Ascus structure of eight yellow, two white and two brown Rhizocarpon species has been investigated by light microscopy. Ultrastructure and function in R. atroflavescens subsp. pulverulentum and R. montagnei were studied in TEM.The Rhizocarpon-type ascus clearly differs from all other ascus types observed in the Lecanorales. It is bitunicate, opening with a slight 'J ac k-in-the-box'-mechanism. Its structure and function are related to patellariacean ascus types, but unlike those the ascus wall cytochemistry shows a certain similarity with Lecanora-and Peltigera-type asci. Rhizocarpon-type asci are embedded in a strongly amyloid hymenial gelatine. The nonamyloid ascus wall is surrounded by the strongly amyloid outer layer. The slightly amyloid expansible inner layer (= endoascus) is apically thickened; it shows the banded and pleated ' accordion-structure ' characteristic of bitunicate asci. Prior to dehiscence, the ascus wall and its outer layer burst. Thereafter the pleatings of the expansible inner layer are stretched, forming the rather short beak which reaches the hymenial surface. During expansion gliding occurs between the expansible inner layer and an outer part of the endoascus, here described as the ' inner layer '. In a few sections of aldehyde-fixed material of R. atroflavescens a small laminated plug was observed in the apex of the endoascus.Rhizocarpon-lype asci are considered to be the most archaic in the Lecanorales. This supports a hypothesis that Rhizocarpon is a phylogenetically basal group, linking the evolved Lecanorineae, and possibly also the Peltigerineae and Teloschistineae with not yet recognized bitunicate ancestral forms similar to those occurring in the Patellariaceae.
IntroductionLiving in symbiosis with algae is a nutritional specialization of various unrelated higher fungi, many of which belong to orders containing both lichenized and nonlichenized species (Henssen & Jahns 1973;Barr 1976). The Lecanorales, however, are probably anciently lichenized and form the main group of lichenized fungi with more than 10 000 species, and seem to have very few non-lichenized members; their taxonomic position as well as their phylogenetic origin are a matter of debate.On the basis of ontogenetic and physiologic criteria, Nannfeldt (1932) and Dughi (1957) supposed that the Lecanorales evolved from archaic ascomycetes. On the basis of comparative studies of ascus structure and ascocarp ontogeny of lichenized as well as non-lichenized fungi, Chadefaud and his coworkers (Chadefaud, 1973;Chadefaud et al. 1963Chadefaud et al. , 1969Letrouit-Galinou 1973) take the lecanoralean fungi to be the most archaic group amongst extant ascomycetes. These authors support the hypothesis that recent bitunicate as well as unitunicate ascomycetes evolved from lecanoralean ' type archaeasce ' asci (which is considered as bitunicate). From comparative studies of ascus fine structure and function in different lecanoralean * I in Lichenologist 10: 47-67 (1978).