Frontalin: De novo biosynthesis of an aggregation pheromone component by Dendroctonus spp. bark beetles (Coleoptera: Scolytidae)

The pheromone component, frontalin (1,5-dimethyl-6,8-dioxabicyclo[3.2.1]octane) is thought to be formed in Dendroctonus spp. bark beetles through the cyclization of oxygenated 6-methyl-6-hepten-2-one (6-MHO). Unlike many of the isoprenoid pheromone components of bark beetles, there is no obvious immediate host conifer precursor for 6-MHO or frontalin. To elucidate the biosynthetic pathway of frontalin, juvenile hormone-treated male Dendroctonus jeffreyi were injected separately with [1-14C]acetate, [2-14C]mevalonolactone, [1-14C]isopentenol, [1-14C]:[1-3H]isopentenol, and [4,5-3H]leucine. Subsequently volatiles were collected on Porapak Q from these males and abdominal tissues were extracted. Radio-HPLC analyses of extracts from males injected with each radiolabeled substrate showed that radioactivity from the injected precursors eluted in a peak with a retention time that matches that of unlabeled frontalin. In all cases, HPLC fractions containing radiolabel that eluted at the same time as a frontalin standard were analyzed by GC-FID and GC-MS to confirm the presence of frontalin. In a separate study, male D. jeffreyi were injected with [1-13C]acetate and an abdominal tissue extract from these insects was analyzed by tandem gas chromatography-isotope ratio monitoring-mass spectrometry (GC-IRM-MS), which unequivocally showed incorporation of 13C into frontalin. Because mevalonate is the key intermediate in the isoprenoid pathway, its incorporation (as mevalonolactone) into frontalin provides compelling evidence that the biosynthesis of frontalin involves that pathway in some form. In the experiment with [1-14C]:[1-3H]isopentenol, there was no significant difference in the mean percentage incorporation of either radioisotope into frontalin. This supports the role of the classical isoprenoid pathway, as tritium would be lost if only a hybrid pathway were involved. Confirming that de novo synthesis may be general to all Dendroctonus spp., 14C-acetate was also incorporated into frontalin by females of D. rufipennis and D. simplex. A radiolabeled precursor/pathway inhibitor study showed that the fatty acid synthase inhibitor, 2-octynoic acid, increased (although not significantly) the mass of frontalin produced and significantly increased the percentage incorporation of radioactivity from [1-14C]acetate into frontalin. This suggests that as fatty acid biosynthesis is blocked, an increased amount of acetate is funneled into frontalin production via the isoprenoid pathway.