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Within the basidiomycetes, the vast majority of known mycorrhizal species are homobasidiomycetes. It was therefore surprising when molecular and ultrastructural studies revealed a broad diversity of mycorrhizal associations involving members of the heterobasidiomycetous Sebacinaceae, fungi which, due to their inconspicuous basidiomes, have been often overlooked. To investigate the phylogenetic position of the Sebacinaceae within the basidiomycetes and to infer phylogenetic relationships within the Sebacinaceae, we made molecular phylogenetic analyses based on nuclear rDNA. We present a well-resolved phylogeny of the main lineages of basidiomycetes which suggests that the Sebacinaceae is the most basal group with known mycorrhizal members. Since more basal taxa of basidiomycetes consist of predominantly mycoparasitic and phytoparasitic fungi, it seems possible that a mycorrhizal life strategy, which was transformed into a saprotrophic strategy several times convergently, is an apomorphic character for the Hymenomycetidae. Mycorrhizal taxa of Sebacinaceae, including mycobionts of ectomycorrhizas, orchid mycorrhizas, ericoid mycorrhizas, and jungermannioid mycorrhizas, are distributed over two subgroups. One group contains species with macroscopically visible basidiomes, whereas members of the other group probably lack basidiomes. Sebacina appears to be polyphyletic; current species concepts in Sebacinaceae are questionable. Sebacina vermifera sensu Warcup & Talbot consists of a broad complex of species possibly including mycobionts of jungermannioid and ericoid mycorrhizas. This wide spectrum of mycorrhizal types in one fungal family is unique. Extrapolating from the known rDNA sequences in Sebacinaceae, it is evident that there is a cosm of mycorrhizal biodiversity yet to be discovered in this group. Taxonomically, we recognise the Sebacinaceae as constituting a new order, the Sebacinales.

The ontogeny of the fruit bodies of Mycena stylobates was studied by light microscopy and scanning electron microscopy (SEM). The ontogeny was divided into two phases: first the primordium with all the structures of the mature fruit body was established, then the primordial stipe elongated rapidly and the exposed hymenium started producing spores immediately. The first detected stage of fruit body formation was an irregularly arranged hyphal structure within the substrate colonized. After the rupture of the surface of the substrate, the primordium established on the surface. Soon a layer of wrapping hyphae was differentiated, which covered the complete primordium. The structures of the stipe and the cap developed synchronously. The developing stipe, cap and basal disc together formed a secondary ring-like cavity, in which the development of the hymenophore took place. The lower side of the cap was covered by a layer of degenerated hyphae. The development of the hymenophore started with a number of small alveolae on the lower side of the cap, which were covered with a hymenophoral palisade. The margins of these alveolae formed the primary lamellae, which in the first stage of their development were covered by a layer of degenerated hyphae. The hymenophoral palisade spread from the developing alveolae to the lamellar edge; the edge of the primary lamellae was forked in the early stages. Secondary lamellae were formed by the down folding of ridges from the lower side of the cap. In contrast to the primary lamellae, they were covered with hymenophoral palisade from the beginning. Spore production started immediately after the elongation of the stipe. These results were compared with other known modes of ontogeny within the Agaricales and some comments on the terminology used for the description of basidiome morphogenesis are made.