In general, the symbiosis is considered obligatory for successful growth and reproduction of the heterotrophic fungal partner. However, also the autotrophic photobiont obviously benefits from the relationship. Thus the lichen is typically a highly stable association which extends the ecological range of both partners. The body (thallus) of most lichens is quite different from that of either the fungus or alga growing separately, and may to some extent resemble simple plants in form and growth (Sanders 2001).
Approximately one-fifth of all known fungal species form obligate symbiotic associations with a green algae, a cyanobacteria (also known as blue-green algae) or simultaneously with both types of photobionts. Many ascomycetes (ca 42%) and a few basidiomycetes participate in lichen symbioses. Ascomycota is a phylum that includes more than 98% of the known lichenized fungal species. According to Lutzoni & al. 2001 the lichens in this phylum evolved earlier than previously believed, and that gains of lichenization have been infrequent during the evolution of Ascomycota. In contrast multiple independent losses of lichenization have occurred. As a consequence, major Ascomycota lineages of exclusively non-lichen-forming species are derived from lichen-forming ancestors (including well-known fungi such as Penicillium and Aspergillus).
The lichen association is thus believed to be a symbiosis with a long evolutionary history. However, the past has only left behind clues to what once happened. Systematists use these clues and try to reconstruct evolutionary relationships among organisms. The study of evolutionary relationships among organisms based on molecular, mostly DNA sequence data, have provided new insights about the relationships between organisms that often contradict the traditional, primarily morphology-based hypotheses. During the past two decades, contributions to fungal classification based on molecular data have led to major changes in our understanding of the evolution of fungi and of their phylogenetic affinities (see Hibbett & al., 2007 for a review).
Hibbett, D.S. et al. 2007. A higher-level phylogenetic classification of the Fungi. Mycol. Res. 111: 509--547.
Lutzoni, F., Pagel, M. & Reeb, V. 2001. Major fungal lineages are derived from lichen symbiotic ancestors. Nature 411: 937-940.
Sanders, W.B. 2001. "Lichens: interface between mycology and plant morphology." Bioscience 51: 1025-1035.