Beetle With Benefits
Fungi help bark beetles thrive and massively change forest landscapes
When a mountain pine beetle looks at a tree, it sees lunch. We’ve all witnessed the aftermath of this appetite — entire swathes of reddened forest, a blight creeping up to the timberline because warmer winters enable the beetles to climb to higher altitudes.
Destructive though they may be, mountain pine beetles are impressive workers. No larger than a grain of rice, they use chemical communication systems to coordinate mass attacks, turn a tree’s defense mechanisms against itself and manufacture a type of antifreeze to survive winter.
When UM researcher John McCutcheon looks at a tree, he sees two bacteria held up by sticks. He’s a microbiologist and a specialist in symbiosis, so he knows that all complex life has benefited from hosting very small organisms. In the tree’s case, the mitochondria and chloroplasts within its cells were once foreign bacteria that developed such beneficial relationships with their hosts that they eventually became part of the tree itself. McCutcheon studies the complex symbiotic associations between insects and microbes. Recently he’s turned his attention to the mountain pine beetle and two species of fungi that help them thrive on an abundant but nutrient-poor food source: trees.
The beetles have the respect of Diana Six, a UM scientist who’s studied them for 22 years. Sure, they cause problems — “I get mad when they kill trees in my yard, just like anyone else,” she says. “Maybe even more so, because I’m embarrassed.” But she also admires their resourcefulness. “These things have evolved in some really remarkable ways,” she says. Most remarkable to Six is the fact that they wouldn’t survive a day without two species of symbiotic fungi that ride from tree to tree in suitcase-like pouches on the beetles’ mouths.
McCutcheon and Six have teamed up to research the relationship between mountain pine beetles and these two fungi at the smallest possible level: their genetics. With a grant from the Montana Institute on Ecosystems, they are studying how these species help and hinder each other. Their findings may have huge implications on our understanding of pine beetle behavior, where they might strike next and what we might be able to do to stop them.
As far as McCutcheon and Six can determine, the story began millions of years ago with some star-crossed matchmaking between species. McCutcheon surmises that a
beetle picked up a fungal partner that gave the beetle an unanticipated edge.
“When these organisms first got together back in time, it was really beneficial for the insect,” he says. “It allowed them to live in places they ordinarily couldn’t. It allowed the insects to spread across the globe.”
It should come as no surprise that the way to the beetles’ hearts was through their stomachs. The two species of fungi helped the beetles derive nutrients from wood, a plentiful but insubstantial food source. When pine beetles bore into the bark of a tree, the fungi in their mouthparts rub off onto the tunnel walls. The fungi colonize the tree, migrating into the xylem, where they bind nitrogen and amino acids and transport them back to the beetles feeding in the sugary phloem. The beetles snack on the fungi like we might swallow multivitamins.
“If we were going to eat total junk food and live on French fries, we’d have to take serious supplementation to keep going,” Six says. “That’s what this is like. The fungi are the critical link that allows the beetles to use the tree. Without the fungi we wouldn’t have mountain pine beetles.”
Like many cases of symbiosis, the relationship is one of evolving codependence. After millions of years of co-evolution, the beetles can only survive with these two specific fungi, and the fungi can only survive with these beetles, which they use as taxis to transport them from tree to tree. Simply put, the species are getting a little clingy. It’s a relationship that bewilders and fascinates McCutcheon. “They’re kind of like an old married couple,” he says. “You get dependent on each other. That dependency, the way it evolves, is very interesting to us.”
McCutcheon knows from studying other symbiotic relationships that the more dependent a symbiont grows on its host, the more bankrupt its genome becomes. The symbiont no longer needs the genetic variation it would to survive in this world alone.
“They become very comfortable,” Six says. “They lose gene function. Eventually they may become so wimpy that if environmental conditions change, they can’t adjust.”
That dependency can imperil the beetles and their fungi. “Building a symbiotic partnership allows you to do things you couldn’t do before,” Six says. “But there’s a cost. They become domesticated.” With fewer genes, the fungi are less likely to have something in their arsenal to adapt to changing conditions. “Symbionts are at a higher risk of a rapidly changing environment than other organisms,” Six says. “That puts their hosts at risk as well.”
For now, climate change appears to be helping mountain pine beetles. Higher temperatures put stress on pine forests and allow the beetle larvae to survive the winter. But warmer weather will not be as kind to the fungi. When Six designed a model for a site in Idaho, she found that one degree of warming over several years would cause the beetle-fungi symbiosis to disintegrate. “That, of course, would
be the end of the beetle at that site,” Six says.
To find out how the fungi might adapt to environmental change, McCutcheon, Six and their Montana Institute on Ecosystems graduate fellow, Daniel Vanderpool, sent away their genomes to be sequenced. Sorting the resulting data will be like assembling six 20- to 40-million-piece puzzles, McCutcheon says, but it will start shedding light on what adaptations these fungi are capable of, and how they might eventually limit the fitness of mountain pine beetles.
With McCutcheon’s background in genomics and symbiosis, and the two decades Six has spent researching these beetles and their fungi in the woods and in the lab, you could say the scientists themselves have a symbiotic relationship. It’s the sort of interdisciplinary duo the Montana Institute on Ecosystems was created to connect.
“We’re on the cusp of asking questions about how these things evolved,” McCutcheon says. “I’m excited. I’m really excited.”
— By Jacob Baynham