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.