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

A genome-level understanding of how the fungus Grosmannia clavigera, a symbiont of the mountain pine beetle, withstands its host tree's chemical defenses could help ecological modelers better forecast the range and extent of the epidemic



Dezene Huber

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 the 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.

Scientists are not sure whether the beetle is able to kill trees by itself, because the symbiotic microorganisms are inherently associated the insect, Bohlmann explains, and "nobody has been able to do conclusive experiments where you have a bark beetle that has been reared without any microorganisms to test if it can damage trees." On the other hand, previous research has shown that the fungus, when inoculated in large amounts below the bark of pine trees, can kill without any help from the beetle.

In nature, of course, the fungus could not penetrate the tree’s outer layer without help from the little black insect, which chews through the bark and burrows into the inner nutrient-rich tissue where it feeds and lays eggs. What the bug gets out of the relationship is not as clear, but the fungi may make the tree's nutrients more available to the beetle and/or protect the insect by neutralizing the tree's robust set of defenses. In response to surface damage, the trees release pitch, or resin. "This gooey, sticky secretion," Bohlmann explains, "in addition to being physically repellant, contains a mixture of dozens of chemicals that have insecticidal and antimicrobial properties."

Pitch doesn't kill Grosmannia clavigera, however, and Bohlmann and his colleagues set out to locate the parts of its genome that make such resistance possible. They exposed the fungus to tree defenses and, by using ultrahigh throughput sequencing technology, took what he describes as "rapid snapshots" of its genomic response over short time periods of hours or days. "We found a whole suite of genes that are being activated," likely related to mechanisms that help the fungus evade, undermine or overcome the chemical defenses of the host tree, Bohlmann says.

When the researchers then knocked out a gene, selected from this set because they deemed it especially critical, the altered fungus was unable to tolerate the tree's chemical weapons—reinforcing the notion that the fungus's pathogenicity arises from only a small subset of its genome.

Bohlmann notes that "although it is widely recognized that this fungus is an important player, it has not yet been used for devising tools for coping with or predicting mountain pine beetle epidemics." He says his group's result sets the stage for more specific investigations that could lead to strategies for curbing the epidemic, including more precise forecasting models.

Brian Aukema, an assistant professor of entomology at the University of Minnesota who was not part of this research, agrees. For example, he explains, one of the most important uncertainties in ecological risk modeling for the mountain pine beetle is the extent to which it may be able to colonize pine species it hasn't before, such as jack pine, whose native range is east of the Rocky Mountains. The answer to this question would shed light on just how far east the beetle epidemic could spread. A species like jack pine could serve as a conduit through the northern coniferous forest, which the insects could use to "go from the west coast of Canada to the east coast of Canada, and potentially down into the United States through different types of pines," Aukema explains.

"Every time it goes to a different host, there's a whole new set of questions you have to ask yourself," Aukema notes, and given this new genomic information, "you can start looking at commonalities across tree species." Most pine species have well-developed defense systems, and this work indicates that Grosmannia clavigera may have the "machinery in place to deal with similar defensive responses across pine species."

To this point, the beetle has generally remained in lodgepole and ponderosa pine forests in the North AmericanWest, and the Rocky Mountains have served as a geoclimatic barrier to the infestation's eastward movement. But that is changing. "We've never seen such numbers of beetles that have successfully gotten over the Rocky Mountains in northwestern Alberta," Aukema says, "and that's where jack pine, which stretches from the east to the west, hybridizes with lodgepole pine. The beetle is successfully reproducing in this hybrid zone, which we've never seen, and it's moving eastward through Alberta."

There have been hints in the scientific literature over the past few decades that this could happen, but "it's kind of stunning to think that in our lifetimes we are actually seeing this now," Aukema says. "It's anybody's guess as to how well [the beetle] is going to do as it moves eastward."

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