Cuticular units

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. 

The Cryptocercus punctatus wood-feeding cockroach complex in the eastern United States was recently revised based on cuticular hydrocarbon profiles, chromosome count and DNA sequencing (16S and COII mitochondrial genes and ITS nuclear gene). Findings showed that there were five distinct hydrocarbon phenotypes that were not... 

Everaerts, C., Maekawa, K., Farine, J.-P., Shimada K., Luykx, P., Brossut, R. and Nalepa, C. A. (2008). The Crypotcercuspunctatus species complex (Dictyoptera Cryptocercidae) in the eastern United States comparison of cuticular hydrocarbons, chromosome number and DNA sequences. Mol. Phyl. Evol., 47, 950-959. 

Haverty, M. I., Forschler, B. T. and Nelson, L. J. (1996a). An assessment of the taxonomy of Reticulitermes (Isoptera Rhinotermitidae) from the southeastern United States based on cuticular hydrocarbons. Sociobiology, 28, 287-318. 

Haverty, M.I., Nelson, L.J. and Forschler, B.T. (1999a). New cuticular hydrocarbon phenotypes of Reticulitermes (Isoptera Rhinotermitidae) from the United States. Sociobiology, 33, 1-21. 

Haverty, M.I., Nelson, L. J. and Page, M. (1990a). Cuticular hydrocarbons of four populations of Coptotermes formosanus Shiraki in the United States. Similarities and origins of introductions../. Chem. Ecol., 16,1635-1647. 

Jones, T. H., Moran, M. D. and Hurd, L. E. (1997). Cuticular extracts of five common mantids (Mantodea Mantidae) of the Eastern United States. Comp. Biochem. Physiol., 116B, 4I9M22. 

The computer program Absolv, has, inter alia, a log Kcw prediction module. For the chemicals used by Sabljib et al. (1990), Absolv gave reasonably good predictions. For example, for partitioning into the cuticular membrane of the tomato (Lycopersicon esculentum), for which data for 13 chemicals were available, 8 Absolv predictions were within 0.5 log units of the measured values, 4 were between 0.5 and 1.0 units in error, and 1 was more than 1 log unit in error. The measured and Absolv-predicted values correlated well (R2 = 0.954, s = 0.425, F = 230.2). 

It is well established that propionate can be utilized for the methyl branch unit in methyl branched hydrocarbons in insects (4-7), and recent data have shown that the methyl branches are inserted early during chajij elongation rather than toward the end of the process (13,16). C-NMR analysis demonstrated that propionates labeled with C in either the 1, 2 or 3 positions are incorporated into the methyl branched alkanes of insect cuticular lipids. C-3 of propionate becomes the branching methyl carbon, C-2 becomes the tertiary carbon and C-l the carbon adjacent to the tertiary carbon (13,16,17) in these methyl branched hydrocarbons. 

The rate of water vapor diffusion per unit leaf area, Jw> equals the difference in water vapor concentration multiplied by the conductance across which Acm occurs (// = g/Ac - Eq. 8.2). In the steady state (Chapter 3, Section 3.2B), when the flux density of water vapor and the conductance of each component are constant with time, this relation holds both for the overall pathway and for any individual segment of it. Because some water evaporates from the cell walls of mesophyll cells along the pathway within the leaf, is actually not spatially constant in the intercellular airspaces. For simplicity, however, we generally assume that Jm, is unchanging from the mesophyll cell walls out to the turbulent air outside a leaf. When water vapor moves out only across the lower epidermis of the leaf and when cuticular transpiration is negligible, we obtain the following relations in the... 

Powdery mildew infection of wheat leaves was accompanied by increased production of cuticular waxes but did not result in consistent changes in the proportions of the component fractions. The amount of wax present per unit area of leaf surface of healthy plants was greater in resistant than in susceptible cultivars, while the partially resistant cultivar investigated was intermediate in this respect. Although exudates from Erysiphe graminis f sp. hordei have been shown to have esterase and cutinase activity which is likely to be of importance in the initiation of infection (Kunoh et a/., 1990 Nicholson et al, 1993), the present results support other evidence for the importance of the thickness of the cuticular wax layer in resistance to powdery mildew. 

Using the same range of C-labeled compounds, Kerler and Schon-herr determined the permeability of astromatous cuticular membranes from the leaves of C. aurantium by measuring permeability coefficients (flow per unit area, time, and driving force) and extrapolated holdup ... 

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