ANALYSIS OF GENES-CODING FOR SMALL-SUBUNIT RIBOSOMAL-RNA SEQUENCES INSTUDYING PHYLOGENETICS OF DEMATIACEOUS FUNGAL PATHOGENS

Because of their ability to display yeast-like growth forms in various environmental conditions, dematiaceous (melanized) hyphomycetes of th e form-genera Exophiala, Rhinocladiella, and Wangiella have been infor mally termed ''black yeasts.'' Cladistic analysis of 1,050 bp of the g enes coding for small-subunit rRNA (SSU rDNA) supported a close relati onship among species of these black yeasts with other dematiaceous hyp homycetes in the form-genera Fonsecaea, Phialophora, and Ramichloridiu m. The conventional categories of these fungi based on asexual states are not supported by phylogenetic analysis of SSU rDNA sequences. Isol ates exhibiting annellidic modes of blastic conidiogenesis (e.g., Exop hiala spp.) were not monophyletic and were placed as sister taxa to is olates that produce phialides or sympodulae. The results indicated ver y close relationships between isolates of Wangiella dermatitidis and E xophiala 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 ascomyc etes, Eurotiales and Onygenales. The etiological agents of chromoblast omycosis were found to be a closely related group (clade), while the a gents of phaeohyphomycosis displayed a broader distribution on the SSU rDNA tree.