Methyl-branched hydrocarbons, major components of the waxy material coating the embryos of the viviparous cockroach Diploptera punctata

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The viviparous cockroach Diploptera punctata carries a wax-coated batch of embryos in a brood sac. When the embryos are expelled into saline, flakes of wax from the surface of the embryos float to the surface. In contrast, embryos of the ovoviviparous species such as Rhyparobia maderae are not nourished by the mother during embryogenesis and do not have a copious waxy coating. As a first step in determining the function of this copious wax layer on the batch of embryos of D. punctata, its composition was compared to that of the waxy material on the outer cuticular surface of the mother (female cuticle) by thin-layer chromatography (TLC) and gas chromatography-mass spectrometry. The major lipid class on the embryos was hydrocarbons with lesser amounts of wax esters and long-chain alcohols. Hydrocarbons from both sources had similar elution times and chemical composition, but were markedly different in the amounts of the major methyl-branched hydrocarbon components. A mixture of 3,X-dimethyl alkanes were 44% of the hydrocarbons on the embryos and were only 29% on the female cuticle. However, trimethylalkanes were only 22% of the hydrocarbons on the embryos and were 34% of the hydrocarbons on the female cuticle. The major hydrocarbons from both sources were mixtures of methyl-branched alkanes with backbones of 33 and 35 carbon atoms. Methyl-branched tritriacontanes were 59% of embryo and 35% of female cuticular hydrocarbons; methyl-branched pentatriacontanes were 19% of embryo and 42% of female hydrocarbons. The difference in proportions of the similar hydrocarbons on the outer cuticular surface of the female and those covering the embryos may suggest that the evolution of copious nutrient secretion for the embryos was accompanied by selection for a mixture of hydrocarbons that prevents water loss by the embryos and protects them against invasion by microorganisms without preventing the movement of nutrient fluid into the embryos.