Editing Ascomycota (section)

== Reproduction ==

=== Asexual reproduction ===
Asexual reproduction is the dominant form of propagation in the Ascomycota, and is responsible for the rapid spread of these fungi into new areas. It occurs through vegetative reproductive spores, the [[conidium|conidia]]. The conidiospores commonly contain one nucleus and are products of [[mitosis|mitotic]] cell divisions and thus are sometimes called mitospores, which are genetically identical to the mycelium from which they originate. They are typically formed at the ends of specialized [[hypha]]e, the ''conidiophores''. Depending on the species they may be [[Seed dispersal|dispersed]] by wind or water, or by animals.

==== Asexual spores ====
Different types of asexual spores can be identified by colour, shape, and how they are released as individual spores. Spore types can be used as taxonomic characters in the classification within the Ascomycota. The most frequent types are the single-celled spores, which are designated ''amerospores''. If the spore is divided into two by a cross-wall ([[septum#In mycology|septum]]), it is called a ''didymospore''.
{| align="right" cellspacing="0" cellpadding="0"
|[[File:Trichoderma aggressivum.jpeg|thumb|right|250px|Conidiospores of ''Trichoderma aggressivum'', diameter approx. 3μm]]
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|[[File:Aspergillus.jpg|thumb|right|250px|Conidiophores of molds of the genus ''Aspergillus''; conidiogenesis is blastic-phialidic]]
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|[[File:Trichoderma harzianum.jpg|thumb|right|200px|Conidiophores of ''[[Trichoderma harzianum]]''; conidiogenesis is blastic-phialidic]]
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|[[File:Trichoderma fertile.jpg|thumb|right|200px|Conidiophores of ''Trichoderma fertile'' with vase-shaped phialides and newly formed conidia on their ends (bright points)]]
|}

When there are two or more cross-walls, the classification depends on spore shape. If the septae are ''transversal'', like the rungs of a ladder, it is a ''phragmospore'', and if they possess a net-like structure it is a ''dictyospore''. In ''staurospores'' ray-like arms radiate from a central body; in others (''helicospores'') the entire spore is wound up in a spiral like a spring. Very long worm-like spores with a length-to-diameter ratio of more than 15:1, are called ''scolecospores''.

==== Conidiogenesis and dehiscence ====
Important characteristics of the anamorphs of the Ascomycota are ''conidiogenesis'', which includes spore formation and dehiscence (separation from the parent structure). Conidiogenesis corresponds to [[Embryology]] in animals and plants and can be divided into two fundamental forms of development: ''blastic'' conidiogenesis, where the spore is already evident before it separates from the conidiogenic hypha, and ''thallic'' conidiogenesis, during which a cross-wall forms and the newly created cell develops into a spore. The spores may or may not be generated in a large-scale specialized structure that helps to spread them.

These two basic types can be further classified as follows:
* ''blastic-acropetal'' (repeated budding at the tip of the conidiogenic hypha, so that a chain of spores is formed with the youngest spores at the tip),
* ''blastic-synchronous'' (simultaneous spore formation from a central cell, sometimes with secondary acropetal chains forming from the initial spores),
* ''blastic-sympodial'' (repeated sideways spore formation from behind the leading spore, so that the oldest spore is at the main tip),
* ''blastic-annellidic'' (each spore separates and leaves a ring-shaped scar inside the scar left by the previous spore),
* ''blastic-phialidic'' (the spores arise and are ejected from the open ends of special conidiogenic cells called [[phialide]]s, which remain constant in length),
* ''basauxic'' (where a chain of conidia, in successively younger stages of development, is emitted from the mother cell),
* ''blastic-retrogressive'' (spores separate by formation of crosswalls near the tip of the conidiogenic hypha, which thus becomes progressively shorter),
* ''thallic-arthric'' (double cell walls split the conidiogenic hypha into cells that develop into short, cylindrical spores called ''arthroconidia''; sometimes every second cell dies off, leaving the arthroconidia free),
* ''thallic-solitary'' (a large bulging cell separates from the conidiogenic hypha, forms internal walls, and develops to a ''phragmospore'').

Sometimes the conidia are produced in structures visible to the naked eye, which help to distribute the spores. These structures are called "conidiomata" (singular: [[Conidium|conidioma]]), and may take the form of ''[[Pycnidium|pycnidia]]'' (which are flask-shaped and arise in the fungal tissue) or ''acervuli'' (which are cushion-shaped and arise in host tissue).

Dehiscence happens in two ways. In ''schizolytic'' dehiscence, a double-dividing wall with a central lamella (layer) forms between the cells; the central layer then breaks down thereby releasing the spores. In ''[[rhexolytic]]'' dehiscence, the cell wall that joins the spores on the outside degenerates and releases the conidia.

=== Heterokaryosis and parasexuality ===
Several Ascomycota species are not known to have a sexual cycle. Such asexual species may be able to undergo genetic recombination between individuals by processes involving ''heterokaryosis'' and ''parasexual'' events.

Parasexuality refers to the process of heterokaryosis,<ref>{{cite book |last1=Cole |first1=Garry T. |title=Medical Microbiology |date=1996 |publisher=University of Texas Medical Branch at Galveston |isbn=978-0-9631172-1-2 |edition=4th |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK8099/ |chapter=Basic Biology of Fungi }}</ref> caused by merging of two hyphae belonging to different individuals, by a process called ''[[anastomosis]]'', followed by a series of events resulting in genetically different [[Cell nucleus|cell nuclei]] in the [[mycelium]].<ref>{{harvnb|Deacon|2005|pp=164–6}}</ref>
The merging of nuclei is not followed by [[meiosis|meiotic events]], such as [[gamete]] formation and results in an increased number of [[chromosome]]s per nuclei. ''[[Mitotic crossover]]'' may enable [[genetic recombination|recombination]], i.e., an exchange of genetic material between [[homologous chromosome]]s. The chromosome number may then be restored to its [[ploidy#Haploid and monoploid|haploid]] state by [[Telophase|nuclear division]], with each daughter nuclei being genetically different from the original parent nuclei.<ref>{{harvnb|Deacon|2005|pp=167–8}}</ref> Alternatively, nuclei may lose some chromosomes, resulting in [[aneuploid]] cells.  ''Candida albicans'' (class Saccharomycetes) is an example of a fungus that has a parasexual cycle (see [[Candida albicans]] and [[Parasexual cycle]]).

=== Sexual reproduction ===
[[File:Hypocrea virens.jpg|thumb|right|115px|Ascus of ''Hypocrea virens'' with eight two-celled Ascospores]]
Sexual reproduction in the Ascomycota leads to the formation of the ''ascus'', the structure that defines this fungal group and distinguishes it from other fungal phyla. The ascus is a tube-shaped vessel, a ''meiosporangium'', which contains the sexual spores produced by [[meiosis]] and which are called ''ascospores''.

Apart from a few exceptions, such as ''[[Candida albicans]]'', most ascomycetes are [[ploidy#Haploid and monoploid|haploid]], i.e., they contain one set of [[chromosome]]s per nucleus. During sexual reproduction there is a [[ploidy#Diploid|diploid]] phase, which commonly is very short, and meiosis restores the haploid state.  The sexual cycle of one well-studied representative species of Ascomycota is described in greater detail in ''[[Neurospora crassa]]''. Also, the adaptive basis for the maintenance of sexual reproduction in the Ascomycota  [[fungus|fungi]] was reviewed by Wallen and Perlin.<ref name="pmid29619017">{{cite journal |vauthors=Wallen RM, Perlin MH |title=An Overview of the Function and Maintenance of Sexual Reproduction in Dikaryotic Fungi |journal=Front Microbiol |volume=9 |pages=503 |date=2018 |pmid=29619017 |doi=10.3389/fmicb.2018.00503 |pmc=5871698 |doi-access=free }}</ref>  They concluded that the most plausible reason for the maintenance of this capability is the benefit of [[DNA repair|repairing DNA damage]] by using  [[homologous recombination|recombination]] that occurs during [[meiosis]].<ref name="pmid29619017" />  DNA damage can be caused by a variety of stresses such as nutrient limitation.

==== Formation of sexual spores ====
The sexual part of the life cycle commences when two hyphal structures [[Mating|mate]]. In the case of ''[[homothallic]]'' species, mating is enabled between hyphae of the same fungal [[cloning|clone]], whereas in ''[[heterothallic]]'' species, the two hyphae must originate from fungal clones that differ genetically, i.e., those that are of a different [[mating type]]. Mating types are typical of the fungi and correspond roughly to the sexes in plants and animals; however one species may have more than two mating types, resulting in sometimes complex [[Allorecognition#Allorecognition phenomena|vegetative incompatibility]] systems. The [[adaptive function of mating type]] is discussed in ''[[Neurospora crassa]]''.

[[Gametangia]] are sexual structures formed from hyphae, and are the generative cells. A very fine hypha, called [[:wikt:trichogyne|trichogyne]] emerges from one gametangium, the ''[[ascogonium]]'', and merges with a gametangium (the ''antheridium'') of the other fungal isolate. The nuclei in the antheridium then migrate into the ascogonium, and [[plasmogamy]]—the mixing of the [[cytoplasm]]—occurs. Unlike in animals and plants, plasmogamy is not immediately followed by the merging of the nuclei (called ''[[karyogamy]]''). Instead, the nuclei from the two hyphae form pairs, initiating the ''dikaryophase'' of the sexual cycle, during which time the pairs of nuclei synchronously divide. Fusion of the paired nuclei leads to mixing of the genetic material and [[genetic recombination|recombination]] and is followed by [[meiosis]]. A similar sexual cycle is present in the [[red algae]] (Rhodophyta). A discarded hypothesis held that a second karyogamy event occurred in the ascogonium prior to ascogeny, resulting in a tetraploid nucleus which divided into four diploid nuclei by meiosis and then into eight haploid nuclei by a supposed process called [[brachymeiosis]], but this hypothesis was disproven in the 1950s.<ref name="Carlile">{{cite journal |last1=Carlile |first1=Michael J. |title=Two influential mycologists: Helen Gwynne-Vaughan (1879–1967) and Lilian Hawker (1908–1991) |journal=Mycologist |date=August 2005 |volume=19 |issue=3 |pages=129–131 |doi=10.1017/S0269-915X(05)00305-8 }}</ref>

[[File:Hypomyces chrysospermus.jpg|thumb|left|150px|Unitunicate-inoperculate Asci of ''Hypomyces chrysospermus'']]
From the fertilized ascogonium, ''dinucleate'' hyphae emerge in which each cell contains two nuclei. These hyphae are called ''ascogenous'' or fertile hyphae. They are supported by the vegetative mycelium containing uni– (or mono–) nucleate hyphae, which are sterile. The mycelium containing both sterile and fertile hyphae may grow into fruiting body, the ''[[ascocarp]]'', which may contain millions of fertile hyphae.

An ascocarp is the fruiting body of the sexual phase in Ascomycota. There are five morphologically different types of ascocarp, namely:

* Naked asci: these occur in simple ascomycetes; asci are produced on the organism's surface.
* [[Ascocarp#Perithecium|Perithecia]]: Asci are in flask-shaped ascoma (perithecium) with a pore (ostiole) at the top.
* [[Ascocarp#Cleistothecium|Cleistothecia]]: The ascocarp (a cleistothecium) is spherical and closed.
* [[Ascocarp#Apothecium|Apothecia]]: The asci are in a bowl shaped ascoma (apothecium). These are sometimes called the "cup fungi". 
* [[Ascocarp#Pseudothecium|Pseudothecia]]: Asci with two layers, produced in pseudothecia that look like perithecia. The ascospores are arranged irregularly.<ref>{{Cite web|url=https://www.microscopemaster.com/ascomycota.html|title=Ascomycota – Characteristics, Nutrition and Significance|website=MicroscopeMaster|language=en|access-date=4 March 2019}}</ref>

The sexual structures are formed in the fruiting layer of the ascocarp, the [[hymenium]]. At one end of ascogenous hyphae, characteristic U-shaped hooks develop, which curve back opposite to the growth direction of the hyphae. The two nuclei contained in the apical part of each hypha divide in such a way that the threads of their [[mitotic spindle]]s run parallel, creating two pairs of genetically different nuclei. One daughter nucleus migrates close to the hook, while the other daughter nucleus locates to the basal part of the hypha. The formation of two parallel cross-walls then divides the hypha into three sections: one at the hook with one nucleus, one at the basal of the original hypha that contains one nucleus, and one that separates the U-shaped part, which contains the other two nuclei.
[[File:Ascocarp2.png|thumb|right|alt=Cross-section of a cup-shaped structure showing locations of developing meiotic asci (upper edge of cup, left side, arrows pointing to two gray-colored cells containing four and two small circles), sterile hyphae (upper edge of cup, right side, arrows pointing to white-colored cells with a single small circle in them), and mature asci (upper edge of cup, pointing to two gray-colored cells with eight small circles in them)|Diagram of an [[apothecium]] (the typical cup-like reproductive structure of ascomycetes) showing sterile tissues as well as developing and mature asci.]]

Fusion of the nuclei (karyogamy) takes place in the U-shaped cells in the hymenium, and results in the formation of a diploid [[zygote]]. The zygote grows into the [[ascus]], an elongated tube-shaped or cylinder-shaped capsule. Meiosis then gives rise to four [[ploidy#Haploid and monoploid|haploid]] nuclei, usually followed by a further mitotic division that results in eight nuclei in each ascus. The nuclei along with some cytoplasma become enclosed within membranes and a cell wall to give rise to ascospores that are aligned inside the ascus like peas in a pod.

Upon opening of the ascus, ascospores may be dispersed by the wind, while in some cases the spores are forcibly ejected form the ascus; certain species have evolved spore cannons, which can eject ascospores up to 30&nbsp;cm. away. When the spores reach a suitable substrate, they germinate, form new hyphae, which restarts the fungal life cycle.

The form of the ascus is important for classification and is divided into four basic types: unitunicate-operculate, unitunicate-inoperculate, bitunicate, or prototunicate. See the article on [[Ascus|asci]] for further details.