Composite organisms known as lichens often emerge from the symbiotic relationship of an algae and/or cyanobacteria that live among filaments of a fungus.
For one and a half century, these mutually helpful lichens have been considered as the model organisms for symbiosis.
Now, however, a new study has revealed that there might be a third partner involved in this supposed biological pairing. In fact, the third party may have been hiding right under our noses all this time.
Hidden Third Partner
For a lichen to be produced, a fungus either joins with an algae or cyanobacteria in a symbiotic relationship that benefits both organisms.
The new research, which was led by scientists from the University of Montana in cooperation with Purdue University, shows that lichens from six continents also contain single-celled fungi known as basidiomycete yeasts.
What's remarkable about the yeasts is that they possibly create chemicals that essentially help lichens repel microbes and ward off predators.
Researchers say the finding may explain why many genetically similar lichens possess wildly different physical features.
It may also explain why experts have failed to synthesize lichens in the laboratory, even when they combine species that partner successfully in nature.
Professor M. Catherine Aime, a mycologist and co-author of the new study, says the discovery overturns their longstanding assumptions about lichen symbiosis.
Aime says the yeasts make up a whole lineage that no one ever knew even existed. And yet, the yeasts exist in a variety of lichens in every continent on the planet as a third symbiotic partner.
How Investigations Pushed Through
Scientists began their research as they wondered why two lichen species seemed genetically identical but contained distinct attributes.
For instance, the yellow lichen Bryoria tortuosa produces a toxic substance called vulpinic acid. On the other hand, the dark brown Bryoria fremontii, which is comprised of the same alga and fungus, produces no acid.
When Aime and colleagues expanded their study to include all fungi, they discovered indications that genes were being expressed in various ways and unexpectedly.
This was done by a fungus that seemed to belong to Basidiomycota, a different phylum than the fungal partner. This suggested the presence of a second fungus — a previously unknown yeast species in a class of early-evolving basidiomycetes.
"It took a long time to convince myself that I wasn't dealing with a contamination," says Toby Spribille, the study's first author.
Once Spribille, Aime and colleagues started looking for traces of similar yeasts in other lichens, they identified related lineages in 52 lichens across the planet, as well as molecular evidence that tells of a shared evolutionary history between the partners.
Spribille says the yeasts were always there and that they somehow managed to make it crack open. The yeasts are invaluable because it allows for the production of an acid that defends the organism from microbe invasion.
Meanwhile, although Aime says they have barely touched the tip of the iceberg in studying the importance of microbes in biological systems, the research points to similar beneficial roles in plant systems.
Details of the new report are published in the journal Science.