Functional Genomics of Mountain Pine Beetle, Dendroctonus ponderosa, Related to Pheromone Biosynthesis
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AbstractDendroctonus spp. bark beetles are among the most destructive pests of North America coniferous forests causing annual losses of billions of dollars to the timber industry, and increasing the risk of forest fires and subsequent suppression cost. Recent outbreaks of mountain pine beetle (MPB), Dendroctonus ponderosae, in Alaska, Canada, and the western U.S destroy millions of acres of forest annually. Locally, during the 1990's, MPB and D. jeffreyi together with Scolytus scolytus killed nearly a third of the conifer trees in the Lake Tahoe basin. Like all bark beetles, MPBs rely on pheromones for host selection, attack, and colonization. Three major pheromone components have identified in D. ponderosae. trans-Verbenol, an á-pinene hydroxylation product, is synthesized by pioneer female during arrival and feeding on the host tree. This terpene alcohol attracts more males and females to the new site. Newly arriving males produce a fatty acid-derived pheromone component, exo-brevicomin, which attracts more males and females to the tree, resulting in mass attack that ultimately kills the tree by overwhelming its defense mechanism. A third pheromone component, frontalin, produced by males feeding in the presence of females, terminates the attack and functions as an anti aggregation pheromone component preventing overcrowding. Frontalin and exo-brevicomin are synthesized de novo in the MPB and their precursors have been identified. Frontalin is produced in the anterior midgut, while exo-brevicomin is thought to be synthesized in the fat body. However, the enzymes involved in last few steps of the synthesis of these pheromones and their regulation are yet to be determined. To that end, we initiated a functional genomics study of the mountain pine beetle midguts and fatiibodies in order to identify candidate genes for the biosynthesis of their pheromone components and subsequently study their regulation. An annotated EST library of midguts and associated fat bodies of fed and juvenile hormone III (JH III) treated MPB was generated and deposited in GenBank providing the first publicly-available functional genomics resources for this economically important forest pest. These ESTs were also used in a gene expression study by microarray and several candidate genes in pheromone production were identified and their regulation was studied. Among the genes identified in this project are several mevalonate and fatty acid synthesis pathway genes. Several cytochromes P450 with expression and tissue distribution profiles matching those of fronatlin or exo-brevicomin biosynthesis were identified and are being characterized. The long term goal of this project is ultimately to develop targeted strategies for direct and more efficient control of this devastating insect pest. Traditional pesticides spraying methods have shown limited success in managing bark beetle population because the beetles spend most of their life cycle underneath the bark, physically protected from the sprayed pesticides by the very trees they attack. In addition bark beetles attack vast areas of mostly public lands, making the use of pesticides economically non-viable.