Any single-celled fungus. The ‘true’ or budding yeasts constitute the subphylum Saccharomycotina of the phylum Ascomycota. They occur as single cells or as groups or chains of cells; true yeasts reproduce asexually by budding and sexually by producing ascospores. Yeasts of the genus Saccharomyces ferment sugars; they are used in the baking and brewing industries (see baker’s yeast), and in genetics as experimental model organisms.