A story I wrote in July of last year described a huge change in our understanding of one of the best known cases of symbiosis: the view that a lichen is a symbiotic partnership between a fungus and an alga. That happens to be true—the alga photosynthesizes, producing food for the fungus, while the fungus provides support, water, and minerals for the alga. But there’s more.
The alga/fungus partnership was discovered in 1868 by the Swiss botanist Simon Schwendener, and it’s what all of us who took biology were taught—until last year. For that’s when a paper came out in Science by Toby Spribille et al. (see my summary of it here) showing that there was a third partner in the symbiosis: another fungus, but a basidiomycete (a group that includes yeasts) along with the well-known ascomycete (the two groups of fungi diverged about 400 million years ago),
A piece by Ed Yong in last year’s Atlantic, called to my attention by reader Hempenstein, tells how Spribille, raised in a trailer park in Montana, came to the discovery that overturned over 150 years of conventional wisdom. It’s a cool tale, involving sequencing the genome of two lichens that seemed to involve the same species of ascomycete fungus but one of which appeared to contain basiodiomycete genes as well. The failure to recognize this third partner in the relationship explains why scientists had encountered difficulties trying to make synthetic lichens by combining the constituent ascomycete and alga in the lab. They were simply missing a partner.
Of course Spribille wasn’t working by himself: there are 12 other authors on the Science paper describing the finding. And we’re not sure if all lichens have these three constituents: so far Spribille has found this situation to obtain in most of the “macrolichens”, but not all of them. And there are other groups of lichens that haven’t yet been examined. Finally, this whole revision may lead to a revision of what each of the partners provides for the others. Clearly the algal role is well defined, but what about the two types of fungi?
At any rate, the story shows the surprises that await us, and how easily long-established paradigms can change in an instant—especially now that we have the ability to sequence DNA. I’ll give just one excerpt from Yong’s piece, “How a guy from a Montana trailer park overturned 150 years of biology”. (I have to say that’s a bit clickbait-y given that Spribille did get a Ph.D. and did his work in a great lab (the McCutcheon group) working on symbiosis; and, moreover, it was a collaboration and not just the work of one guy.)
Down a microscope, a lichen looks like a loaf of ciabatta: it has a stiff, dense crust surrounding a spongy, loose interior. The alga is embedded in the thick crust. The familiar ascomycete fungus is there too, but it branches inwards, creating the spongy interior. And the basidiomycetes? They’re in the outermost part of the crust, surrounding the other two partners. “They’re everywhere in that outer layer,” says Spribille.
Despite their seemingly obvious location, it took around five years to find them. They’re embedded in a matrix of sugars, as if someone had plastered over them. To see them, Spribille bought laundry detergent from Wal-Mart and used it to very carefully strip that matrix away.
And even when the basidiomycetes were exposed, they weren’t easy to identify. They look exactly like a cross-section from one of the ascomycete branches. Unless you know what you’re looking for, there’s no reason why you’d think there are two fungi there, rather than one—which is why no one realised for 150 years. Spribille only worked out what was happening by labeling each of the three partners with different fluorescent molecules, which glowed red, green, and blue respectively. Only then did the trinity become clear.
Here’s a photo from the science paper showing the yeasts:
And the mysteries that remain:
“But really, we don’t know what they do,” says McCutcheon. “And given their existence, we don’t really know what the ascomycetes do, either.” Everything that’s been attributed to them might actually be due to the other fungus. Many of the fundamentals of lichenology will need to be checked, and perhaps re-written. “Toby took huge risks for many years,” says McCutcheon. “And he changed the field.”