Multiple origins of lichen symbioses in fungi suggested by SSU rDNA phylogeny.

Phylogenetic hypotheses provide a context for examining the evolution of heterotrophic lifestyles. The lichen lifestyle, which is the symbiotic association of fungi with algae, is found in various representatives of Dicaryomycotina, both Ascomycetes and Basidiomycetes. A highly resolved parsimony analysis of small subunit ribosomal DNA (SSU rDNA) sequences suggests at least five independent origins of the lichen habit in disparate groups of Ascomycetes and Basidiomycetes. Because lichen associations arose from parasitic, mycorrhizal, or free-living saprobic fungi, neither mutualism nor parasitism should be construed as endpoints in symbiont evolution.