, little pheromone is produced when less hydrocarbon is available, for example in starved females. 

The eggs of B. germanica contain the same types of hydrocarbons as the hemolymph, HDLp, and cuticle of the adult female. Only 150 pg of hydrocarbons accumulate on the epicuticular surface whereas up to 450 pg accumulate within the female during the period of egg maturation (Fan et al., 2002). The internal hydrocarbons are divided primarily between the ovaries, fat body, and 150 pg of HDLp-bound hydrocarbons in the hemolymph. During oocyte maturation ovarian hydrocarbons increase by more than 65-fold - from 3.5 pg on day-1 to 232 pg on day-8 (Fan et al., 2002). However, after oviposition on day-9, ovarian hydrocarbons decline to only 8.2 pg, demonstrating that hydrocarbons were associated with the ovulated oocytes. Radiotracing results indicate that they serve as components of the cuticular lipids of the embryos and first instars (Fan and Schal, unpublished results). 

The ability of insects to withstand desiccation was recognized in the 1930s to be due to the epicuticular layer of the cuticle. Wigglesworth (1933) described a complex fatty or waxy substance in the upper layers of the cuticle which he called cuticulin . The presence of hydrocarbons in this wax of insects was suggested by Chibnall et al. (1934) and Blount et al. (1937), and over the next few decades the importance of hydrocarbons in the cuticular wax of insects was established (Baker et al., 1963 and references therein). The first relatively complete chemical analyses of the hydrocarbons from any insect, the American cockroach, Periplaneta americana (Baker et al., 1963), occurred after the development of gas-liquid chromatography (GLC). The three major components of the hydrocarbons of this insect, //-pen taco sane, 3-methylpentacosane and (Z,Z)-6,9-heptacosadiene, represent the three major classes of hydrocarbons on insects, n-alkanes, methyl-branched alkanes and alkenes. Baker and co-workers (1963) were able to identify n-pentacosane by its elution time on GLC to a standard and its inclusion in a 5-angstrom molecular sieve. 3-Methylpentacosane... 

Aliphatic hydrocarbons such as n-alkanes and n-alkenes have been successfully used to distinguish between algal, bacterial, and terrestrial sources of carbon in estuarine/coastal systems (Yunker et al., 1991, 1993, 1995 Canuel et al., 1997). Saturated aliphatic hydrocarbons are considered to be alkanes (or paraffins) and nonsaturated hydrocarbons which exhibit one or more double bonds are called alkenes (or olefins)—as indicated in the simple structures of hexadecane and 1,3-butadiene, respectively (figure 9.7). It should also be noted that, n-alkanes tend to be odd-numbered as they result from enzymatic decarboxylation of fatty acids. Long-chain n-alkanes (LCH) (e.g., C27, C29, and C31) are generally considered to be terrestrially derived, originating from epicuticular waxes... 

Apart from the n-alkanes discussed above, epicuticular wax hydrocarbons contain sesqui- and diterpenoids. These compounds are based on the structural skeletons of cadinane and abietane, respectively, which are shown in Fig. 7-24. The sesquiterpenoids recovered by Simoneit and Mazurek (1982) in the rural aerosol were calamenene, tetrahydrocadalane, and cadalene. These compounds presumably are degradation products of cadinane derivatives (various isomers of cadinenes and cadinols), which are ubiquitous in essential oils of many higher plants (Simonsen and Barton, 1961). The major diterpenoid hydrocarbons observed in the aerosol samples were dehy-droabietane, dehydroabietin, and retene. The main source of abietane derivatives are coniferous resins. The parent compounds dehydrate fairly rapidly to yield the more stable hydrocarbons found in the aerosols. These may then serve as markers for hydrocarbons arising from vegetation, in addition to the odd-to-even carbon number preference in the n-alkanes. 

In plant tissues Cieo can undergo some other modifications (Figure 3), namely elongation, hydroxylation, oxidation, epoxydation, reduction, oxidative decarboxylation, etc. As a result of these modifications many different lipophilic substances are produced. Among these substanees very long-chain FAs (VLCFAs, C>2o), different unusual FAs (hydroxy-, epoxy-, acetylenic, dicarboxylic), fatty aldehydes and alcohols, hydrocarbons, oxilipins, etc. are formed. Some of them are present in plants in free form (are embedded in the complex cuticular lipid matrix or as a components of epicuticular waxes), the others are used as a substrates for more complex lipids and lipid polymers biosynthesis (see below). 

Brancned, cyclic and unsaturated hydrocarbons m higher plants are formed from appropriate fatty acids by decarboxylation (cf. Kolattukudy, 1980). The process has also been studied in cyanobacteria, insects and other species (cf. Kolattukudy, 1976). For a review of these and other conversions involved in the synthesis of plant epicuticular waxes see von Wettstein-Knowles (1979, 1982). These other... 

The epicuticular wax contains 3-dicarbo-nyl compounds with long aliphatic chains and an isolated cis double bond. Tbe hydrocarbon chain length is 16-24 carbons long, including... 

On the basis of experiments with protoplasts from epidermal cells of barley leaf sheaths, the proposal was made that the plasmalemma and/or cell wall was the site of the epicuticular wax synthesizing machinery . The observations summarized above are pertinent in this respect. The cer-cqu determined polypeptide contains the enzymatic activity for the final step in the associated pathway consisting of at least the apparent decarboxylation and hydroxylation reactions. The latter would be expected to occur close to the plant surface. This is where the decarbonylation activity which is the final step in hydrocarbon synthesis in peas has been located more specifically in a cutin containing fraction of a microsomal preparation. That the condensing activity of the 0-ketoacyl elongase, which is... 

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