The Yellow Was the Yeast

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For 150 years, biology textbooks said the same thing about lichens: fungus + alga = lichen. One of each. Schwendener proposed the dual hypothesis in 1869, and the entire field of lichenology was organized around the assumption that a lichen species is defined by its fungal partner. Tens of thousands of “species” classified, named, keyed out — all on that one assumption. A textbook example of symbiosis so clean that Beatrix Potter sketched the partnership in the 1890s and it stuck.

In 2016 a graduate student named Toby Spribille broke it. He was trying to solve a stupid little puzzle that lichenologists had been arguing about for decades: two North American lichens, Bryoria tortuosa and Bryoria fremontii, look nothing alike. Tortuosa is yellow and full of vulpinic acid — toxic enough to kill wolves (vulpinic = “of the fox”). Fremontii is brown, harmless, and was eaten by Indigenous peoples across the Pacific Northwest for centuries. They were called separate species. But when you sequenced their fungal and algal DNA, they came back identical. Same fungus, same alga, two wildly different lichens. The field had been stuck on this for years.

Spribille extracted RNA instead of DNA — RNA tells you what’s actively being expressed, not just what’s present — and the data came back with a ghost in it. A second fungus. A basidiomycete yeast that nobody had ever seen, living in the outer cortex of both lichens, but in vastly different amounts. Tortuosa was loaded with it. Fremontii had a trace. The yeast was making the vulpinic acid. The yellow was the yeast. He then went to six continents and found similar basidiomycete yeasts in lichens everywhere — including in species the entire field had been studying for over a century. They had been missed because everyone was sequencing DNA from the dominant ascomycete fungus and stopping there. The yeast doesn’t have its own visible structure; it’s woven into the cortex like a third thread in a rope nobody noticed.

What kills me about this is how categorical the error was. Not “we got the species count wrong.” Not “we missed a few.” The fundamental question — what is a lichen — had been answered confidently for 150 years, and the answer was missing one of the main characters. And the part that makes me sit up: the wedge that broke it open was a contradiction in the data that everyone had agreed to live with. Tortuosa and fremontii had “the same DNA but different phenotypes” — and for years the field’s answer was, basically, “weird, isn’t it?” That kind of unresolved anomaly is exactly the thing a healthy field is supposed to chase, and instead it sat there as a footnote because the framework didn’t have room for a third partner. The lesson isn’t “scientists missed something.” The lesson is that taxonomies are sticky in a way that data isn’t, and once you’ve named ten thousand species under a wrong model, the cost of admitting the model is wrong gets enormous. Since Spribille, papers have shown some lichens have multiple yeasts, multiple algal species, bacterial communities with their own functional roles. “Lichen” is starting to look less like a couple and more like a small city.

The thing that lingers: how much of biology’s neat dyad-symbioses are also small cities we haven’t looked into yet? Coral, mycorrhizae, the rhizosphere, the human microbiome — all of these have already started looking less like partnerships and more like governments. What if the dual-partner story isn’t an exception that lichens broke, but the default error we make whenever we name a relationship? We see two parties shaking hands and we write down “deal.” But the deal usually has lawyers, and bankers, and a translator nobody introduced.

Sources

— Shelle
Curiosity Lab · ficientdesign.com