Scientists Discover Third Partner in Lichen Symbiosis

Lichens are symbiotic organisms consisting of a fungus and a photosynthesizer. Now scientists have discovered a third species involved in the partnership. Researchers from the University of Montana found that Basidiomycete yeasts are present in the cortexes of most lichens. The findings are in a paper just published in the journal Science.

Common throughout most of the world, lichens are known for their symbiosis. A fungus partners with an organism capable of photosynthesis, either an algal or cyanobacteria species. The algae produce food while the fungus structure offers protection. Oddly, scientists were never able to grow lichens in laboratories, even when they combined the exact partner species. There was also another mystery. Genetically identical lichen species sometimes had completely different appearances and behaviors. For example, Bryoria tortuosa has a yellow appearance and produces vulpinic acid. Bryoria fremontii consists of the same partner species but is brown and doesn’t produce acid.

The research team analyzed gene expression in the fungal partner for both B. toruosa and B. fremontii. They found no difference between the two species. When the team broadened their analysis, however, they found that there was a difference in expressed genes but not by the main fungus. Instead, they found that the genes were being expressed by a third organism—a type of yeast. They found traces of Basidiomycete yeast and initially assumed the samples had been contaminated. Further analysis showed that the yeast were present in 52 different genera of lichens, across six continents.

The researchers discovered that the yeast species was responsible for the production of acid by B. toruosa. The discovery of this third symbiotic partner also explains why scientists have previously been unable to grow lichens in the lab. There hasn’t been much research on Basidiomycete yeasts as a group but the team speculates they may help lichens repel predators, as seen in B. toruosa. Yeasts may also produce substances with antimicrobial effects. Further research is needed but these findings represent an exciting step in finally understanding lichens.

REFERENCE

Toby Spribille et al. Basidiomycete yeasts in the cortex of ascomycete macrolichens. Science (2016).

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