The saclike structure that gives Ascomycota its name and produces sexual spores is the ascus.
An ascus is a microscopic sexual spore-bearing cell found in ascomycete fungi. The name Ascomycota comes from the Greek askos, meaning “sac” or “wineskin,” describing this characteristic structure. The spores produced inside an ascus are called ascospores, and they are typically released when the ascus opens.
A typical ascus contains eight ascospores. This number results when a diploid nucleus undergoes meiosis to produce four haploid nuclei, followed by a mitotic division that produces eight. In many fungi, asci occur in a fertile layer inside a larger reproductive structure called an ascocarp or ascoma.
The ascus is not the same as a conidium, which is an asexual spore, or a basidium, the sexual spore-bearing cell characteristic of Basidiomycota. A sporangium is a broader term for a spore-producing structure and is not the defining structure of Ascomycota. Asci vary in form, and their structure helps mycologists classify ascomycete fungi.