Ascus

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An ascus (plural asci) is the spore-bearing container produced in ascomycete fungi. Each ascus normally contains either four or eight ascospores, produced by a meiotic cell division followed, in some species, by a mitotic cell division. In some cases the asci are formed in a regular layer, the hymenium, in a fruiting body which is visible to the naked eye, here called an ascocarp or ascoma. In other cases, such as single-celled yeasts, no such structures are found.

Asci generally release their spores by bursting. When one ascus bursts, it triggers the bursting of all the other asci in the ascocarp. This is an example of positive feedback.

Asci, notably those of Neurospora crassa, have been used in laboratories for studying the process of meiosis, because the four cells produced by meiosis line up in regular order. By modifying a gene coding for spore color, the biologist can study crossing over and other phenomena.

[edit] Ascus classification

The form of the ascus, the capsule which contains the sexual spores, is important for classification of the Ascomycota and is divided into four basic types.

A unitunicate-operculate ascus has a "lid", the Operculum, which breaks open when the spores ripen and in this way sets them free. Unitunicate-operculate asci only occur in those ascocarps which have apothecia, for instance the morels. 'Unitunicate' means 'single-walled'.
Instead of an operculum, a unitunicate-inoperculate ascus has an elastic ring which functions like a pressure valve. On ripening it briefly extends and so lets the spores shoot out. This type appears both in apothecia and in perithecia; an example is the illustrated Hypomyces chrysospermus.
A bitunicate ascus is enclosed in a double wall. This consists of a thin brittle outer shell and a thick elastic inner wall. When the spores are ripe the shell splits open so that the inner wall can take up water. As a consequence this begins to extend with its spores until it protrudes above the rest of the ascocarp so that the spores can escape into free air without being obstructed by the bulk of the fruiting body. Bitunicate asci occur only in pseudothecia and are found only in the classes Dothideomycetes and Chaetothyriomycetes (which were formerly united in the old class Loculoascomycetes). Examples: Venturia inaequalis (Apple Scab) and Guignardia aesculi (Brown Leaf Mold of Horse Chestnut).
Prototunicate asci are mostly spherical in shape and they have no active dispersal mechanism at all. The ripe ascus wall simply dissolves so that the spores can escape, or it is broken open by other influences such as animals. Asci of this type can be found both in perithecia and in cleistothecia, for instance with Dutch elm disease (Ophiostoma). This is something of a catch-all term for cases which do not fit into the other three ascus types, and they probably belong to several independent groups which evolved separately from unitunicate asci.