Sites of synthesis and transport pathways of insect hydrocarbons: cuticle and ovary as target tissues

Abstract

The outer surface of insects is covered with a lipid layer that provides water-proofing and protection against environmental stresses. Hydrocarbons (HC) are major constituents of this epicuticular wax and they also serve as semiochemicals. In some insects HC are also exploited as biosynthetic precursors for pheromones. HC are synthesized by oenocytes which are situated in the integument or hemocoel. Shuttling of HC to the epicuticle, fat body, and gonads requires transport through an aqueous medium. Insects, unlike vertebrates, use a versatile lipoprotein to effect lipid transport and to selectively deliver lipids to specific tissues. A high-density hemolymph lipoprotein (lipophorin [Lp]) serves this function. In adult females of the German cockroach (Blattella germanica), Lp carries both HC and a contact sex pheromone. Lipophorin is a multi-functional lipid carrier serving also as a juvenile hormone binding protein in many insects. Studies of the interactions between Lp and HC are beginning to unravel the routes used in delivering HC to target tissues. We discuss the pathways and dynamics of loading of Lp with HC and HC-derived pheromones, their transport through the hemolymph, and deposition in various tissues, including the epicuticle, ovaries, and pheromone-emitting glands.