Peculiarities of lichen-forming fungiThe biology of sexual reproduction is very poorly understood in lichen-forming fungi; this group of nutritional specialists, which derive fixed carbon from a minute algal or cyanobacterial symbiont, belong to approx. 99% to the ascomycetes (sac fungi), almost 40% of sac fungi being lichenized. More than 10% of terrestrial ecosystems are lichen-dominated; these are the climatically extreme sites where plants can no longer compete (high mountain, arctic, antarctic or desert ecosystems). Lichen-forming ascomycetes are the ancestors of many biotechnologically important non-lichenized sac fungi. Aims of the present investigations Non-lichenized and lichenized ascomycetes have a relatively simple mating system, only one MAT locus being involved. Studies on the biology of sexual reproduction in lichen-forming ascomycetes, as performed in the current project, help us to understand 1) processes involved in the formation of new species. The most common and widespread Xanthoria species, as studied in our project, derive from cross-fertile ancestors and have switched to self-fertility; thus they can form ascospores at any time. 2) the genetics of populations. Many species of lichen-forming fungi are critically endangered, their populations being strongly fragmented. If all thalli (individuals) of a small, isolated population of a cross-fertile species belong to the same mating type they cannot produce fruiting bodies, because spermatia of the opposite mating type are needed to form a dikaryon, whence sporangia (asci= sacs) with sexually produced sprores (ascospores) derive. With molecular tools we characterized the mating systems of numerous species of lichen-forming ascomycetes, the focus being on Teloschistaceae and Parmeliaceae. 3) developmental processes involved in fruiting body formation. Light and electron microscopy techniques applied to lichens, as collected in the wild, and to their sterile cultured fungi helped us to understand why early stages of fruiting body formation are so different from the model organisms among non-lichenized ascomycetes, as found in text books. Instead of one ascogonium with trichogyne per fruiting body initial, as typically formed by non-lichenized ascomycetes of the well-investigated genera Neurospora, Sordaria etc., lichen-forming species differentiate a branched hyphal system with many ascogonia and trichogynes per primordium, thus enhancing the chance of successful dikaryon formation.