The nucleus from a male cell, called antheridium, passes through a bridge into the female cell, an ascogonium. The male and female cells may be from the same, self-compatible colony or from two colonies of opposite mating types. Once the male and female nuclei fuse to form a zygote, the cell becomes an ascus (plural, asci). The diploid zygote nucleus divides by meiosis to form haploid nuclei, which in turn divide by mitosis to form eight nuclei. Each new nucleus walls off inside the ascus to form an ascospore.
Fungi associated with the class Ascomycetes are characterized by the production of sexual spores known as ascospores. Ascospores are formed within a saclike structure known as an ascus. It is important to note, however, that they are usually identified on the basis of characteristic asexual structures.