Phylogenetic Origins of the Asexual Mycorrhizal SymbiontCenococcum geophilumFr. and Other Mycorrhizal Fungi among the Ascomycetes

The phylogenetic relationship of the asexual mycorrhizal fungusCenococcum geophilumFr. among sexual ascomycetes was examined by phylogenetic analysis of nucleotide sequence data from the nuclear small subunit (18S) ribosomal RNA genic region. A specific focus of this study was to test the hypothesis that the genusElaphomycesis the closest sexual relative ofC. geophilum.Thus nucleotide sequence data of fiveC. geophilumisolates, threeElaphomycesspecies, and 44 additional genera of ascomycetes were included in the phylogenetic analyses. The percentage of similarity among the 18S rDNA sequences of theC. geophilumisolates examined was 99.8 to 100%, indicating thatC. geophilumis monophyletic. Percent similarity of nucleotide sequence among the threeElaphomycesspecies was also high and ranged from 99.4 to 99.5%. DNA parsimony and distance analysis of the sequence data separated these 2 genera on distant clades when sequence from 44 additional genera of ascomycetes was included. Parsimony and distance analyses positionedC. geophilumas a basal, intermediate lineage between the two Loculoascomycete orders, the Pleosporales and the Dothidiales, and strongly supportedElaphomycesto be of Plectomycete origin. Among the sexual Ascomycetes examined, which included representative taxa from four classes of filamentous Ascomycetes (Plectomycetes, Pyrenomycetes, Discomycetes, and Loculoascomycetes), no close sexual relative toC. geophilumwas identified. At least four independent lineages of mycorrhizal fungi were identified among the ascomycetes examined.