BIOSYNTHESIS OF CONIFEROPHAGOUS BARK BEETLE PHEROMONES AND CONIFER ISOPRENOIDS: EVOLUTIONARY PERSPECTIVE AND SYNTHESIS

Abstract

In this overview we compare the significance and evolutionary history of two interacting biological systems, the conifer-feeding bark beetles (Coleoptera: Scolytidae) and their host conifers (Gymnospermae: Coniferales and Taxales). Isoprenoid natural products play key roles in the aggregation of the bark beetles and in the defence of the conifers. Our approach is to couple the most recent advances in the biochemical and molecular literature on these systems with ecological and behavioral data to compare monoterpenoid pheromone biosynthesis in scolytids with monoterpene biosynthesis in conifers. This synthesis reveals and evaluates the evolutionary redundancy occurring in the biochemical systems of the insect and host. Although host monoterpenes may be utilized directly or as derivatives in aggregation by scolytids, oxygenated monoterpenes that are behaviorally active for scolytids have been rarely identified from their coniferous hosts.De novomonoterpenoid biosynthesis in the Scolytidae, a process that is likely to be rare among metazoans, is substantially different from monoterpene biosynthesis in the conifers. The pathways appear to be shared only at the late-stage reactions that follow the formation of isopentenyl diphosphate. Little is known of the regulation of monoterpene biosynthesis in conifers, but scolytids positively regulate monoterpenoid biosynthesis using a sesquiterpenoid hormone, juvenile hormone, which does not occur in conifers. Little is known of the subcellular site of synthesis of monoterpenoids in scolytids, but conifer monoterpene biosynthesis is compartmentalized in the plastids, which do not occur in scolytid cells. In addition to bark beetles and conifers, the vertebrate model presents one of the few systems in which isoprenoid synthesis has been studied enough to provide a meaningful comparison. Possible unique features of monoterpenoid pheromone biosynthesis in scolytids relative to isoprenoid biosynthesis in vertebrates include the following: (1) a monoterpenoid end product; (2) a hypothetically scolytid-specific prenyl transferase (= geranyl diphosphate synthase) that catalyzes the condensation of two five-carbon (C₅) units, but does not catalyze additional condensation reactions with the C₅monomelic unit; (3) a scolytid-specific monoterpene (myrcene) synthase; and (4) a scolytid-specific, transcriptional-level sesquiterpenoid isoprenoid regulatory mechanism. Features 2 and 3 may be shared with conifers. This review also updates the 1985 landmark scientific paper by John Borden by listing the references and species of coniferophagous Scolytidae for which aggregation pheromones have been identified since 1985.