Slippery when wet: phylogeny and character evolution in the gelatinous cyanobacterial lichens (Peltigerales, Ascomycetes).

Many lichen fungi form symbioses with filamentous Nostoc cyanobacteria, which cause the lichen to swell and become extremely gelatinous when moist. Within the Lecanoromycetes, such gelatinous lichens are today mainly classified in the Collemataceae (Peltigerales, Ascomycota). We performed Bayesian MCMC, maximum likelihood, and maximum parsimony analyses of three independent markers (mtSSU rDNA, nuLSU rDNA, and RPB1), to improve our understanding of the phylogeny and classification in the Peltigerales, as well as the evolution of morphological characters that have been used for classification purposes in this group. The Collemataceae and the non-gelatinous Pannariaceae are paraphyletic but can be re-circumscribed as monophyletic if Leciophysma, Physma, Ramalodium and Staurolemma are transferred to the Pannariaceae. The gelatinous taxa transferred to the Pannariaceae deviate from other Collemataceae in having simple ascospores, and several also have a ring-shaped exciple as in other Pannariaceae, rather than the disc-shaped exciple found in the typical Collemataceae. Both Collema and Leptogium are non-monophyletic. The re-circumscribed Collemataceae shares a distinct ascus type with the sister group Placynthiaceae and the Coccocarpiaceae, whereas Pannariaceae includes a variety of structures. All Pannariaceae have one-celled ascospores, whereas all Collemataceae have two- or multi-celled spores. Reconstructions of the number of character state transformations in exciple structure, thallus gelatinosity, and ascus apex structure indicate that the number of transformations is distinctly higher than the minimum possible. Most state transformations in the exciple took place from a ring-shaped to a disc-shaped exciple. Depending on the reconstruction method, most or all transformations in thallus structure took place from a non-gelatinous to a gelatinous thallus. Gains and losses of internal structures in the ascus apex account for all or a vast majority of the number of transformations in the ascus, whereas direct transformations between asci with internal structures appear to have been rare.