ASCOMYCOTA is a division or phylum of the kingdom
The ascomycetes are a monophyletic group, i.e. it contains all
descendants of one common ancestor. This group is of particular
relevance to humans as sources for medicinally important compounds,
such as antibiotics and for making bread, alcoholic beverages, and
cheese, but also as pathogens of humans and plants. Familiar examples
of sac fungi include morels , truffles , brewer\'s yeast and baker\'s
yeast , dead man\'s fingers , and cup fungi . The fungal symbionts in
the majority of lichens (loosely termed "ascolichens") such as
Cladonia belong to the Ascomycota. There are many plant-pathogenic
ascomycetes, including apple scab , rice blast , the ergot fungi ,
black knot , and the powdery mildews . Several species of ascomycetes
are biological model organisms in laboratory research. Most famously,
Neurospora crassa , several species of yeasts , and Aspergillus
species are used in many genetics and cell biology studies.
* 1 Asexual reproduction in ascomycetes and their characteristics
* 2 Modern classification of
* 7 Reproduction
* 7.1 Asexual reproduction
* 7.1.1 Asexual spores * 7.1.2 Conidiogenesis and dehiscence
* 7.2 Heterokaryosis and parasexuality
* 7.3.1 Formation of sexual spores
* 8 Ecology
* 8.1 Lichens * 8.2 Mycorrhizal fungi and endophytes * 8.3 Symbiotic relationships with animals
* 9 Importance for humans
* 9.1 Harmful interactions * 9.2 Positive effects
* 10 See also * 11 Notes * 12 Cited texts
ASEXUAL REPRODUCTION IN ASCOMYCETES AND THEIR CHARACTERISTICS
Ascomycetes are 'spore shooters'. They are fungi which produce microscopic spores inside special, elongated cells or sacs, known as 'asci', which give the group its name.
Asexual reproduction is the dominant form of propagation in the
Ascomycota, and is responsible for the rapid spread of these fungi
into new areas. Asexual reproduction of ascomycetes is very diverse
from both structural and functional points of view. The most important
and general is production of conidia, but chlamydospores are also
frequently produced. Furthermore,
1) CONIDIA FORMATION:
Asexual reproduction may occur through vegetative reproductive spores, the conidia. Asexual, non-motile haploid spore of a fungus, which is named after the Greek word for dust; conia and hence also known as conidiospores and mitospores. The CONIDIOSPORES commonly contain one nucleus and are products of mitotic cell divisions and thus are sometimes call MITOSPORES, which are genetically identical to the mycelium from which they originate. They are typically formed at the ends of specialized hyphae, the conidiophores. Depending on the species they may be dispersed by wind or water, or by animals. Conidiophores may simply branch off from the mycelia or they may be formed in fruiting bodies.
The hypha that creates the sporing (conidiating) tip can be very similar to the normal hyphal tip, or it can be differentiated. The most common differentiation is the formation of a bottle shaped cell called a PHIALIDE , from which the spores are produced. As all of these asexual structures are not single hyphae. In some groups, the conidiophores (the structures that bear the conidia) are aggregated to form a thick structure.
E.g. In the order Moniliales, all of them are single hyphae with the exception of the aggregations, termed as coremia or synnema. These produce structures rather like corn-stokes, with many conidia being produced in a mass from the aggregated conidiophores.
The diverse conidia and conidiophores sometimes develop in asexual sporocarps with different characteristics (e.g. aecervulus, pycnidium, sporodochium). Some species of Ascomycetes form their structures within plant tissue, either as parasite or saprophytes. These fungi have evolved more complex asexual sporing structures, probably influenced by the cultural conditions of plant tissue as a substrate. These structures are called the SPORODOCHIUM. This is a cushion of conidiophores created from a pseudoparenchymatous stroma in plant tissue. The PYCNIDIUM is a globose to flask-shaped parenchymatous structure, lined on its inner wall with conidiophores. The ACERVULUS is a flat saucer shaped bed of conidiophores produced under a plant cuticle, which eventually erupt through the cuticle for dispersal.
Asexual reproduction process in ascomycetes also involves the budding which we clearly observe in YEAST. This is termed a “blastic process”. It involves the blowing out or blebbing of the hyphal tip wall. The blastic process can involve all wall layers, or there can be a new cell wall synthesized which is extruded from within the old wall.
The initial events of budding can be seen as the development of a ring of chitin around the point where the bud is about to appear. This reinforces and stabilizes the cell wall. Enzymatic activity and turgor pressure act to weaken and extrude the cell wall. New cell wall material is incorporated during this phase. Cell contents are forced into the progeny cell, and as the final phase of mitosis ends a cell plate, the point at which a new cell wall will grow inwards from, forms.
CHARACTERISTICS OF ASCOMYCETES:
· There are 2000 identified genera and 30,000 species of Ascomycota.
· The unifying characteristic among these diverse groups is the presence of a reproductive structure known as the ASCUS, though in some cases it has a reduced role in the life cycle.
· Many ascomycetes are of commercial importance. Some play a beneficial role, such as the yeasts used in baking, brewing, and wine fermentation, plus truffles and morels, which are held as gourmet delicacies.
· Many of them cause tree diseases, such as
Dutch elm disease
· Some of the plant pathogenic ascomycetes are apple scab, rice blast, the ergot fungi, black knot, and the powdery mildews.
· The yeasts are used to produce alcoholic beverages and breads. The mold PENICILLIUM is used to produce the anti-biotic penicillin.
· Almost half of all members of the phylum
· Others, such as morels (a highly prized edible fungi), form important MYCHORRHIZAL relationships with plants, thereby providing enhanced water and nutrient uptake and, in some cases, protection from insects.
· Almost all ascomycetes are terrestrial or parasitic. However, a few have adapted to marine or freshwater environments.
· The cell walls of the hyphae are variably composed of CHITIN and β-glucans, just as in Basidiomycota. However, these fibers are set in a matrix of glycoprotein containing the sugars galactose and mannose.
· The mycelium of ascomycetes is usually made up of SEPTATE HYPHAE. However, there is not necessarily any fixed number of nuclei in each of the divisions.
· The septal walls have septal pores which provide cytoplasmic continuity throughout the individual hyphae. Under appropriate conditions, nuclei may also migrate between septal compartments through the septal pores.
· A unique character of the
MODERN CLASSIFICATION OF ASCOMYCOTA
There are three subphyla that are described and accepted:
OUTDATED TAXON NAMES
Several outdated taxon names—based on morphological features—are
still occasionally used for species of the Ascomycota. These include
the following sexual (teleomorphic ) groups, defined by the structures
of their sexual fruiting bodies : the
Discomycetes , which included
all species forming apothecia ; the
Pyrenomycetes , which included all
sac fungi that formed perithecia or pseudothecia , or any structure
resembling these morphological structures; and the Plectomycetes,
which included those species that form cleistothecia . Hemiascomycetes
included the yeasts and yeast-like fungi that have now been placed
Some ascomycetes do not reproduce sexually or are not known to produce asci and are therefore anamorphic species. Those anamorphs that produce conidia (mitospores) were previously described as Mitosporic Ascomycota . Some taxonomists placed this group into a separate artificial phylum , the Deuteromycota (or "Fungi Imperfecti"). Where recent molecular analyses have identified close relationships with ascus-bearing taxa, anamorphic species have been grouped into the Ascomycota, despite the absence of the defining ascus. Sexual and asexual isolates of the same species commonly carry different binomial species names, as, for example, Aspergillus nidulans and Emericella nidulans, for asexual and sexual isolates, respectively, of the same species.
Most species grow as filamentous, microscopic structures called
hyphae or as budding single cells (yeasts). Many interconnected hyphae
form a thallus usually referred to as the mycelium , which—when
visible to the naked eye (macroscopic)—is commonly called mold .
During sexual reproduction, many
In lichenized species, the thallus of the fungus defines the shape of the symbiotic colony. Some dimorphic species, such as Candida albicans , can switch between growth as single cells and as filamentous, multicellular hyphae. Other species are pleomorphic , exhibiting asexual (anamorphic) as well as a sexual (teleomorphic) growth forms.
Except for lichens, the non-reproductive (vegetative) mycelium of
most ascomycetes is usually inconspicuous because it is commonly
embedded in the substrate, such as soil, or grows on or inside a
living host, and only the ascoma may be seen when fruiting.
Pigmentation , such as melanin in hyphal walls, along with prolific
growth on surfaces can result in visible mold colonies; examples
Cladosporium species, which form black spots on bathroom
caulking and other moist areas. Many ascomycetes cause food spoilage,
and, therefore, the pellicles or moldy layers that develop on jams,
juices, and other foods are the mycelia of these species or
Large masses of yeast cells, asci or ascus-like cells, or conidia can also form macroscopic structures. For example. Pneumocystis species can colonize lung cavities (visible in x-rays), causing a form of pneumonia . Asci of Ascosphaera fill honey bee larvae and pupae causing mummification with a chalk-like appearance, hence the name "chalkbrood". Yeasts for small colonies in vitro and in vivo , and excessive growth of Candida species in the mouth or vagina causes "thrush", a form of candidiasis .
The cell walls of the ascomycetes almost always contain chitin and β-glucans , and divisions within the hyphae, called "septa ", are the internal boundaries of individual cells (or compartments). The cell wall and septa give stability and rigidity to the hyphae and may prevent loss of cytoplasm in case of local damage to cell wall and cell membrane . The septa commonly have a small opening in the center, which functions as a cytoplasmic connection between adjacent cells, also sometimes allowing cell-to-cell movement of nuclei within a hypha. Vegetative hyphae of most ascomycetes contain only one nucleus per cell (uninucleate hyphae), but multinucleate cells—especially in the apical regions of growing hyphae—can also be present.
In common with other fungal phyla, the
Owing to their long evolutionary history, the
DISTRIBUTION AND LIVING ENVIRONMENT
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 conidia . The conidiospores commonly contain one nucleus and are products of 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 hyphae , the conidiophores. Depending on the species they may be dispersed by wind or water, or by animals.
Different types of asexual spores can be identified by colour, shape,
and how they are released as individual spores.
Conidiospores of Trichoderma aggressivum, Diameter approx. 3µm
Conidiophores of molds of the genus Aspergillus, conidiogenesis is blastic-phialidic
Conidiophores of Trichoderma harzianum , conidiogenesis is blastic-phialidic
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
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 phialides , 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: conidioma ), and may take the form of 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
Parasexuality refers to the process of heterokaryosis, 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 nuclei in the mycelium . The merging of
nuclei is not followed by meiotic events , such as gamete formation
and results in an increased number of chromosomes per nuclei. Mitotic
crossover may enable recombination , i.e., an exchange of genetic
material between homologous chromosomes . The chromosome number may
then be restored to its haploid state by nuclear division , with each
daughter nuclei being genetically different from the original parent
nuclei. Alternatively, nuclei may lose some chromosomes, resulting in
Apart from a few exceptions, such as
Formation Of Sexual Spores
The sexual part of the life cycle commences when two hyphal
structures mate . In the case of homothallic species, mating is
enabled between hyphae of the same fungal 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 .
Gametangia are sexual structures formed from hyphae, and are the generative cells. A very fine hypha, called 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 recombination and is followed by meiosis . A similar sexual cycle is present in the blue green 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. 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.
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 spindles 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. 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
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 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 asci for further details.
The fruiting bodies of the
Many ascomycetes also form symbiotic relationships with other organisms, including plants and animals.
Probably since early in their evolutionary history, the Ascomycota
have formed symbiotic associations with green algae (Chlorophyta), and
other types of algae and cyanobacteria . These mutualistic
associations are commonly known as lichens , and can grow and persist
in terrestrial regions of the earth that are inhospitable to other
organisms and characterized by extremes in temperature and humidity,
MYCORRHIZAL FUNGI AND ENDOPHYTES
Members of the
Endophytic fungi live inside plants, and those that form mutualistic or commensal associations with their host, do not damage their hosts. The exact nature of the relationship between endophytic fungus and host depends on the species involved, and in some cases fungal colonization of plants can bestow a higher resistance against insects, roundworms (nematodes), and bacteria ; in the case of grass endophytes the fungal symbiont produces poisonous alkaloids , which can affect the health of plant-eating (herbivorous) mammals and deter or kill insect herbivores.
SYMBIOTIC RELATIONSHIPS WITH ANIMALS
Several ascomycetes of the genus Xylaria colonize the nests of leafcutter ants and other fungus-growing ants of the tribe Attini , and the fungal gardens of termites (Isoptera). Since they do not generate fruiting bodies until the insects have left the nests, it is suspected that, as confirmed in several cases of Basidiomycota species, they may be cultivated.
Bark beetles (family Scolytidae) are important symbiotic partners of ascomycetes. The female beetles transport fungal spores to new hosts in characteristic tucks in their skin, the mycetangia . The beetle tunnels into the wood and into large chambers in which they lay their eggs. Spores released from the mycetangia germinate into hyphae, which can break down the wood. The beetle larvae then feed on the fungal mycelium, and, on reaching maturity, carry new spores with them to renew the cycle of infection. A well-known example of this is Dutch elm disease , caused by Ophiostoma ulmi , which is carried by the European elm bark beetle, Scolytus multistriatus .
IMPORTANCE FOR HUMANS
Tree attacked by the Bluestain fungus, Ophiostoma minus
Ascomycetes make many contributions to the good of humanity, and also have many ill effects.
One of their most harmful roles is as the agent of many plant diseases. For instance:
Claviceps purpurea on rye (Secale cereale)
* The originally Asian Cryphonectria parasitica is responsible for
attacking Sweet Chestnuts (
Castanea sativa ), and virtually eliminated
On the other hand, ascus fungi have brought some important benefits to humanity.
* The most famous case may be that of the mould Penicillium
* Some ascomycete fungi can be altered relatively easily through
genetic engineering procedures. They can then produce useful proteins
such as insulin , human growth hormone , or TPa, which is employed to
dissolve blood clots.
* Several species are common model organisms in biology, including
* ^ Cavalier-Smith, T. (1998). "A revised six-kingdom system of
Life". Biological Reviews of the Cambridge Philosophical Society. 73
(3): 203–266. PMID 9809012 . doi :10.1111/j.1469-185X.1998.tb00030.x
* ^ Kirk et al., p. 55.
* ^ Lutzoni F; et al. (2004). "Assembling the fungal tree of life:
progress, classification, and evolution of subcellular traits".
American Journal of Botany. 91 (10): 1446–80. PMID 21652303 . doi
* ^ James TY; et al. (2006). "Reconstructing the early evolution of
* Alexopoulos CJ, Mims CW, Blackwell M (1996). Introductory Mycology. John Wiley and Sons. ISBN 0-471-52229-5 . * Deacon J. (2005). Fungal Biology. Cambridge, MA: Blackwell Publishers. ISBN 1-4051-3066-0 . * Jennings DH, Lysek G (1996). Fungal Biology: Understanding the Fungal Lifestyle. Guildford, UK: Bios Scientific Publishers Ltd. ISBN 978-1-85996-150-6 . * Kirk PM, Cannon PF, Minter DW, Stalpers JA (2008). Dictionary of the Fungi. 10th ed. Wallingford: CABI. ISBN 0-85199-826-7 . * Taylor EL, Taylor TN (1993). The Biology and Evolution of Fossil Plants. Englewood Cliffs, N.J: Prentice Hall. ISBN 0-13-651589-4 .
* v * t * e
* Glaucophyta * Rhodophyta
VIRIDIPLANTAE OR PLANTAE SENSU STRICTO
* Chlorophyta * Streptophyta
* Acavomonadia * Ciliates * Colponemidia
INCERTAE SEDIS Kamera lens
* Diphyllatea * Discocelida * Glissodiscea * Micronucleariida * Rigifilida
* Cristidiscoidea * TRUE FUNGI
Major kingdoms are underlined. See also: protist . Sources and alternative views: Wikispecies.
* v * t * e
Ascomycota (sac fungi)
Basidiomycota (with basidia )
Zoosporic fungi (paraphyletic)
Fungal phyla are underlined. See also: fungi imperfecti (polyphyletic group).