**2.2.2 The evolution of fungal symbioses**

Some fungi live in symbiotic associations with photosynthetic partners, obtaining carbohydrates from their symbionts and providing water, nutrients, or protection in return. The two most remarkable types of symbioses involving fungi are lichens and mycorrhizal associations. In lichens fungi associate with green algae or cyanobacteria (or both) to form a vegetative structure called thallus. Lichens are remarkably successful, colonizing all kinds of habitats and regions (Nash, 1996). Lutzoni et al. (2000) used a phylogenetic framework to study the evolution of lichenization in the *Ascomycota* (including most of the lichenized fungi) and found that this life-style arouse early in the evolutionary history of the phylum and that it has been easier for lineages to loose the ability to be lichenized than it is to become lichens. These results led them to conclude that many non-lichenized *Ascomycota* lineages (including important well-known fungi such as *Penicillium* and *Aspergillus*) descend from lichenized ancestors.

Mycorrhizal associations are symbioses involving fungi and plant roots. The mycorrhizal condition is the natural state for most plants under most ecological conditions. Mycorrhizas (the structure constituted by the root and the fungus) are the main organs of nutrient uptake in land plants (Smith & Read, 2008). The evolution of mycorrhizal associations had a tremendous impact on terrestrial ecosystems and is thought to have facilitated the initial colonization of land by plants (Pirozynski & Malloch, 1975). Ectomycorrhizal (ECM) fungi are one of the major groups of mycorrhizal fungi. They are mainly basidiomycetes and associate with about 30 plant families, including oaks, pines, poplars, and dipterocarps (Smith & Read, 2008). A broad phylogenetic analysis of mycorrhizal and free-living mushroom-forming fungi (*Homobasidiomycetes*, within *Agaricomycotina*) revealed that the ancestor of this group was free-living, and that ectomycorrhizal symbioses were lost and gained a number of times within the clade (Hibbett et al., 2001). This means that ectomycorrhizal fungal symbionts have evolved repeatedly from decomposer percursors and that there have been several reversals to this latter stage, with half of all Homobasiomycetes potentially deriving from ectomycorrhizal ancestors. Such findings suggest that although ECM symbioses are widespread and play relevant ecological roles in nature, they are an evolutionary unstable mutualism.
