Juvenile hormone regulates de novo isoprenoid aggregation pheromone biosynthesis in pine bark beetles, Ips spp., through transcriptional control of HMG-CoA reductase.

Evidence is presented for transcriptional regulation of de novo pheromone biosynthesis in Ips spp. bark beetles, but the comparative biochemical and molecular approach reveals a dichotomy between species in the pini and grandicollis subgeneric groups. Radiotracer studies with 14C-acetate demonstrate that feeding on host phloem stimulates biosynthesis in males of three Ips spp. However, treatment with juvenile hormone III (JH III) stimulates biosynthesis only in Ips pini. Thus, two species in the grandicollis subgeneric group (I. grandicollis and I. paraconfusus) appear to have a different mode of regulation related to JH III than does I. pini. Between 16 and 20 hr after feeding has commenced, pheromone production, as measured by accumulation in abdominal tissue, is stimulated about 150- (I. pini) and 350-times (I. paraconfusus) above the control level of 1-10 ng/male measured at 0 hr. Treatment with JH III results in accumulation in I. pini that is 3-4 times more than in phloem-fed males, whereas the identical treatment results in only weak accumulation in I. paraconfusus (45-times less than phloem-fed males). Comparative studies of gene expression and enzyme activity related to biosynthesis also support different modes of JH III-related regulation in I. pini and I. paraconfusus. In males of both species, feeding on host phloem results in increased transcript abundance and increased activity for the key de novo isoprenoid pathway enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-R). However, while JH III treatment results in comparable maximal increases in HMG-R transcript levels in both species (similar to feeding), the activity of HMG-R in crude extracts from JH III-treated male I. paraconfusus is low in comparison with male I. pini. Hypothetical explanations for the interspecific dichotomy in the regulation of pheromone biosynthesis include a second hormone or factor in grandicollis group species that functions either alone or with JH III; in both cases acting after HMG-R has been transcribed.

Midgut tissue of male pine engraver, Ips pini, synthesizes monoterpenoid pheromone component ipsdienol de novo.

For over three decades the site and pathways of bark beetle aggregation pheromone production have remained elusive. Studies on pheromone production in Ips spp. bark beetles have recently shown de novo biosynthesis of pheromone components via the mevalonate pathway. The gene encoding a key regulated enzyme in this pathway, 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-R), showed high transcript levels in the anterior midgut of male pine engravers, Ips pini (Say) (Coleoptera:Scolytidae). HMG-R expression in the midgut was sex, juvenile hormone, and feeding dependent, providing strong evidence that this is the site of acyclic monoterpenoid (ipsdienol) pheromone production in male beetles. Additionally, isolated midgut tissue from fed or juvenile hormone III (JH III)-treated males converted radiolabeled acetate to ipsdienol, as assayed by radio-HPLC. These data support the de novo production of this frass-associated aggregation pheromone component by the mevalonate pathway. The induction of a metazoan HMG-R in this process does not support the postulated role of microorganisms in ipsdienol production.