Dennis R. Nelson

Long-Chain Methyl-Branched Hydrocarbons: Occurrence, Biosynthesis, and Function

Publisher Summary This chapter focuses on the hydrocarbon components of the surface lipids, particularly to the long-chain internally branched methylalkanes and methylalkenes. These compounds have been extensively investigated since 1970, when di- and tri-methylalkanes were identified in an insect, and the technique of identifying mixtures of the methylalkanes from their mass spectra was elucidated. The majority of studies of the occurrence and function of the long-chain hydrocarbons has been done with insects. The surface lipids of plants and insects are important because; (1) they allow the uptake of water but prevent excessive water loss when available moisture is low; (2) they prevent the penetration of inorganic chemicals; (3) they act as a barrier against microorganisms; (4) they affect the absorption of agricultural; (5) they may serve as a sex attractant; and (6) they may serve as a kairomone for insect parasites and predators. The studies of biosynthesis of alkanes and the origin of the methyl groups have been done largely with plants and microorganisms though some of the more recent investigations have involved insects and other arthropods.

Cuticular hydrocarbons ofReticulitermes virginicus (Banks) and their role as potential species- and caste-recognition cues.

The cuticular hydrocarbon components of four castes ofReticulitermes virginicus (Banks) have been identified and quantitated. Components identified includen-alkanes; 2-, 3-, 11-, 13-, and 15-methyl-alkanes; 11,15-dimethylalkanes, (Z)-9-alkenes; (Z,Z)-7,9-dienes; and (E/Z)-6,9-dienes ranging in carbon number from C21 to C40. All caste forms ofR.virginicus contained the same components, but showed caste-specific proportions. Comparison of these hydrocarbons with those of the sympatric termiteR. flavipes (Kollar) suggest that cuticular hydrocarbons might serve as species- and caste-recognition cues. A bioassay was developed to test this species-recognition hypothesis, with the experimental results supporting the hypothesis.

Cuticular hydrocarbons of the house fly, Musca domestica

The cuticular hydrocarbons, some of which serve as components of the sex pheromone, of the house fly Musca domestica L. were identified by gas chromatography-mass spectrometry (GC-MS). The types of components identified in both sexes were n-alkanes, (Z)-9-alkenes, terminally (2-, 3- and 4-) and internally branched monomethylalkanes, and 3, x-, 4, x- and internally branched dimethylalkanes. The methylalkanes were minor components in males, whereas they were major components in females. (Z)-9-Tricosene, (Z)-9-hentriacontene and (Z)-9-tritriacontene were identified in females but not in males. In the male, 2- and 3-methyloctacosanes together with 2- and 3-methyltriacontanes were identified, whereas in the female the corresponding 4- and 3-methyl isomers were identified.

Re-analysis of the cuticular methylalkanes of Solenopsis invicta and S. richteri

Gas chromatographic retention times and electron impact and chemical ionization mass spectrometry were used to identify two new series of dimethylalkanes in Solenopsis invicta Buen and Solenopsis richteri Forel. The first series consisted of internally branched dimethylalkanes with one methylene group between the branch points and were identified in an extract of cuticular lipids from S. invicta and S. richteri. The second new series consisted of 3,x-dimethylalkanes with odd carbon numbers, where x = 7, 9, or 11, and 4,x-dimethylalkanes with even numbers, where x = 8, 10, or 12. The mass spectrum published by Loket al. (1975), and which was tentatively identified as being that of 10,12-dimethyltricosane, is re-interpreted as being that of a mixture of 3,9- and 3,11-dimethyltricosanes. Internally branched monomethylalkanes, and 3- and 4-methylalkanes were found in the cuticular lipids of both species. 3,7,11-Trimethylalkanes were tentatively identified as being present in S. invicta. The n-alkanes and alkenes also were identified in S. richteri.

Application of methoxymercuration-demercuration followed by mass spectrometry as a convenient microanalytical technique for double-bond location in insect-derived alkenes

The positions of double bonds in olefins can be readily determined by a sodium borohydride reduction of their methoxymercuration products followed by mass spectrometry. Fragmentation of the methoxy derivative in the mass spectrometer results in cleavage on either side of the methoxy group to give intense fragment ions which are characteristic of each isomer. This simple and convenient microanalytical technique was applied to several synthetic standards and insect derived olefins, including the alkenes from the cuticular lipids of the honeybeeApis mellifera L.

Biosynthesis of the hydrocarbon components of the sex pheromone of the housefly, Musca domestica L.☆

Abstract Biosynthesis of hydrocarbons, including components of the sex pheromone of the housefly Musca domestica L., was investigated. In vitro studies with isolated tissues from adult flies showed that the hydrocarbon components of the pheromone were synthesized primarily by the epidermal cells in abdominal segments two to seven. The incorporation of [3H or 14C]-labelled acetate, palmitate, stearate and oleate into the saturated and unsaturated hydrocarbon components showed that (Z)-9-tricosene (muscalure) was synthesized de novo by female insects and the distribution of label was consistent with a pathway in which oleic acid was elongated and then decarboxylated. A comparison of the incorporation and distribution of labelled acetate, propionate and succinate into hydrocarbons indicated that the mono- and dimethylalkanes were formed by the substitution of a methylmalonyl-CoA for malonyl-CoA during chain elongation. The incorporation of radioactivity from [1-14C]-propionate increased dramatically in female insects two days after adult emergence, which corresponds in time to the production of methyl branched alkanes. In contrast, this substrate was not efficiently incorporated at any time into male insects.

Insect hydrocarbons: Corroboration of structure by synthesis and mass spectrometry of mono- and dimethylalkanes

Standard mono- and dimethyl-branched hydrocarbons were synthesized from substituted thiophenes for the purpose of comparing their mass spectra to those of hydrocarbons isolated from the tobacco hornworm,Manduca sexta L. The mass spectra of the standard compounds confirmed the structures of the partially characterized natural products.

Cuticular hydrocarbons of the tsetse flies Glossina morsitans morsitans, G. austeni and G. pallidipes

Abstract Methylalkanes were identified in the surface lipids of 3 species of tsetse flies including several that had new sequences of methyl branching. Males had a simpler composition than females. The methylalkanes of G. m. morsitans had all branch points separated by 3 methylenes in both the di- and trimethylakanes. G. austeni had 3 or 5 methylenes and G. pallidipes had 3, 7 or 9 methylenes between the branch points of the dimethylalkanes. In G. pallidipes a 3,5 sequence was identified in trimethylpentacosane and trimethylheptacosane. Sequences of 3,3, 3,5, 5,3 and 7,3 methylenes between adjacent methyl branch points were identified in the trimethylalkanes of female G. austeni . There was some variation in isomeric composition, particularly in dimethyltritriacontane of female G. pallidipes , i.e. females from England, Kenya, Mozambique and Zambia had 11,15-dimethyltritriacontane as the major isomer whereas females from Uganda and Zimbabwe had 13,21-dimethyltritriacontane as the major isomer.

THE COMPOSITION OF THE CUTICULAR LIPIDS FROM NYMPHS AND EXUVIAE OF THE SILVERLEAF WHITEFLY, BEMISIA ARGENTIFOLII

Abstract The surface lipids of Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae) nymphs and exuviae were characterized. The identification and distribution of lipids were determined by capillary gas chromatography (CGC) and CGC-mass spectrometry (CGC-MS). The quantity of lipid from exuviae was 80% greater than that from nymphs. The composition for the lipid classes and the chain-length distribution of each lipid class were similar for both nymphs and exuviae: wax esters (86%), long-chain aldehydes (7%), hydrocarbons (3–4%) and long-chain alcohols (3–4%). The wax esters were composed of even-numbered-carbon compounds ranging from C 38 to C 64 . Structural analyses of the wax esters by CGC-MS with single ion monitoring revealed that the carbon numbers for the major acid and alcohol moieties ranged from C 14 –C 28 and C 24 –C 36 , respectively, and the major wax ester constituents were the C 52 ester, dotriacontanyl icosanoate (27%) and the C 54 ester, tetratriacontanyl icosanoate (14%). The major aldehyde was dotriacontanal (C 32 ), and the major alcohols were near equal amounts of dotriacontan-1-ol (C 32 ) and tetratriacontan-1-ol (C 34 ). The major hydrocarbons for both nymphs and exuviae were odd-carbon-numbered C 25 –C 35 n-alkanes. Lipids extracted from the exuviae from laboratory-reared sweetpotato whiteflies, Bemisia tabaci (Gennadius), were essentially the same as the lipids from B. argentifolii .

The lipid composition of the wax particles from adult whiteflies, Bemisia tabaci and Trialeurodes vaporariorum

Abstract Adult whiteflies are characterized by the presence of copious amounts of wax particles covering all surfaces of the body except the eyes. The lipid composition was determined for wax particles removed from the surfaces of the sweetpotato whitefly, Bemisia tabaci (Gennadius), and the greenhouse whitefly, Trialeurodes vaporariorum (Westwood). The lipid components in the wax particles of both species were mostly mixtures of long-chain aldehydes and long-chain primary alcohols. The major wax particle components for B. tabaci were C 34 aldehyde and C 34 alcohol and small amounts of C 32 aldhyde and alcohol. For the wax particles from T. vaporariorum , C 32 aldehyde and C 32 alcohol were the major components with lesser amounts of the C 30 components. These findings were compared to the surface lipids of fully-waxed B. tabaci and T. vaporariorum adults that contained, in addition to the major amounts of long-chain aldehydes and alcohols, quantities of long-chain wax esters. Wax esters were not present in lipid extracts from the surface of B. tabaci whiteflies at the time of adult emergence (prior to deposition of wax particles). Thus, the appearance of wax esters on the cuticular surfaces occurred during the period of deposition of wax particles. The quantities of wax esters in the surface lipid extracts of wing tissues separated from the bodies of adult whiteflies indicated that the wing surfaces were a major site of wax ester deposition.