Scientists unveil unexpected new player in lichen symbiosis

For nearly 150 years, lichens have been known as a classic example of symbiosis between a fungus and a photosynthetic partner, either an alga or cyanobacteria, but researchers reported Thursday many lichens actually are composed of three partners, not the widely recognized two.

By using recent advances in genomic sequencing, the new study, published as the cover article in the U.S. journal Science, showed that many lichens contain not only the expected fungus and the photosynthesizing partner, but also a previously unknown second fungus that had never before been detected.

"This is a pretty fundamental shake-up of what we thought we knew about the lichen symbiosis," first author Toby Spribille of the University of Montana said in a statement.

"It forces a reassessment of basic assumptions about how lichens are formed and who does what in the symbiosis."

The discovery came about when Spribille, working as part of the university's microbiologist John McCutcheon's team, set out to investigate why two lichen species seemed genetically identical but had distinctive attributes.

The lichen Bryoria tortuosa is yellow and produces a toxic substance known as vulpinic acid while B. fremontii -- made up of the same fungus and alga -- is dark brown and produces no such acid.

An analysis of gene expression of the known fungal partner in the two species showed no differences, so the researchers broadened the data search to include all fungi.

Unexpectedly, they found indications that genes were being expressed by another fungus that appeared to belong to the Basidiomycota, a completely different phylum than the known fungal partner.

What is more, the toxic lichens contained far more of the extra fungus, which the researchers identified as a previously unknown form of yeast.

Spribille, also a postdoctoral researcher at the University of Graz, said he was initially skeptical of the results.

"It took a long time to convince myself that I wasn't dealing with a contamination," he said.

But once the researchers began looking for traces of similar yeasts in other lichens, they found related lineages in 52 genera of lichens worldwide and molecular evidence that indicates a long, shared evolutionary history between the symbiotic partners.

"It's everywhere," McCutcheon said. "This thing has basically been hidden in plain sight for more than 100 years."

Since the new fungus is globally distributed and seems to be an integral part of the symbiosis, the researchers will next explore what it really does.

"The word symbiosis in part comes from the study of lichens," McCutcheon said. "The textbook definition of lichen has always been restricted to one fungus and one fungus only. Our work shows that this definition doesn't seem to be correct." Endit