August 19, 2008

Asian Longhorned Beetles Employ a Fungal Mutualist in Digesting Wood

I wrote recently about the appearance of the Asian longhorned beetle in the Worcester area. The insect is a highly problematic invasive species that's likely to do a great deal of damage to hardwood tree populations in the area before it's brought to heel. In fact, the unfortunate truth is that it is likely to be many years before it's stopped, and if it's like most invasives it's probably here to stay, maintaining at least small populations that will allow for outbreaks from time to time when conditions are favorable for elevated rates of reproduction.

It behooves us, therefore, to understand the biology of Anoplophora glabripennis as thoroughly as possible. One of the fundamental questions on the road to understanding the ecology of any organism is, what does it eat and how does it eat it? When it comes to beetles, a fungus or two (or twenty) is frequently involved. Fungi are everywhere in the biosphere, but there are few interkingdom mutualisms as old and as intimate as those between Coleoptera and fungi. A forthcoming paper in the Proceedings of the National Academy of Science has found evidence that a different sort of symbiosis is at work in the Asian longhorned beetle, giving it the ability to digest living wood — a very unusual ability among beetles and one that is key to Anoplophora's destructive potential.

Novel fungus helps beetles to digest hard wood

A little known fungus tucked away in the gut of Asian longhorned beetles helps the insect munch through the hardest of woods according to a team of entomologists and biochemists. Researchers say the discovery could lead to innovative methods of controlling the invasive pest, and potentially offer more efficient ways of breaking down plant biomass for generating biofuels.

Microbes in the gut of insects are known to break down cellulose, but little is known about how, or whether, insects degrade lignin. This natural polymer helps plants stay upright and protects them from most forms of microbial attack.

"Lignin is nature's plastic and any organism that wants to get to the sugars in a plant has to be able to get past this protective barrier," said Ming Tien, study co-author and Penn State professor of biochemistry and molecular biology. "We suspect that the fungus produces enzymes that help the beetles degrade lignin."

Before this report, it was thought that insects are unable to extensively break down lignin, and that they get around the problem either by feasting on wood that has already degraded, or by living close to fungi that can degrade the wood for them...

The researchers compared the chemical structure of non-degraded wood before and after it had passed through the gut of two wood-eating insects. To measure the degree of change in the lignin, they first fed pin oak wood to Asian longhorned beetles. Next they fed ponderosa pine wood to the Pacific dampwood termite, an insect that typically eats only dead wood.

Chemical analyses of feces from the two bugs indicated that they are able to alter the chemical structure of lignin by selectively adding or removing certain groups of molecules from the polymer...

"This fungus has genes that then make enzymes," explained Hoover, whose team's findings appear today (Aug. 18) in the Proceedings of the National Academy of Sciences. "We have been able to detect messages from the [fungal] DNA, which get translated into enzymes." [NOTE: I have checked the PNAS Early Edition and the paper is not available yet; this is typical for PNAS. It's a vexing problem that I really wish they would correct. It takes far too long for material to appear after authors are led to believe their publication will be made available on a particular date.]

While the researchers have identified the fungus residing in the gut of the Asian longhorned beetle, they have yet to find one in the gut of the termite.

"The types of chemical changes we see in the beetle are similar to those seen in the white-rot fungus," said Geib. "Changes that we see in the termite are similar to those in the brown-rot fungus. The chemical changes to the lignin are similar..."

"This type of fungus [in the Asian longhorned beetle] is known to cause disease in plants," said Hoover, whose work is funded by the Alphawood Foundation and the Penn State College of Agricultural Sciences. "But this particular strain appears to be unique. It looks like these insects somehow acquired the fungus to live in their gut and help them break down wood..."
Because of the way that this release has been written, a lot of information has to be inferred. I look forward to the day when the actual paper is made available, whenever that may come, so that we can learn more about what's been discovered — including the identity of the fungus, which is only referred to as a "novel fungus" in the release itself.

Still, it sounds from the release that the fungus is likely to be a basidiomycete. The reference to it as being like a white-rotter means that it likely produces two different varieties of peroxidases, perhaps both lignin peroxidase and manganese peroxidase if it's anything like saprophytic polypores like Ganoderma and Polyporus. As the release mentions that the fungus is capable of living freely, outside the guts of Asian longhorned beetles, as a pathogen, there's a strong likelihood that it's not a yeast, such as those found in the digestive tracts of Bolitotherus cornutus (see this paper by Suh et al. for a background on yeasts in beetle bellies). It would be very unusual for a yeast to produce lignin-degrading enzymes, particularly in large enough quantities to be useful to a wood-eating insect. Termites get around the problem of lignin's refractory nature with a battery of dinoflagellates and bacteria. The use of a mutualist fungus is an exciting new find in symbiology.

From the release, it also sounds like the researchers suspect that the longhorned beetles may have multiple allies working on their behalf. Gut flora is generally a complex thing, so that's likely, and the release itself doesn't make any mention of the relative importance of the novel fungus in the overall picture. Nonetheless, the discovery of the fungus could provide one more target in the attempt to control a particularly dangerous invasive species. In tinkering with ecology, whether on the scale of a digestive system or a forest, removing just one organism frequently has a ripple effect on total health. Whether this might turn out to be the case here — handcuff the fungus, kill the beetle — remains to be seen.

I anticipate finding out the identity of the new fungus and where it fits into the taxonomy of lignin degraders. In the meantime, I plan on being up on Wachusett on Wednesday to look for polypores, tenebs... and Asian longhorned beetles, unfortunately.

Sphere: Related Content