Mycologists have made numerous attempts to categorise fungi according to their vegetative and reproductive characteristics. Fungi are divided into four classes according to conventional classifications: Ascomycetes, Phycomycetes, Basidiomycetes, and Deuteromycetes. The Phycomycetes comprise fungi like Oomycetes, Chytridiomycetes, and Zygomycetes, lower fungi with an algal origin.
The identification of yeasts has been traditionally based on morphological characteristics, but due to their poor differentiation, distinctive reactions in a standardized set of fermentation and assimilation tests have been more reliable. However, genetic crosses as well as molecular studies have demonstrated that those phenotypic characteristics are not adequate for defining taxa. Therefore, systematics have turned increasingly to molecular approaches to distinguish and identify yeast species and to develop a system of classification based on phylogeny. Kurtzman and colleagues analyzed partial sequences of SSU and LSU rRNAs and were the pioneers in yeast phylogenetic studies; they provided numerous and valuable data to assess taxonomic relationships between these fungi (296, 298, 301, 302). 2b1af7f3a8