Trimorphic stepping stones pave the way to fungal virulence

The fungal kingdom encompasses ≈1.5 million species (1) as diverse as single-celled yeasts, pathogens of animals/plants, and plant root symbionts. Fungi are eukaryotic, closely aligned with metazoans (2, 3). Animals and fungi diverged ≈1 billion years ago; their last common ancestor was unicellular, motile, and aquatic. Some fungi grow as unicellular yeasts, but most are filamentous multicellular organisms. Importantly, some fungi are dimorphic, growing as both yeast and filamentous forms (i.e., Saccharomyces cerevisiae). Yet others are trimorphic and grow as yeast, hyphae, and pseudohyphae (i.e., Candida albicans). S. cerevisiae pseudohyphal growth long eluded detection—it requires special conditions/strains and was lost during domestication (4). How pseudohyphae are related to yeast and hyphae (as a distinct fate or a continuum) was unknown until the report of Carlisle et al. (5) in this issue of PNAS. They reveal that pseudohyphae are intermediate between yeast and hyphae, with implications for pathogen–host interactions and fungal evolution.

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