Chapter 1 - Definition, Classification and Nomenclature of the Yeasts

Abstract

Yeasts can be defined as those fungi whose asexual growth predominantly results from budding or fission, and which do not form their sexual states within or upon a fruiting body. An imprecise distinction is made between yeasts and those dimorphic filamentous fungi that often produce abundant yeast-like growth. For ascomycetous yeasts, this distinction is substantiated by molecular comparisons, which demonstrate that budding and fission yeasts are phylogenetically distinct from one another and from the euascomycetes. Yeasts, whether ascomycetes or basidiomycetes, have sexual states that are not enclosed in fruiting bodies. Rules for taxonomy of the yeasts and other fungi fall under the authority of the International Code of Botanical Nomenclature. From gene sequence comparisons, strains that represent new species are usually easily recognized. Nonetheless, some would argue against description of a new species based on a single strain. The argument is that a single strain does not reflect the genetic variation that might be found in a species, and that little can be learned of the ecology of a species when only a single strain is available.