Spruce Up: Researchers Pinpoint Genes That Give Pine-Killing Fungus Immunity to Host Tree Defenses

In western North America the mountain pine beetle—the most destructive of the many species collectively known as bark beetles—is on a pine tree–killing spree. Since the 1990s swarms of the tiny killer, spurred in part by a streak of relatively mild winters that don't kill the insect, along with dry summers that leave trees more vulnerable to attack, have destroyed huge swaths of pine forests—around 16 million hectares (an area larger than Florida) in British Columbia alone. The beetles are now threatening to move eastward, and research ecologists are working to rapidly build a better understanding of exactly how insect invasion kills trees, searching for insights that might allow forestry workers better cope with the epidemic.

One thing that has become clear is that the beetles don't work solo but in tandem with various microorganisms with which they have symbiotic relationships. In fact, explains Joerg Bohlmann, a professor of forest science and botany at the University of British Columbia, without these microorganisms the insect "would perhaps never be able to kill trees."

In 2009 Bohlmann and colleagues sequenced the genome of the symbiont seemingly most critical to the tree-killing process, a fungus known as Grosmannia clavigera. Now, he and 22 other authors report in the January 25 issue of Proceedings of the National Academy of Sciences that they have identified, within that genome, a specific gene cluster that activates in response to a pine tree's chemical defenses, detoxifying them and allowing the fungus to survive in an environment toxic to most microorganisms. The finding will help ecologists better understand the interactions among the beetle, the fungus and the pine tree, and could lead to more precise ecological forecasting models to predict the potential range and extent of the current infestation.