The Lichen/Non-lichen Connection

© Else C. Vellinga
Lab
Original publication: Mycena News, September 2005

One thing we knew for certain about lichens: the fungal partner of the lichen symbiosis could not live its whole life cycle without a photosynthesizing slave providing sugars to the fungus. These enslaved, encapsulated algae or cyanobacteria (formerly known as blue algae) can live without the fungi, but the fungus had to find an algal or bacterial partner, the photobiont.

Now we have to rethink this certainty, as it was recently shown that the same fungal species can exist both with and without photobionts and in each case the fungus forms fruitbodies and sexual spores. Both lifestyles were known: the surprise is in their identification. Stictis species were originally the ones without a partner, growing on branches stripped of bark. Conotrema was the name for a lichenized species, growing on the bark of trees. Now, there can only be one name, and this is Stictis, since it is the older name.

If a spore of the fungus lands on bare branches they will grow as a lonely fungus, as no algal cells are found in this habitat. If a spore lands on the bark of a tree it will find an algal partner and go on as a lichen. This strategy of optional lichenization has many advantages in fast changing ecosystems experiencing disturbance or succession. Of course the finding that one and the same fungal species can live either as a fungus or as a lichen, raises many new issues; how widespread is this phenomenon, where does the fungus find its carbon when it is living without photobionts, and what is the role of lichenization in the evolution of the fungi?

The Stictis/Conotrema connection was unraveled by researchers from Sweden who had extensively collected on Populus tremula, a trembling aspen species in the northern boreal region of Scandinavia. They show their surprise and enthusiasm for their discovery in the following paper: