Animals use sterols as membrane inserts, as precursors to steroid hormones, and in developmental gene regulation. Insects are unable to synthesize sterol molecules de novo and must therefore obtain sterols or sterol precursors directly from their diet. Detailed physiological studies have demonstrated that there is significant diversity in how plant-feeding insects metabolize phytosterols into biologically active sterol molecules (e.g., cholesterol), but to date there has been no investigation of the sterol profiles of a naturally occurring phytophagous insect community feeding on a single host plant. The neutral sterol profiles of the ecological model plant Solidago altissima (late goldenrod), six of its common above-ground herbivorous insect associates, and the fungal symbiont of one of those herbivores were determined as an initial step to elucidating this crucial aspect of insect nutritional ecology in this system.
Results/Conclusions The primary detectable sterols of S. altissima leaf tissue were the phytosterols spinasterol, 22-dihydrospinasterol, and avenasterol. Many S. altissima herbivores appeared to have converged on a metabolic pathway that metabolizes Solidago sterols into lathosterol and/or cholesterol. The treehopper Acutalis tartarea appeared to be unable to efficiently metabolize sterols with Δ22 double bonds. Two species, the leaf-beetle Trirhabda virgata and the stem-galling fly Eurosta solidaginis, contained extremely low to undetectable amounts of cholesterol, suggesting that they may be unable to fully metabolize 28/29 carbon Δ7 phytosterols and/or that the Δ7 sterol lathosterol is substituted for cholesterol in these species. The fungal ectosymbiont of the leaf gall midge Asteromyia carbonifera contained primarily ergosterol and two ergosterol precursors. The larvae and pupae of A. carbonifera lacked phytosterols, phytosterol metabolites, or cholesterol, instead containing an ergosterol metabolite in addition to unmetabolized ergosterol and erogsterol precursors, demonstrating the crucial role that their fungal symbiont plays in their nutritional ecology. These data are discussed in the context of sterol metabolism in insects and the potential ecological and evolutionary implications.
