[Dimorphism in fungi--a gray zone for taxonomy?]

Dimorphic fungi can grow as mycelial phase and as yeast phase. The change of growth form is effected by an altered programme in gene expression, which is induced either in certain stages of ontogenesis or by environmental factors. Therefore it is necessary to distinguish ontogenetically conditioned (irreversible) and environment conditioned (reversible) dimorphism. The first is characteristic for dimorphic Ustomycetes, Basidiomycetes and related anamorphs as well as for Taphrinales; the second for the majority of dimorphic Ascomycetes, Endomycetes, and related anamorphs. The integration of dimorphic fungi in the systems of filamentous fungi and of yeasts--which originally have been elaborated rather independently--was difficult in many cases. In recent times, the study of certain ultrastructural and biochemical-physiological characters has much facilitated the taxonomic arrangement of dimorphic fungi and has allowed to demonstrate some phylogenetic relations among filamentous, dimorphic, and yeast fungi. The authors hold the concept that yeasts have evolved from filamentous resp. dimorphic fungi by neotenic simplification.