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An ascus (from Ancient Greek ἀσκός (askós) 'skin bag, wineskin'; pl.: asci) [1] is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division.
There are 2000 identified genera and 30,000 species of Ascomycota. The unifying characteristic among these diverse groups is the presence of a reproductive structure known as the ascus, though in some cases it has a reduced role in the life cycle. Many ascomycetes are of commercial importance.
An ascocarp, or ascoma (pl.: ascomata), is the fruiting body of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores.
Epigeous sporocarps have mycelia that extend underground far beyond the mother sporocarp. There is a wider distribution of mycelia underground than sporocarps above ground. [2] Hypogeous fungi are usually called truffles or false truffles. There is evidence that hypogeous fungi evolved from epigeous fungi. [3]
This binuclear cell initiates ascus formation and is called an "ascus-initial" cell. Next the two uninucleate cells on either side of the first ascus-forming cell fuse with each other to form a binucleate cell that can grow to form a further crozier that can then form its own ascus-initial cell. This process can then be repeated multiple times.
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The ascus itself forms as a radiating spur branch at the top of the hook. Each nucleus divides, resulting in the formation of a pair of compatible nuclei, i.e. a dikaryon, in the ascus, which is now the penultimate space. Two sister nuclei remain, one in the basal cell and the other in the crozier tip.