Molecular approaches to the taxonomy of ballistosporous yeasts based on the analysis of the partial nucleotide sequences of 18S ribosomal ribonucleic acids.

Abstract

The partial nucleotide sequences of 18S ribosomal RNA (positions 1451 to 1618 in Saccharomyces cerevisiae) of 48 strains representing 46 species of ballistosporous yeasts and a related genus were determined by dideoxy sequencing with reverse transcriptase. The sequence data were analyzed by neighbor-joining method and several unrooted phylogenetic trees were drawn. Ballistosporous yeast species were separated into two groups: a of xylose-containing species and a of xylose-lacking species. All of the species of Bullera and Kockovaella are included in the former group, and all of the species of Ballistosporomyces, Bensingtonia, Sporobolomyces, and Sporidiobolus are included in the latter group. Species of the genus Bullera are scattered on several branches of the phylogenetic trees and species of Cryptococcus are located at several positions close to those of species in several groups of Bullera. Species of piricola group of Bullera, Bullera megalospora, Bullera punicea and Bullera piricola, are considered to be closely related and are far removed from where the remaining species of the genus are located. These three species are characterized by the production of extremely large asymmetrical (bilaterally symmetrical) ballistospores and are considered to be included in a different genus from Bullera. A close relationship of Kockovaella to Fellomyces is suggested. Species of Sporobolomyces are scattered on several branches of the phylogenetic trees. The same phenomenon is observed in the case of Bensingtonia and Ballistosporomyces. These observations clearly suggest that the mode of vegetative reproduction such as the productivity of ballistospores, stalked conidia and budding yeast cells, does not reflect the phylogenetic relationship among basidiomycetous yeasts and is considered to be a less significant taxonomic criterion than hitherto believed.