The term “symbiosis” was coined to capture the concept of unlike organisms “living together”. Some of the latter-day outcomes of symbiosis are familiar to anyone with a basic familiarity with biology – eukaryotic cells, photosynthesizing plants, coral reefs and lichens, for instance, all came about by different organisms forming a long-term beneficial codependence in the distant past. In the evolutionary give-and-take of organisms “living together”, however, not all outcomes are mutually beneficial all the time, and some are outright antagonistic. To arrive at these outcomes, the ancestors of today’s symbionts exchanged valuable metabolic goods in a microscopic, biofilm marketplace. How this all happens, and what it is doing to the partner genomes in the process, is central to the modern study of symbiosis.
For as long as anyone can remember, “lichen” has been used as a catch-all term for symbioses of fungi and photosynthetic partners (algae or cyanobacteria). Fungi that have entered into such symbioses hark from disparate evolutionary lineages, however, and are very old: many likely predate the emergence of four-footed terrestrial vertebrates. The lichens that have resulted from these separate events have in many cases settled on partner arrangements that are so distinct they are every bit as worthy of their own categories as are lungfish, amphibians, birds, reptiles or mammals.
We are interested in the innovations in fungal symbiosis associated with major forks in the evolutionary tree of life. What happens when fungi enter a preferential association with algae (“lichens”), and why do many fungi revert to lifestyles without algae (“saprotrophs”), and are they really so different? What enables some fungal-algal assemblages, and not others, to push upwards and outwards from their biofilm-like states and form complex, three-dimensional thalli (macrolichens)? And how are their characteristic architectures determined?