Analysis of genes coding for small-subunit rRNA sequences in studying phylogenetics of dematiaceous fungal pathogens

Because of their ability to display yeast-like growth forms in various environmental conditions, dematiaceous (melanized) hyphomycetes of the form-genera Exophiala, Rhinocladiella, and Wangiella have been informally termed "black yeasts." Cladistic analysis of 1,050 bp of the genes coding for small-subunit rRNA (SSU rDNA) supported a close relationship among species of these black yeasts with other dematiaceous hyphomycetes in the form-genera Fonsecaea, Phialophora, and Ramichloridium. The conventional categories of these fungi based on asexual states are not supported by phylogenetic analysis of SSU rDNA sequences. Isolates exhibiting annellidic modes of blastic conidiogenesis (e.g., Exophiala spp.) were not monophyletic and were placed as sister taxa to isolates that produce phialides or sympodulae. The results indicated very close relationships between isolates of Wangiella dermatitidis and Exophiala mansonii and between Rhinocladiella aquaspersa and Exophiala jeanselmei. This clade of dematiaceous hyphomycetes was a sister group to a clade comprising members of two orders of cleistothecial ascomycetes, Eurotiales and Onygenales. The etiological agents of chromoblastomycosis were found to be a closely related group (clade), while the agents of phaeohyphomycosis displayed a broader distribution on the SSU rDNA tree.