This is a large and diverse group, which is characterised by the formation of club-shaped cells, or asci, within which the ascospores are formed. Of the 2000 known genera, only a small number produce visible fruiting bodies, and only some of these are covered in these web pages.
The ascospores are a result of fusion between two nuclei,
either from the same individual or from two different but compatible mating strains. Meiosis
follows fusion to produce four nuclei embedded in a common cytoplasm
(the young ascus). In some species the cytoplasm becomes condensed around the nuclei to form four ascospores, but more commonly, a mitotic division takes place to form eight nuclei, resulting in the formation of eight ascospores. In some species, a series of mitosis
divisions follows one another, resulting in the formation of twenty-four, thirty-two, sixty-four or more ascospores inside a much-inflated ascus
As ascospores are usually forcibly shot some distance from the ascus, the hymenium
therefore develops on top of the ascocarp
facing upwards, and outwards or is spread over the surface of variously shaped structures. Ascomycetes
are more numerous than Basidiomycetes
, but few of them produce visible fruiting bodies.
Ascomycetes are divided into a number of classes. Those which produce large fruiting bodies, are classified as Euascomycetes, which have unitunicate asci, each with a single wall. Euascomycetes are then divided into 6 different orders based upon the shape of the ascocarp. Only two of which are covered here.
Discomycetes in which the asci are exposed on dish-shaped ascocarps (apothecia). Pyrenomycetes in which the asco-carps are bottle-shaped, enclosing the asci.
Ascomycetes of this class have an ascus wall composed of a single layer (unitunicate). Inside each ascus flask, eight ascospores develop, bathed in a fluid, which is retained under pressure. When the spores are ripe, the neck of the ascus is designed to open with a shock stimulus. This is achieved either by the opening of a lid (operculum), or they may simply burst or break at maturity. A raindrop falling on the cup surface usually triggers this. The spores are then forcibly discharged into the damp air currents.