Ascus

An ascus (plural: asci) is the spore-producing cell of fungi in the phylum Ascomycota — the sac fungi. It is a sac-shaped structure, typically cylindrical, in which spores (ascospores) are produced internally. Each ascus usually contains eight ascospores, the product of a single meiotic division followed by one mitotic division. The spores develop inside the ascus and are released when the ascus opens — in some species by a forceful discharge that can propel spores several centimeters, in others by passive disintegration of the ascus wall.

The Ascomycota are the largest phylum of fungi, with over 64,000 described species. The group includes morels and truffles (prized in cuisine), yeasts (essential for fermentation), most lichen-forming fungi (the mycobiont in the vast majority of lichen species), and many plant pathogens — powdery mildews, ergot, Dutch elm disease, chestnut blight. Despite this ecological and morphological diversity, the ascus unites them: it is the diagnostic feature of the phylum.

Asci are typically arranged in a fertile layer called the hymenium, which may line the surface of cup-shaped or flask-shaped fruiting bodies (apothecia and perithecia). In truffles, the asci are enclosed within a solid fruiting body that never opens — the spores are dispersed only when an animal digs up and eats the truffle, attracted by volatile compounds (see fungal chemical ecology). In yeasts, the ascus is the entire organism: a single cell that undergoes meiosis and produces ascospores directly, without forming a multicellular fruiting body at all. Compare with the basidium, the equivalent structure in the Basidiomycota, which produces spores externally rather than internally.

Related terms

• Basidium — the spore-producing structure of Basidiomycota, the parallel to the ascus
• Spore — the reproductive units that asci produce
• Fruiting Body — the macroscopic structures that bear asci
• Lichen — most lichen-forming fungi are Ascomycota
• Fungal Reproduction — sexual reproduction and the ascus life cycle
• Fungal Taxonomy — the ascus defines the phylum Ascomycota
• Fungal Chemical Ecology — truffle VOCs as a dispersal strategy for enclosed asci