Ascus
220446
203266955
2008-04-04T11:42:53Z
Heliocybe
3833834
[[WP:UNDO|Undid]] revision 203065000 by [[Special:Contributions/165.234.104.11|165.234.104.11]] ([[User talk:165.234.104.11|talk]])
[[Image:Hypomyces chrysospermus.jpg|thumb|right|250px|Asci of ''Hypomyces chrysospermus'' (they are unitunicate-inoperculate)]][[Image:ascus.gif|thumb|right|Ascus of ''Saccharomyces cerevisiae'' containing a tetrad of four spores]]
An '''ascus''' (plural '''asci''') is the sexual spore-bearing cell produced in [[Ascomycota|ascomycete]] [[fungi]]. On average, asci normally contain 8 [[ascospore]]s, produced by a [[meiosis|meiotic]] cell division followed, in most species, by a [[mitosis|mitotic]] cell division. However, asci in some genera or species can number 1 (e.g. ''[[Monosporascus cannonballus]]''), 2, 4, or multiples of four. In a few cases, the ascospores can bud off [[conidia]] that may fill the asci (e.g. ''[[Tympanis]]'') with hundreds of conidia, or the ascospores may fragment, e.g. some ''[[Cordyceps]]'', also filling the asci with smaller cells. Ascospores are nonmotile, usually single celled, but not infrequently may be septate, and in some cases septate in multiple planes. Mitotic divisions within the developing spores populate each resulting cell in septate ascospores with nuclei.
In many 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. In rare cases asci of some genera can regularly develop inside older discharged asci one after another, e.g. ''[[Dipodascus]]''.
Asci normally release their spores by bursting at the tip, but they may also digest themselves passively releasing the ascospores either in a liquid or as a dry powder. Typically, actively discharging asci have a specially differentiated tip, either a pore or an operculum. In some hymenium forming genera, when one ascus bursts, it can trigger the bursting of many other asci in the ascocarp resulting in a massive discharge visible as a cloud of spores - the phenomenon called "puffing". This is an example of [[positive feedback]]. A faint hissing sound can also be heard for species of [[Peziza]] and other [[cup fungi]].
Asci, notably those of ''[[Neurospora crassa]]'', have been used in laboratories for studying the process of meiosis, because the four [[cell (biology)|cell]]s produced by meiosis line up in regular order. By modifying a [[gene]] coding for spore color, the biologist can study [[Chromosomal crossover|crossing over]] and other phenomena.
Asci of most [[Pezizomycotina]] develop after the formation of [[crozier (mycology)|croziers]] at their base. The croziers help maintain a brief [[dikaryon]]. The compatible nuclei of the dikaryon merge forming a [[diploid]] nucleus that then under goes [[meiosis]] and ultimately internal ascospore formation. Members of the [[Taphrinomycotina]] and [[Saccharomycotina]] do not form croziers.
===Ascus classification===
The form of the '''ascus''', the capsule which contains the sexual spores, is important for classification of the [[Ascomycota]]. There are four basic types of ascus.
*A '''unitunicate-operculate''' ascus has a "lid", the [[Operculum (botany)|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 [[morel]]s. 'Unitunicate' means 'single-walled'.
*Instead of an operculum, a '''unitunicate-inoperculate''' ascus has an elastic ring that functions like a [[Relief valve|pressure valve]]. On ripening it briefly expands and so lets the spores shoot out. This type appears both in [[Apothecium|apothecia]] and in [[Perithecium|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 [[Pseudothecium|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|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 [[Perithecium|perithecia]] and in [[Cleistothecium|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.
==External links==
* [http://www.ilmyco.gen.chicago.il.us/Terms/ascus134.html IMA Mycological Glossary: Ascus]
* [http://www.apsnet.org/education/IllustratedGlossary/PhotosA-D/ascus.htm APSnet Illustrated Glossary of Plant Pathology: Ascus]
==References==
* The section on classification was translated from the [[:de:Ascomycota|German article on Ascomycota]].
[[Category:Mycology]]
[[Category:Fungal morphology and anatomy]]
[[cs:Vřecko]]
[[de:Ascus]]
[[es:Ascus]]
[[eo:Asko]]
[[fr:Asque (cellule)]]
[[nl:Sporenzakje]]
[[pt:Ascus]]
[[wa:Aske]]