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Anascus (from Ancient Greek ἀσκός (askós) 'skin bag, wineskin';pl.:asci)[1] is the sexualspore-bearingcell produced inascomycetefungi. Each ascus usually contains eightascospores (or octad), produced bymeiosis followed, in most species, by amitotic cell division. However, asci in some genera or species can occur in numbers of one (e.g.Monosporascus cannonballus), two, four, or multiples of four. In a few cases, the ascospores can bud offconidia that may fill the asci (e.g.Tympanis) with hundreds of conidia, or the ascospores may fragment, e.g. someCordyceps, also filling the asci with smaller cells. Ascospores are nonmotile, usually single celled, but not infrequently may be coenocytic (lacking aseptum), and in some cases coenocytic in multiple planes. Mitotic divisions within the developing spores populate each resulting cell in septate ascospores with nuclei. The termocular chamber, oroculus, refers to the epiplasm (the portion ofcytoplasm not used in ascospore formation) that is surrounded by the "bourrelet" (the thickened tissue near the top of the ascus).[2]
Typically, a single ascus will contain eight ascospores (or octad). The eight spores are produced bymeiosis followed by amitotic division. Two meiotic divisions turn the originaldiploidzygote nucleus into fourhaploid ones. That is, the single original diploid cell from which the whole process begins contains two complete sets ofchromosomes. In preparation for meiosis, all theDNA of both sets is duplicated, to make a total of four sets. The nucleus that contains the four sets divides twice, separating into four new nuclei – each of which has one complete set of chromosomes. Following this process, each of the four new nuclei duplicates its DNA and undergoes a division by mitosis. As a result, the ascus will contain four pairs of spores. Then the ascospores are released from the ascus.
In many cases the asci are formed in a regular layer, thehymenium, in a fruiting body which is visible to the naked eye, here called anascocarp orascoma. In other cases, such as single-celledyeasts, 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. Discharging asci usually 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 ofpositive feedback. A faint hissing sound can also be heard for species ofPeziza and othercup fungi.
Asci, notably those ofNeurospora crassa, have been used in laboratories for studying the process of meiosis, because the fourcells produced by meiosis line up in regular order. By modifyinggenes coding for spore color and nutritional requirements, the biologist can studycrossing over and other phenomena. The formation of asci and their use in genetic analysis are described in detail inNeurospora crassa.
Asci of mostPezizomycotina develop after the formation ofcroziers at their base. The croziers help maintain a briefdikaryon. The compatible nuclei of the dikaryon merge forming adiploid nucleus that then undergoesmeiosis and ultimately internal ascospore formation. Members of theTaphrinomycotina andSaccharomycotina do not form croziers.
The form of the ascus, the capsule which contains the sexual spores, is important for classification of theAscomycota. There are four basic types of ascus.
An ascospore is aspore contained in an ascus, or that was produced inside an ascus. This kind of spore is specific tofungi classified asascomycetes (Ascomycota).
The ascospores ofBlumeria graminis are formed and released under the humid conditions.[3] After landing onto a suitable surface, unlikeconidia, ascospores ofBlumeria graminis showed a more variable developmental patterns.[3]
The fungiSaccharomyces produces ascospores when grown on V-8 medium, acetate ascospore agar, or Gorodkowa medium. These ascospores are globose and located in asci. Each ascus contains one to four ascospores. The asci do not rupture at maturity. Ascospores are stained with Kinyoun stain and ascospore stain. When stained with Gram stain, ascospores are gram-negative while vegetative cells are gram-positive.
The fission yeastSchizosaccharomyces pombe is a single-celled haploid organism that reproduces asexually by mitosis and fission. However, exposure to the DNA damaging agent hydrogen peroxide induces pair-wise mating of haploid cells of opposite mating type to form transient diploid cells that then undergo meiosis to form asci, each with four ascospores.[4] The production of viable ascospores depends on successful recombinational repair during meiosis.[5] When this repair is defective a quality control mechanism prevents germination of damaged ascospores. These findings suggest that mating followed by meiosis is an adaptation for repairing DNA damage in the parental haploid cells in order to allow production of viable progeny ascospores.
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