The members of some fungal species have the ability to grow in the form of yeasts or mycelium
depending on the environmental and some internal conditions. This phenomenon, denominated
“dimorphism” is not exclusive of some fungal taxa, but examples represent almost all groups. Fungal
dimorphism is an important phenomenon from both applied and basic concepts. In the former aspect
because a significant number of species pathogenic for plants, animals, and specially humans are dimorphic,
the causal agents displaying different forms during their saprophytic and pathogenic stages, and more
specifically because it has been demonstrated that inhibition of the dimorphic transition by drugs or
mutation blocks the pathogenic development. Regarding the second aspect, the dimorphic transition has all
the elements to consider it as an example of cell differentiation, and as such it constitutes a basic model for
the study of this important biological phenomenon. In this chapter we analyze dimorphism on this line and
propose its study through its division into four blocks of reactions: stimulus reception, translation of the
stimulus, change in the developmental program and final outcome.
Keywords: Dimorphism, fungi, yeast, mycelium, hyphae, pseudomycelium, cell differentiation,
development, morphogenesis, molecular taxonomy, phylogeny, morphology index, virulence, cell wall.
