The acceptance by the mycological and lichenological communities, more than 12 years ago, that basidiomycetes tightly associated with green algae or cyanobacteria are lichens, simultaneously established, for the first time, that lichens (including both asco- and basidiolichens) are derived from multiple independent origins. Because approximately one-fifth of all known fungi are lichenized and more than 98% of the diversity of lichens is within the ascomycetes, one crucial question remains to be answered: How many independent origins of lichens took place during the evolution of the ascomycetes? To address this question we have sequenced a 1 kb and a 1.4 kb fragment at the 5' end of the small and large subunits of the nuclear ribosomal DNA, respectively, for 54 species representing 31 orders of ascomycetes. The combined maximum parsimony analysis revealed two equally most parsimonious trees. The ancestral character states (lichenized versus non-lichenized) were reconstructed for every node using maximum likelihood. The confidence levels for each of the ancestral character states were estimated by reconstructing the evolution of lichenization on a large number of trees randomly sampled within the confidence envelope surrounding the best trees.

Key words: ancestral character state reconstruction, comparative methods, lichens, macroevolution, phylogenetics, symbiosis