MOLECULAR TAXONOMY OF THE YEASTS

The term 'yeast' is often taken as a synonym for Saccharomyces cerevis iae, but the phylogenetic diversity of yeasts is illustrated by their assignment to two taxonomic classes of fungi, the ascomycetes and the basidiomycetes. Subdivision of taxa within their respective classes is usually made from comparisons of morphological and physiological feat ures whose genetic basis is often unknown. Application of molecular co mparisons to questions in yeast classification offers an unprecedented opportunity to re-evaluate current taxonomic schemes from the perspec tive of quantitative genetic differences. This review examines the imp act of molecular comparisons, notably rRNA/rDNA sequence divergence, o n the current phenotypically defined classification of yeasts. Princip al findings include: 1) budding ascomycetous yeasts are monophyletic a nd represent a sister group to the filamentous ascomycetes, 2) fission yeasts are ancestral to budding and filamentous ascomycetes, 3) the m olecular phylogeny of basidiomycetous yeasts is generally congruent wi th type of hyphal septum, presence or absence of teliospores in the se xual state, and occurrence of cellular xylose.