The common wisdom of fungus evolution has been flipped on its head by a new study. Researchers were surprised to find that many of today's free-wheeling fungi are descended from species that lived in symbiosis with other organisms as lichens.
A lichen is a collaboration between a fungus and a photosynthetic partner, which can be either green algae, cyanobacteria, or both. The algae or bacteria produce carbohydrates through photosynthesis, some of which are absorbed by the fungi. In return, the fungi provide shelter that allows its partners to survive otherwise intolerable conditions. Up to 20% of fungal species are thought to live as lichens. Scientists assumed lichenization had arisen independently many times, because of the diversity of species that form lichens and also because many branches of the fungus family tree have both lichenized and nonlichenized species.
A new genetic analysis has changed all that. Researchers from the Field Museum of Natural History in Chicago and the University of Reading in the United Kingdom analyzed the Ascomycota fungi--a phylum including 98% of known lichenized species and about 75% of all known fungi. The researchers analyzed RNA from 52 species and found that instead of evolving many times, lichenized fungi arose just a few times in the deep history of the group. On many subsequent occasions, the fungi ditched their photosymbiont partners, the researchers report in the 20 June issue of Nature.
Why do fungi so often turn to a solitary life? Team member Mark Pagel, an evolutionary biologist at the University of Reading, speculates that "symbioses are evolutionarily unstable ... as it's inevitable that natural selection causes the partners to exploit one another." And when one partner is too sorely taken advantage of, it doesn't benefit by continuing the relationship.
Other evolutionary biologists applaud the study. "This is the first solid piece of work to suggest lichen symbiosis is reversible," says David Hibbett, a fungal evolutionary biologist at Clark University in Worcester, Massachusetts. "It's a dynamite study ... which promotes the view that underlying symbioses are conflicts of interest between the partners."