Day 86: Root-living fungi and their trees enjoy long-lasting partnerships (e.g., hundreds of millions of years)

Photo by Laurie McGavran/Friends of Bandelier National Monument

A friendly reminder: Earth Day is on Wednesday, so be the best steward you can be! I’m working on a post or two with Earth Day specifically in mind, but I won’t claim this is one of them. A lot of my posts have to do with a year-round fascination with Earth and the living that’s done here; this is one of those.

It’s about partnerships in nature.

Specifically, it’s about a two-way interaction between trees and the fungi that are often tenants on their roots. The arrangement works out pretty well: the roots provide a home for the fungi and feed it sugars and starches, and the fungi help stabilize the roots, enhance their mass and affording increased access to nutrients and water. 

I studied a three-way version of these mutualisms in grad school in Flagstaff, Arizona. I was working on genetic studies of Sciurus aberti, the Abert (aka “tassel-eared”) squirrel. The preferred food of the squirrel, when it’s available in the warm seasons, is the fruiting bodies — called truffles — of fungi associated with ponderosa pine trees. It’s a love triangle that works like a charm: in addition to the relationship between the trees and the fungi, the squirrels get fat on the truffles and then scamper about the forest, inoculating new trees with their feces. Research has shown that the spores stay viable as they pass through the squirrels’ digestive tract.

You might say I took a shine to those little rodents; I raised two from hairless thumb-sized orphans (brought to me by others). I carried them around in my shirt pockets and woke every two hours to feed them until they grew up to chew my books and cache nuts atop the fridge. Both were released in timely fashion (several years apart) at the safe haven of Lowell Observatory. 

But back to the mutualisms that don’t include humans. Or Abert squirrels.

The plant-fungi mutualisms mostly involve various pine trees and flowering members of the rosid clade, a wide and varied assemblage of plants that includes, for example, mustard, mango trees, St. John’s Wort, geraniums and of course roses. 

David Hibbett, a biologist at Clark University in Worcester, Massachusetts and his co-author, P. Brandon Matheny, of the University of Tennessee in Knoxville, got curious about how the partnerships came about — and which of the players evolved first.

Symbiotic fungi on tree roots. Credit: Iowa State University Extension

Past studies trying to get to the bottom of the mutualisms had come up with vastly different results, the authors write, “ranging from scenarios with many relatively recent origins of the symbiosis … a single ancient origin … and many subsequent transitions to the free-living condition; or multiple gains and losses of the association.”

So the researchers analyzed the genes of both the trees and the fungi using the “molecular clock” technique, which exploits differences in the DNA between species to track the time since they diverged from a common ancestor. 

It’s clear that plant-fungus mutualisms arose not just once in nature, but separately at least 16 times — eight each in flowering plants and in the pines, Hibbett and Matheny report.

They also found that one class of mutualistic fungi — called the Agaricomycetes – is much older than either the rosids or pines. In those cases, the ancestor of the mutualistic fungi couldn’t have been mutualistic, because their modern hosts — the plants in the rosid clade — didn’t exist yet.  The fungal ancestors had to be free-living. 

* Hibbetts emailed to remind me that “Agaricomycetes contains more than just mutualists. This is a huge group that includes mycorrhizal fungi, but also many decayers and some pathogens.”

The results are less clear for other members of a large sub-class within the Agaricomycetes, called Agaricomycetidae. They’re still significantly older than the rosids, but in mutualisms involving those fungi and pines, it’s sometimes hard to tell which species — the tree or the fungus — evolved first.

All the white roots on the larch seedlings are symbiotic fungi roots; the thicker red/brown roots are the larch's roots. Credit: Buckingham Nurseries online catalogue, www.hedging.co.uk

The researchers say the symbiotic relationships between fungi and pines probably first occurred between 270 and 130 million years ago, because that’s the span of time between the emergence of pines as a recognizable group, and their diversification. (Rosids, by comparison, are about 100 million years old.)

“The first … symbioses may have involved Pinaceae (pines), which are older than rosids, ” the authors wrote, “but several major clades of Agaricomycetes … are young enough to have been [originally] associated with either rosids or Pinaceae, suggesting that some contemporary … partnerships could be of very ancient origin.”

The authors aren’t able to confirm their findings with fossil evidence, because the oldest fossilized plant roots found to be associated with fungi are from the Middle Eocene era, 50 million years ago. 

Looking forward, they hope to do more genetic analyses to clear up the history of pine-fungi associations. And time will tell whether enough fossil evidence will trickle in to support the genetic findings: “it will eventually be necessary to perform calibrated analyses to place the organismal phylogeny in the context of geological history,” the authors write.

Source: The original paper is here, at the open-access journal BMC Biology. For more information, check out the Hibbett lab.