Lichen: Apparently happy couple really a threesome

July 21, 2016) Since the discovery of their true nature 140 years ago, lichens have been the poster children for symbiosis. In the textbook definition of a lichen, the filaments of a single fungus provide protection for photosynthetic algae or cyanobacteria, which in turn provide food for the fungus. But 140 years after the term "symbiosis" was coined to describe lichen, it turns out there's a third party involved in the relationship - a yeast that may help provide the structure found in large "leafy" and "branching" lichens.

"These yeast are sort of hidden just below the surface," says John McCutcheon, a fellow of the Integrated Microbial Biodiversity program at the Canadian Institute for Advanced Research (CIFAR), and a genome biologist at the University of Montana. "People had probably seen these cells before and thought they were seeing something else. But the molecular techniques we used happened to be especially good for spotting the signal of a separate organism, and after years of looking at the data it finally occurred to us what we were seeing."

McCutcheon and colleagues found a variety of yeast called a Basidiomycete living in the cortex - the tough outer layer - of all of the lichens they sampled from a number of locations on six different continents.

Lichens come in two basic flavors. One forms a thin, film-like layer on rocks and trees. The other kind is composed of "macrolichens" that grow big leafy, branching or vine-like structures. It's the latter that seem to harbor the yeast.

The discovery started when Toby Spribille, a postdoctoral fellow in McCutcheon's lab, was studying two lichen species collected from the mountains around the Missoula, Montana campus - Bryoria fremontii and B. tortuosa. The two species are distinguished by the presence of vulpinic acid in B. tortuosa, which also gives it a yellow colour. However, genetic tests showed that the known fungus and alga in both lichen species were identical.

But Spribille and McCutcheon found the genetic signature of a third species - a basidiomycete yeast, present in both of the lichen species but more abundant in the yellow version. They and their colleagues went on to test 56 different lichens from around the world, and found each had its own distinct variety of basidiomycete yeast.

McCutcheon says it seems possible that the newly discovered yeast is necessary in part to create the large structures found in macrolichens. They tend to be embedded in crystals inside the lichen, and could have a role in forming them. This might solve the mystery of why macrolichens are so hard to grow in the lab when you start with only a fungus and an alga - they could be missing a vital third member.

"This doesn't prove that they're necessary to create the structure of the macrolichens, or that they do anything else for that matter. But its early days. It took a lot of work just to discover that they were there. We're interested if the yeast is making these important compounds, or possibly enabling the other fungus to make them. We don't know, but it's the obvious next question," McCutcheon says.

The paper "Basidiomycete yeasts in the cortex of ascomycete macrolichens" will be published in Science on July 22.

Source: Canadian Institute for Advanced Research