ascus

An ascus, plural asci, is a sac-like cellular compartment in sac fungi (Ascomycota) that contains the sexual spores, known as ascospores. The ascus is a defining feature of the phylum, which includes a vast range of fungi from yeasts to molds. Asci develop within an ascocarp, a fruiting body that can take several forms, such as an open cup (apothecium), a flask-shaped cavity with a pore (perithecium), or a completely enclosed sphere or network (cleistothecium).

During sexual reproduction, karyogamy occurs inside each ascus, followed by meiosis and usually one round of mitosis, producing typically eight haploid ascospores. Eight is the common spore count, though some asci exhibit variations with fewer or more spores. Asci then release their spores when mature, either through specialized openings, a lid, or by rupture of the ascus wall, depending on the group.

Asci come in several structural types. Unitunicate asci have a single functional wall that often expands to discharge spores through a pore; bitunicate asci possess two walls that separate during spore release; some asci are operculate, featuring a movable lid, while others release spores without a lid.

Ecology and significance: ascospores enable the widespread dispersal of Ascomycota, which includes ecologically and economically important fungi such as edible truffles, baker’s and brewing yeasts, plant pathogens, and many decomposers. The ascus is a central taxonomic and developmental feature used to identify and classify these fungi.