2 Department of Plant and Microbial Biology, University of California, Berkeley, California 94720-3102
The basidial morphology of the two species in the genus Filobasidiella, F. neoformans and F. depauperata is very similar when observed with light microscopy. The two species can be separated by the presence or absence of clamp-connections and the shape of their basidiospores. F. neoformans produces clamp-connections on the hyphae and its basidiospores are subglobose to barrel-shaped, while F. depauperata lacks clamp-connections and produces oblong to pentagonal basidiospores. Although the basidial morphology undoubtedly places the two species in the same genus, there are fundamental differences between them. Unlike F. neoformans, F. depauperata does not have a yeast stage and lacks dikaryotic hyphae. F. depauperata also lacks the biochemical markers of F. neoformans such as urease activity, myo-inositol assimilation and starch formation, as well as growth at 37 degree C. Most importantly, F. neoformans is a human pathogen while F. depauperata is not. A phylogenetic tree constructed on the basis of 18S rDNA sequence indicates that F. depauperata is most closely related to F. neoformans, and that the two species are closer related to the Tremellaceae than to the Filobasidiaceae. Hyphal septa of F. depauperata have dolipores with parenthesomes consisting of cupulate vesicles typical of Tremella species and Filobasidium capsuligenum. The nucleus in the basidium undergoes meiosis prior to spore formation. The spindle pole bodies (SPBs) of F. depauperata are nearly identical to those reported for F. neoformans and Tremella globispora. Molecular studies indicate that F. depauperata lacks the CNLAC1 gene which encodes laccase in F. neoformans. The present study indicates that the genus Filobasidiella belongs to the Tremella-lineage and that differences between the two species with regard to the presence or absence of an ontogenetic yeast stage and certain biochemical characteristics are less important for defining the genus than are basidium morphology, the mariner of basidiospore production, and rDNA nucleotide sequences.