Nuclear ribosomal DNA sequences (ITS region) of fifteen species in the hyphomycete genus Oidiodendron and ten species from the 4 genera in the Myxotrichaceae, Byssoascus, Gymnostellatospora, Myxotrichum, and Pseudogymnoascus, were analysed to: (i) reveal the levels of intra- versus interspecific sequence variation within the genus Oidiodendron, clarify species delimitation and examine the usefulness of some morphological characters used for identification; (ii) assess the possible conspecificity of documented ericoid mycorrhizal strains of Oidiodendron; and (iii) test the hypothesis based on morphological inference that the genus Oidiodendron belongs with the genus Myxotrichum in the Myxotrichaceae (Onygenales). Comparison of molecular and morphological data for multiple strains of O. griseum, O. tenuissimum and O. maius revealed that conidiophore length and the production of a diffusing pigment are not reliable key characters for the genus. Several historically important ericoid mycorrhizal strains, documented as O. griseum, were reidentified as O. maius. Parsimony analyses of 23 Oidiodendron strains showed that three highly supported monophyletic groups, each one consisting of a pair of species, are resolved within the genus. A low level of sequence divergence between the species in these pairs suggests conspecificity for each pair. Other interspecific relationships were not well-supported by bootstrap values. Parsimony analysis of a second dataset composed of mitotic and meiotic taxa showed that Oidiodendron, Myxotrichum and Byssoascus form a well-supported monophyletic group within the Myxotrichaceae, and that this group has diverged significantly from the two other genera in the family. Relationships in the monophyletic group were correlated with anamorph state produced; three Myxotrichum spp., and B. striatosporus, all with distinct or reported Oidiodendron states, nested with anamorphic species, while species of Myxotrichum with other anamorph states were excluded. In addition, sequence divergence measures between the meiotic and mitotic species clustered in two monophyletic clades were found to be comparable to the intraspecific levels for Oidiodendron spp. These results support the initial hypothesis that Oidiodendron is closely related phylogenetically to the genus Myxotrichum and suggest that the generic concept of Byssoascus needs reexamination.
