5.19 Yeast-mycelial dimorphism

The majority of fungi grow in one of two growth forms: either as rounded or spherical cells (yeast fungi) or as a polarised branching mycelium (filamentous fungi). However, some fungi are capable of growing in both forms depending on the local environmental conditions. Such fungi are found throughout the fungal kingdom and are termed ‘dimorphic’. Dimorphism is common amongst pathogens because the different growth forms represent different advantageous strategies for the pathogen: the yeast form is ideal for distribution around the host through fluid circulation systems (water-conducting tissues of plants, blood or lymphatic circulation in animals), whilst the hyphal growth form can penetrate solid tissues. Examples include the human and animal pathogens Candida albicans, Paracoccidioides brasiliensis, Histoplasma capsulatum and Blastomyces dermatitidis, and plant pathogens like Ustilago maydis, Ophiostoma ulmi and Rhodosporidium sphaerocarpum.

Whilst most dimorphic fungi are members of the Ascomycota, dimorphism also occurs in other phyla (examples are the common mould Mucor rouxii, a member of the Mucoromycota, and Ustilago and Rhodosporidium, which are in the Basidiomycota). Umbilicaria muhlenbergii is the only known dimorphic lichenised fungus that grows in the hyphal form in lichen thalli but as yeast cells in axenic cultures. Wanga et al. (2020) showed that nutrient limitation and hyperosmotic stress trigger the dimorphic change in U. muhlenbergii and found that cAMP signalling had a role in regulating dimorphism in this species. Whereas in smuts (Ustilaginomycotina), lipid/hydrophobicity is a potential common cue for dimorphic transition in plant-associated dimorphic fungi (Kijpornyongpan & Aime, 2020). Comparison of transcriptomic profiles during yeast and hyphal growth in four species of plant pathogens (Tilletiopsis washingtonensis, Meira miltonrushii, Ustilago maydis and Ophiostoma novo-ulmi) revealed differences across all four species suggesting species-specific programmes for dimorphic transition and hyphal growth (Kijpornyongpan & Aime, 2021).

Candida albicans is an important opportunistic pathogen of humans and the majority of research has focused on understanding the mechanisms involved in dimorphism in this organism. Normally, infections are relatively superficial and restricted to mucosal membranes causing both oral and vaginal candidiasis (commonly known as thrush). However, for individuals who are immunocompromised, for example following immunosuppressive therapy after transplant surgery or as a result of HIV infection, Candida infections often become systemic and invasive and have a high rate of fatality. Invasive candidiasis has been associated with the presence of the hyphal form of the organism, whereas superficial infections are generally associated with the yeast phase, so implicating the transition from yeast to hypha as an important event in the pathology of the organism. However, a non-hyphal mutant strain of C. albicans was still able to cause infections and death in mice, though the extent and spread of the organism was reduced; and the human pathogens Histoplasma capsulatum, and Paracoccidioides brasiliensis are pathogenic only in the yeast phase.

A wide range of environmental parameters have been shown to induce a yeast-to-hypha transition in Candida, including serum, N-acetylglucosamine, proline and a shift from an acidic to a more alkaline medium. This suggests that a number of independent signal transduction systems exist within Candida for each factor, particularly as mutants that have lost the ability to undergo a yeast-mycelium transition by one stimulus are still capable of forming hyphae or germ tubes when exposed to other stimuli (Gauthier, 2015; Nigg & Bernier, 2016).

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Updated May, 2021