The
Ascomycota are a
Division/PhylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
of the kingdom Fungi, and subkingdom
DikaryaDikarya is a subkingdom of Fungi that includes the phyla Ascomycota and Basidiomycota, both of which in general produce dikaryons, may be filamentous or unicellular, but are always without flagella. The Dikarya are most of the so called "Higher Fungi", but also include many anamorphic species that...
. Its members are commonly known as the
Sac Fungi. They are the largest phylum of Fungi, with over 64,000 species. The defining feature of this fungal group is the "
ascusAn ascus is the sexual spore-bearing cell produced in ascomycete fungi. On average, asci normally contain 8 ascospores, produced by a meiotic cell division followed, in most species, by a mitotic cell division. However, asci in some genera or species can number 1 , 2, 4, or multiples of four...
" (from
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
:
(
askos), meaning "sac" or "wineskin"), a microscopic
sexual structureSexual reproduction is characterized by processes that pass a combination of genetic material to offspring, resulting in diversity. The main two processes are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration...
in which nonmotile
sporeIn biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions...
s, called ascospores, are formed. However, some
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
of the Ascomycota are asexual, meaning that they do not have a sexual cycle and thus do not form asci or ascospores. Previously placed in the Deuteromycota along with asexual species from other fungal taxa, asexual (or anamorphic) ascomycetes are now identified and classified based on
morphologicalIn biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...
or physiological similarities to ascus-bearing
taxa|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a...
, and by phylogenetic analyses of
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
sequences.
The ascomycetes are a monophyletic group, i.e., all of its members trace back to 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
pathogenA pathogen , infectious agent, or germ, is a biological agent that causes disease or illness to its host....
s of humans and plants. Familiar examples of sac fungi include
morelMorchella, the true morels, is a genus of edible mushrooms closely related to anatomically simpler cup fungi. These distinctive mushrooms appear honeycomb-like in that the upper portion is composed of a network of ridges with pits between them....
s, truffles, brewer's yeast and
baker's yeastBaker's yeast is the common name for the strains of yeast commonly used as a leavening agent in baking bread and related products, where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol. The use of potatoes, water from potato boiling, eggs, or sugar in a bread...
,
Dead Man's FingersXylaria is a genus of ascomycetous fungi commonly found growing on dead wood.Two of the common species of the genus are Xylaria hypoxylon and Xylaria polymorpha ...
, and cup fungi. The fungal
symbiontThe term symbosis commonly describes close and often long-term interactions between different biological species...
s in the majority of
lichenLichens are composite organisms consisting of a symbiotic association of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
s (loosely termed
"ascolichens"Lichens are composite organisms consisting of a symbiotic association of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
) such as
CladoniaCladonia is a genus of moss-like lichens in the family Cladoniaceae. They are the primary food source for reindeer and caribou. Cladonia species are of economic importance to reindeer-herders, such as the Sami in Scandinavia or the Nenets in Russia. Antibiotic compounds are extracted from some...
belong to the Ascomycota. There are many plant-pathogenic ascomycetes, including
apple scabApple scab is a disease of apple trees caused by the ascomycete fungus Venturia inaequalis. The disease manifests as dull black or grey-brown lesions on the surface of tree leaves, buds or fruits. Lesions may also appear less frequently on the woody tissues of the tree. Fruits and the undersides...
, rice blast, the
ergot fungiErgot refers to a group of fungi of the genus Claviceps . The most prominent member of this group is Claviceps purpurea. This fungus grows on rye and related plants, and can cause ergotism in humans and other mammals consuming seeds contaminated with the fruiting structure of this fungus, called an...
, black knot, and the
powdery mildewPowdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected plants display white powder-like spots on...
s. Several species of ascomycetes are biological model organisms in laboratory research. Most famously
Neurospora crassa Neurospora crassa is a type of red bread mold of the phylum Ascomycota. The genus name, meaning "nerve spore" refers to the characteristic striations on the spores....
, several species of
yeastYeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with about 1,500 species currently described; they dominate fungal diversity in the oceans. Most reproduce asexually by budding, although a few do so by binary fission...
s, and
AspergillusAspergillus is a genus of a few hundred molds found throughout much of nature worldwide. Aspergillus was first catalogued in 1729 by the Italian priest and biologist Pier Antonio Micheli. Viewing the fungi under a microscope, Micheli was reminded of the shape of an aspergillum , and named the genus...
species are used in many
geneticsGenetics, , a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding...
and
cell biologyCell biology is an academic discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
studies.
PenicilliumPenicillium is a genus of ascomycetous fungi of major importance in the environment, food and drug production...
species on cheeses and those producing antibiotics for treating bacterial
infectious diseaseAn infectious disease is a clinically evident disease resulting from the presence of pathogenic microbial agents, including pathogenic viruses, pathogenic bacteria, fungi, protozoa, multicellular parasites, and aberrant proteins known as prions...
s are examples of taxa that belong to the Ascomycota.
Ascomycetes versus Ascomycota
Before the recognition of the fungal kingdom, the sac fungi were considered to be a
ClassIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
, not a
PhylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
. The original collective term for these
taxa|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a...
was "Ascomycetes", which was first coined in the 1800s for a rankless nonlichenized taxon that possessed asci. The names Ascomycota, Ascomycetes, and others with the same root are based upon the term "ascus". "Ascomycetes" was soon used to include lichenized taxa, and became the standard term, at the class level, for all ascus-bearing species, just as the term "Basidiomycetes" became used for their basidium-bearing counterparts. Elevation of the taxonomic rank of the Ascomycetes resulted in the names Ascomycetae, Ascomycotina, and finally Ascomycota. Together, the Ascomycota and the Basidiomycota form the subkingdom
DikaryaDikarya is a subkingdom of Fungi that includes the phyla Ascomycota and Basidiomycota, both of which in general produce dikaryons, may be filamentous or unicellular, but are always without flagella. The Dikarya are most of the so called "Higher Fungi", but also include many anamorphic species that...
. The more familiar term, Ascomycetes, is still loosely used, e.g. at fungal forays it is often said of a fungus, such as
PezizaPeziza is large genus of saprophytic cup fungi that grow on the ground, rotting wood, or dung. Most members of this genus are of unknown edibility and are difficult to identify as separate species without use of microscopy.-Species:...
, "It is an ascomycete, not a basidiomycete" in reference to their sexual reproductive mode. The terms are further abbreviated to "ascos" and "basidos" which are not officially sanctioned technical names.
Modern classification of Ascomycota
There are three
subphylaIn life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum.Not all phyla are divided into subphyla...
that are described and accepted:
- The Pezizomycotina
Pezizomycotina is a subphylum of the Ascomycota and is more or less synonymous with the older taxon Euascomycota...
is the largest subphylum and contains all ascomycetes that produce ascocarpAn ascocarp, or ascoma , is the fruiting body of an ascomycete fungus. It consists of very tightly interwoven hyphae and may contain millions of asci, each of which contains typically eight ascospores...
s (fruiting bodies), except for one genus, NeolectaNeolecta is a genus of ascomycetous fungi that have fruiting bodies in the shape of unbranched to lobed bright yellowish, orangish to pale yellow-green colored, club-shaped, smooth, fleshy columns up to about 7 cm tall. The species share the English designation "Earth tongues" along with some...
, in the TaphrinomycotinaTaphrinomycotina is one of three subphyla constituting the Ascomycota and is more or less synonymous with the slightly older invalid name Archiascomycetes...
. It is roughly equivalent to the previous taxon, Euascomycetes. The Pezizomycotina includes most macroscopic "ascos" such as truffles, ergotErgot refers to a group of fungi of the genus Claviceps . The most prominent member of this group is Claviceps purpurea. This fungus grows on rye and related plants, and can cause ergotism in humans and other mammals consuming seeds contaminated with the fruiting structure of this fungus, called an...
, ascolichens, cup fungi (discomycetesDiscomycetes is a former taxonomic class of Ascomycete fungi which contains all of the cup, sponge, brain, and some club-like fungi. It includes typical cup fungi like the scarlet elf cup and the orange peel fungus, and fungi with fruiting bodies of more unusual shape, such as morels, truffles and...
), pyrenomycetesThe Sordariomycetes are a class of fungi in the subdivision Pezizomycotina , consisting of 15 orders, 64 families, 1119 genera, and 10564 species.Sordariomycetes generally produce their asci in perithecial fruiting bodies....
, lorchels, and caterpillar fungus.http://www.herbarium.usu.edu/fungi/FunFacts/Caterpillar.htm It also contains microscopic fungi such as powdery mildewPowdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected plants display white powder-like spots on...
s, dermatophyticThe fungus genus Trichophyton is characterized by the development of both smooth-walled macro- and microconidia. Macroconidia are mostly borne laterally directly on the hyphae or on short pedicels, and are thin- or thick-walled, clavate to fusiform, and range from 4 to 8 by 8 to 50 um in size....
fungi, and LaboulbenialesThe Laboulbeniales are an order of fungi within the class Lecanoromycetes.These include the obligate insect parasites, with cellular thalli, predisposing this fungi to certain death without an insect host....
.
- The Saccharomycotina
Saccharomycotina is a subphylum of the phylum Ascomycota , and consists of yeasts - they form no ascocarps , their asci are naked, and they can reproduce asexually by budding....
comprises most of the "true" yeasts, such as baker's yeastBaker's yeast is the common name for the strains of yeast commonly used as a leavening agent in baking bread and related products, where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol. The use of potatoes, water from potato boiling, eggs, or sugar in a bread...
and CandidaCandida is a genus of yeasts. Many species of this genus are endosymbionts of animal hosts including humans. While usually living as commensals, some Candida species have the potential to cause disease...
which are single-celled (unicellular) fungi, which reproduce vegetatively by budding. Most of these species were previously classified in a taxon called Hemiascomycetes.
- The Taphrinomycotina
Taphrinomycotina is one of three subphyla constituting the Ascomycota and is more or less synonymous with the slightly older invalid name Archiascomycetes...
includes a disparate and basalIn phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
group within the Ascomycota that was recognized following molecular (DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
) analyses. The taxon was originally named Archiascomycetes (or Archaeascomycetes). It includes both hyphal fungi (NeolectaNeolecta is a genus of ascomycetous fungi that have fruiting bodies in the shape of unbranched to lobed bright yellowish, orangish to pale yellow-green colored, club-shaped, smooth, fleshy columns up to about 7 cm tall. The species share the English designation "Earth tongues" along with some...
, TaphrinaTaphrina is a fungal genus within the Ascomycota that causes leaf and catkin curl diseases and witch's brooms of certain flowering plants. One of the more commonly observed species causes peach leaf curl...
), fission yeasts (SchizosaccharomycesSchizosaccharomyces is a genus of fission yeasts. The most well-studied species is S. pombe. Like the distantly related Saccharomyces cerevisiae, S. pombe is a significant model organism in the study of eukaryotic cell biology...
), and the mammalian lung parasite, Pneumocystis.
Ribosomal RNARibosomal RNA is the central component of the ribosome, the protein manufacturing machinery of all living cells. The function of the rRNA is to provide a mechanism for decoding mRNA into amino acids and to interact with the tRNAs during translation by providing peptidyl transferase activity.The...
gene sequencing of soil suggests that there may be a fourth subphylum of Ascomycota (termed Soil Clone Group I or SCGI), that has not been described in cultures or based on fruiting bodies. SCGI organisms are only known from
DNA sequenceA DNA sequence or genetic sequence is a succession of letters representing the primary structure of a real or hypothetical DNA molecule or strand, with the capacity to carry information as described by the central dogma of molecular biology....
s found in soils worldwide and are placed between the Taphriomycotina and the Saccharomycotina.
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
DiscomycetesDiscomycetes is a former taxonomic class of Ascomycete fungi which contains all of the cup, sponge, brain, and some club-like fungi. It includes typical cup fungi like the scarlet elf cup and the orange peel fungus, and fungi with fruiting bodies of more unusual shape, such as morels, truffles and...
, which included all species forming apothecia; the Pyrenomycetes, which included all sac fungi that formed perithecia or
pseudotheciaAn ascocarp, or ascoma , is the fruiting body of an ascomycete fungus. It consists of very tightly interwoven hyphae and may contain millions of asci, each of which contains typically eight ascospores...
, or any structure resembling these morphological structures; and the Plectomycetes, which included those species that form
cleistotheciaAn ascocarp, or ascoma , is the fruiting body of an ascomycete fungus. It consists of very tightly interwoven hyphae and may contain millions of asci, each of which contains typically eight ascospores...
.
HemiascomycetesHemiascomycetes is class of fungi of the phylum Ascomycota that includes the yeasts and yeastlike fungi. These are morphologically simple fungi; no ascoma is formed, and the asci are produced free on the host or substrate. Asexual reproduction occurs by the formation of blastospores or, less...
included the yeasts and yeast-like fungi that have now been placed into the
SaccharomycotinaSaccharomycotina is a subphylum of the phylum Ascomycota , and consists of yeasts - they form no ascocarps , their asci are naked, and they can reproduce asexually by budding....
or
TaphrinomycotinaTaphrinomycotina is one of three subphyla constituting the Ascomycota and is more or less synonymous with the slightly older invalid name Archiascomycetes...
, while the Euascomycetes included the remaining species of the Ascomycota which are now in the
PezizomycotinaPezizomycotina is a subphylum of the Ascomycota and is more or less synonymous with the older taxon Euascomycota...
, and the
NeolectaNeolecta is a genus of ascomycetous fungi that have fruiting bodies in the shape of unbranched to lobed bright yellowish, orangish to pale yellow-green colored, club-shaped, smooth, fleshy columns up to about 7 cm tall. The species share the English designation "Earth tongues" along with some...
which are in the Taphrinomycotina.
Some ascomycetes do not reproduce sexually or are not known to produce
asciAn ascus is the sexual spore-bearing cell produced in ascomycete fungi. On average, asci normally contain 8 ascospores, produced by a meiotic cell division followed, in most species, by a mitotic cell division. However, asci in some genera or species can number 1 , 2, 4, or multiples of four...
and are therefore anamorphic species. Those anamorphs that produce
conidiaConidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus; they are also called mitospores due to the way they are generated through the cellular process of mitosis...
(mitospores) were previously described as
Mitosporic AscomycotaThe Mitosporic Ascomycota are a heterogeneous group of ascomycotic fungi whose common characteristic is the absence of a sexual state; many of the pathogenic fungi in humans belong to this...
. Some taxonomists placed this group into a separate
artificial phylumForm taxon is a biological term with two uses:In general taxonomy, it is a kind of wastebasket taxon, either a taxon that is not a natural group but united by shared plesiomorphies, or a presumably artificial group of organisms whose true relationships are not known, being obscured by...
, 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 nidulansAspergillus nidulans is one of many species of filamentous fungi in the phylum Ascomycota. It has been an important research organism for studying eukaryotic cell biologyfor over 50 years,...
and
Emericella nidulans, for asexual and sexual isolates, respectively, of the same species.
Species of the Deuteromycota were classified as Coelomycetes if they produced their conidia in minute flask- or saucer-shaped conidiomata, known technically as
pycnidia and
acervuli. The
HyphomycetesHyphomycetes is a class of fungi in the phylum Fungi imperfecti that lack fruiting bodies. Identification of the Hyphomycetes is primarily based on microscopic morphology including: conidial morphology, especially septation, shape, size, colour and cell wall texture, the arrangement of conidia as...
were those species where the conidiophores (
i.e., the hyphal structures that carry conidia-forming cells at the end) are free or loosely organized. They are mostly isolated but sometimes also appear as bundles of cells aligned in parallel (described as
synnematal) or as cushion-shaped masses (described as
sporodochial).
Morphology
Most species grow as filamentous, microscopic structures called
hyphaA hypha is a long, branching filamentous cell of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or more...
e. Many
interconnectedAn anastomosis is a network of streams that both branch out and reconnect, such as blood vessels or leaf veins. The term is used in medicine, biology, mycology and geology.-Medicine:...
hyphae form a
myceliumthumb|right|Fungal myceliaMycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or in many other substrates...
, which—when visible to the naked eye (macroscopic)—is commonly called
moldMolds include all species of microscopic fungi that grow in the form of multicellular filaments, called hyphae. In contrast, microscopic fungi that grow as single cells are called yeasts...
(or, in botanical terminology, thallus). During sexual reproduction, many Ascomycota typically produce large numbers of
asciAn ascus is the sexual spore-bearing cell produced in ascomycete fungi. On average, asci normally contain 8 ascospores, produced by a meiotic cell division followed, in most species, by a mitotic cell division. However, asci in some genera or species can number 1 , 2, 4, or multiples of four...
. The asci is often contained in a multicellular, occasionally readily visible fruiting structure, the
ascocarpAn ascocarp, or ascoma , is the fruiting body of an ascomycete fungus. It consists of very tightly interwoven hyphae and may contain millions of asci, each of which contains typically eight ascospores...
(also called an
ascoma). Ascocarps come in a very large variety of shapes: cup-shaped, club-shaped, potato-like, spongy, seed-like, oozing and pimple-like, coral-like, nit-like, golf-ball-shaped, perforated tennis ball-like, cushion-shaped, plated and feathered in miniature (
LaboulbenialesThe Laboulbeniales are an order of fungi within the class Lecanoromycetes.These include the obligate insect parasites, with cellular thalli, predisposing this fungi to certain death without an insect host....
), microscopic classic Greek shield-shaped, stalked or sessile. They can appear solitary or clustered. Their texture can likewise be very variable, including fleshy, like charcoal (carbonaceous), leathery, rubbery, gelatinous, slimy, powdery, or cob-web-like. Ascocarps come in multiple colors such as red, orange, yellow, brown, black, or, more rarely, green or blue. Some ascomyceous fungi, such as
Saccharomyces cerevisiae, grow as single-celled yeasts, which—during sexual reproduction—develop into an ascus, and do not form fruiting bodies.
In
lichenLichens are composite organisms consisting of a symbiotic association of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
ized species, the thallus of the fungus defines the shape of the
symbioticThe term symbosis commonly describes close and often long-term interactions between different biological species...
colony. Some
dimorphicPhenotypic switching is switching between two cell-types. An example is Candida albicans, which, when it infects host tissue, switches from the usual unicellular yeast-like form of into an invasive, multicellular filamentous form...
species, such as
Candida albicansCandida albicans is a diploid fungus and a causal agent of opportunistic oral and genital infections in humans. Systemic fungal infections have emerged as important causes of morbidity and mortality in immunocompromised patients...
, 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.
PigmentA pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
ation, such as
melaninMelanin is a class of compounds found in plants, animals, and protists, where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine. Many melanins are insoluble salts and show affinity to water...
in hyphal walls, along with prolific growth on surfaces can result in visible mold colonies; examples include
CladosporiumCladosporium is a genus of fungi including some of the most common indoor and outdoor molds. It produces olive-green to brown or black colonies, and its dark-pigmented conidia are formed in simple or branching chains....
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 occasionally
MucoromycotinaMucormycotina is a fungal grouping.It includes Endogonales, Mucorales, and Mortierellales....
and almost never
BasidiomycotaBasidiomycota is one of two large phyla that, together with the Ascomycota, comprise the subkingdom Dikarya within the Kingdom Fungi...
. Sooty molds that develop on plants, especially in the tropics are the thalli of many species.
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
pneumoniaPneumonia is an inflammatory illness of the lung. Frequently, it is described as lung parenchyma/alveolar inflammation and abnormal alveolar filling with fluid ....
. Asci of
AscosphaeraAscosphaera is a genus of fungi within the Ascosphaeraceae family.-External links:* at Index Fungorum...
fill
honey beeHoney bees are a subset of bees, primarily distinguished by the production and storage of honey and the construction of perennial, colonial nests out of wax. Honey bees are the only extant members of the tribe Apini, all in the genus Apis...
larvaA larva is a young form of animal with indirect development, going through or undergoing metamorphosis ....
e and pupae causing mummification with a chalk-like appearance, hence the name "chalkbrood". Yeasts for small colonies
in vitroA procedure performed in vitro is performed not in a living organism but in a controlled environment, such as in a test tube or Petri dish...
and
in vivoIn vivo refers to experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
, and excessive growth of
CandidaCandida is a genus of yeasts. Many species of this genus are endosymbionts of animal hosts including humans. While usually living as commensals, some Candida species have the potential to cause disease...
species in the mouth or vagina causes "thrush", a form of
candidiasisCandidiasis or thrush is a fungal infection of any of the Candida species, of which Candida albicans is the most common. Candidiasis encompasses infections that range from superficial, such as oral thrush and vaginitis, to systemic and potentially life-threatening diseases...
.
The cell walls of the ascomycetes almost always contain
chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is the main component of the cell walls of fungi, the exoskeletons of arthropods, such as crustaceans Chitin...
and
β-glucansβ-Glucans are polysaccharides of D-glucose monomers linked by glycosidic bonds. Beta-glucans are a diverse group of molecules which can vary with respect to molecular mass, solubility, viscosity, and three-dimensional configuration...
, and divisions within the hyphae, called "
septaIn anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.- In human anatomy :* Interatrial septum, the wall of tissue that separates the left and right atria of the heart...
", 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
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
in case of local damage to cell wall and
cell membraneThe cell membrane is the biological membrane separating the interior of a cell from the outside environment....
. The septa commonly have a small opening in the center, which functions as a
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
ic connection between adjacent cells, also sometimes allowing cell-to-cell movement of
nucleiIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
within a hypha. Vegetative hyphae of most ascomycetes contain only one nucleus per cell (
uninucleate hyphae), but
multinucleateMultinucleate cells have more than one nucleus per cell, which is the result of nuclear division not being followed by cytokinesis. As a consequence, multiple nuclei share one common cytoplasm. This can be the consequence of a disturbed cell cycle control Multinucleate (also multinucleated,...
cells—especially in the apical regions of growing hyphae—can also be present.
Metabolism
In common with other fungal phyla, the Ascomycota are heterotrophic organisms that require organic molecules as energy sources. These are obtained by feeding on a variety of organic substrates including dead matter, foodstuffs, or as symbionts in or on other living organisms. To obtain these nutrients from their surroundings, ascomycetous fungi secrete powerful digestive enzymes which break down organic substances into smaller molecules, which are then taken up into the cell. Many species live on dead plant material such as leaves, twigs, or logs. Several species colonize plants, animals, or other fungi as parasites or
mutualistic symbiontsMutualism is a biological interaction between two organisms, where each individual derives a fitness benefit . Similar interactions within a species are known as co-operation...
and derive all their metabolic energy in form of nutrients from the tissues of their hosts.
Owing to their long evolutionary history, the Ascomycota have evolved the capacity to break down almost every organic substance. Unlike most organisms, they are able to use their own
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at...
s to digest plant
biopolymerBiopolymers are polymers produced by living organisms. Cellulose and starch, proteins and peptides, and DNA and RNA are all examples of biopolymers, in which the monomeric units, respectively, are sugars, amino acids, and nucleotides....
s such as
celluloseCellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
or
ligninLignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
.
CollagenCollagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. It is naturally found exclusively in metazoa, including sponges. In muscle tissue it serves as a major component of endomysium...
, an abundant structural protein in animals, and
keratinKeratins are a family of fibrous structural proteins; tough and insoluble, they form the hard but un-mineralized structures found in reptiles, birds, amphibians, and mammals...
—a protein that forms hair and nails—, can also serve as food sources. Unusual examples include
Aureobasidium pullulans, which feeds on wall paint, and the kerosene fungus
Amorphotheca resinae, which feeds on aircraft fuel (causing occasional problems for the airline industry), and may sometimes blocks fuel pipes. Other species can resist high
osmotic stressOsmotic pressure is the pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane.Jacobus Henricus van 't Hoff first proposed a formula for calculating the osmotic pressure, but this was later improved upon by Harmon Northrop Morse.A related...
and grow, for example, on salted fish, and a few ascomycetes are aquatic.
The Ascomycota is characterized by a high degree of specialization; for instance, certain species of
LaboulbenialesThe Laboulbeniales are an order of fungi within the class Lecanoromycetes.These include the obligate insect parasites, with cellular thalli, predisposing this fungi to certain death without an insect host....
attack only one particular leg of one particular insect species. Many Ascomycota engage in symbiotic relationships such as in lichens—symbiotic associations with green algae or
cyanobacteriaCyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria = blue)...
—in which the fungal symbiont directly obtains products of
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
. In common with many basidiomycetes and
GlomeromycotaGlomeromycota is one of seven currently recognized phyla within the kingdom Fungi, with approximately 200 described species. Members of the Glomeromycota form arbuscular mycorrhizas with the roots or thalli of land plants. Geosiphon pyriformis forms an endocytobiotic association with Nostoc...
, some ascomycetes form symbioses with plants by colonizing the roots to form
mycorrhizaA mycorrhiza is a symbiotic association between a fungus and the roots of a plant. In a mycorrhizal association, the fungus may colonize the roots of a host plant, either intracellularly or extracellularly...
l associations. The Ascomycota also represents several carnivorous fungi, which have developed hyphal traps to capture small
protistProtists , are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy...
s such as
amoebaAmoeba is a genus of protozoan.- Terminology :There are many closely related terms that can be the source of confusion:...
e, as well as roundworms (
Nematoda),
rotiferThe rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
s,
tardigradeTardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773...
s, and small arthropods such as
springtailSpringtails form the largest of the three lineages of modern hexapods that are no longer considered insects...
s (
Collembola).
Distribution and living environment
The Ascomycota are represented in all land ecosystems worldwide, occurring on all continents including
Antarctica| style="border-top:solid 1px #ccd2d9; padding:0.4em 1em 0.4em 0; vertical-align:top;" | 14,000,000 km
2 280,000 km
2 13,720,000 km
2 |-! style="border-top: solid 1px #ccd2d9; padding: 0.4em 1em 0.4em 0; vertical-align: top;...
.. Spores and hyphal fragments are dispersed through the atmosphere and freshwater environments, as well as ocean beaches and tidal zones. The distribution of species is variable; while some are found on all continents, others, as for example the white truffle
Tuber magnatum, only occur in isolated locations in Italy and Eastern Europe. The distribution of plant-parasitic species is often restricted by host distributions; for example,
CyttariaCyttaria is a genus of ascomycete fungi. At least 20 species and subspecies belong to the Genus Cyttaria, found in South America and Australia associated with or growing on Nothofagus....
is only found on
NothofagusNothofagus, also known as the southern beeches, is a genus of about 35 species of trees and shrubs native to the temperate oceanic to tropical Southern Hemisphere in southern South America and Australasia...
(Southern Beech) in the
Southern HemisphereThe Southern Hemisphere is the half of a planet that is south of the equator—the word hemisphere literally means 'half ball'...
.
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
conidiaConidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus; they are also called mitospores due to the way they are generated through the cellular process of mitosis...
. The conidiospores commonly contain one nucleus and are products of
mitoticMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
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
hyphaA hypha is a long, branching filamentous cell of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or more...
e, the
conidiophores. Depending on the species they may be 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 (
septumIn anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.- In human anatomy :* Interatrial septum, the wall of tissue that separates the left and right atria of the heart...
), it is called a
didymospore.
When there are two or more cross-walls, the classification depends on spore shape. If the septa 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
EmbryologyEmbryology is the study of the development of an embryo. An embryo is defined as any organism in an early stage well before birth or hatching, or in plants, before germination occurs....
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.
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 which is 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
Phialide [Gr. phialis, diminutive of phiale, a broad, flat vessel]*A flask-shaped projection from the conidiophore of certain fungi.*Projects from the mycelium without increasing in length without a subsequent increase in the formation of conidia....
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 which 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).
Acervular conidiomata, or
acervuli, are specialized conidia that form within the tissues of a host organism:
- subcuticular, lying under the outer layer of the plant (the cuticle
Plant cuticles are a protective waxy covering produced only by the epidermal cells of leaves, young shoots and all other aerial plant organs without periderm...
),
- intraepidermal, inside the outer cell layer (the epidermis
The epidermis is a single-layered group of cells that covers plants leaves, flowers, roots and stems. It forms a boundary between the plant and the external world. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds, and ...
),
- subepidermal, under the epidermis, or deeper inside the host.
Mostly they develop a flat layer of relatively short conidiophores which then produce masses of spores. The increasing pressure leads to the splitting of the epidermis and cuticle and allows release of the conidia from the tissue.
Pycnidial conidiomata or
pycnidia form in the fungal tissue itself, and are shaped like a bulging vase. The spores are released through a small opening at the apex, the
ostiole.
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 which joins the spores on the outside degenerates and releases the conidia.
Heterokaryosis and parasexuality
Several Ascomycota species have no—or 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, caused by merging of two hyphae belonging to different individuals, by a process called
anastomosisAn anastomosis is a network of streams that both branch out and reconnect, such as blood vessels or leaf veins. The term is used in medicine, biology, mycology and geology.-Medicine:...
, followed by a series of events resulting in genetically different
cell nucleiIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
in the
myceliumthumb|right|Fungal myceliaMycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or in many other substrates...
.
The merging of nuclei is not followed by
meiotic eventsIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
, such as
gameteA gamete is a cell that fuses with another gamete during fertilization in organisms that reproduce sexually...
formation and results in an increased number of
chromosomeA chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions...
s per nuclei.
Mitotic crossoverMitotic crossover is a rare type of genetic recombination that may occur in some types of somatic cells during mitosis. Mitotic crossover may occur in organisms that do not have a cycle of sexual reproduction where chromosomal crossover would normally occur during meiosis to generate genetic...
may enable
recombinationGenetic recombination is the process by which a strand of genetic material is broken and then joined to a different DNA molecule. In eukaryotes recombination occurs in mitosis as a common mechanism of DNA repair and in meiosis as a way of facilitating chromosomal crossover...
, i.e., an exchange of genetic material between
homologous chromosomeHomologous chromosomes are chromosomes in a biological cell that pair during cell division during the creation of gametes .Each chromosome pair contains genes for the same biological features, such as eye color, at the same locations on the chromosome.Each chromosome pair can contain the same...
s. The chromosome number may then be restored to its haploid state by
nuclear divisionTelophase, from the ancient Greek "τελος" and "φασις" , is a stage in either meiosis or mitosis in an eukaryotic cell reversing the effects of prophase and prometaphase events...
, with each daughter nuclei being genetically different from the original parent nuclei. Alternatively, nuclei may lose some chromosomes, resulting in aneuploid cells.
Sexual reproduction
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
meiosisIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
and which are called
ascospores.
Apart from a few exceptions, such as
Candida albicansCandida albicans is a diploid fungus and a causal agent of opportunistic oral and genital infections in humans. Systemic fungal infections have emerged as important causes of morbidity and mortality in immunocompromised patients...
, most ascomycetes are haploid, i.e., they contain one set of
chromosomeA chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions...
s per nuclei. During sexual reproduction there is a diploid phase which commonly is very short, and meiosis restores the haploid state.
Formation of sexual spores
The sexual part of the life cycle commences when two hyphal structures
mateIn biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation and, in social animals, also to raise their offspring. For animals, mating methods include random mating, disassortative mating, assortative mating, or a mating pool....
. In the case of
homothallicHomothallic refers to the possession, within a single organism, of the resources to reproduce sexually.It can be contrasted to heterothallic.It is often used to categorize fungi. In yeast, homothallic cells have mating types a and α...
species, mating is enabled between hyphae of the same fungal
cloneCloning in biology is the process of producing populations of genetically-identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or organisms...
, whereas in
heterothallicHeterothallic species have sexes that reside in different individuals....
species, the two hyphae must originate from fungal clones that differ genetically, i.e., those that are of a different
mating typeMating types occur in eukaryotes that undergo sexual reproduction via isogamy. Since the gametes of different mating types look alike, they are often referred to by numbers, letters, or simply "+" and "-" instead of "male" and "female." Mating can only take place between different mating...
. 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 vegetative incompatibility systems.
GametangiaA gametangium is an organ or cell in which gametes are produced that is found in many multicellular protists, algae, fungi, and the gametophytes of plants...
are sexual structures formed from hyphae, and are the generative cells. A very fine hypha, called
trichogyneTrichogyne is a genus of flowering plants in the daisy family, Asteraceae....
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
plasmogamyPlasmogamy is a stage in the sexual reproduction of fungi. In this stage, the cytoplasm of two parent mycelia fuse together without the fusion of nuclei, as occurs in higher terrestrial fungi. After plasmogamy occurs, the secondary mycelium forms. The secondary mycelium consists of dikaryotic...
—the mixing of the
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
—occurs. Unlike in animals and plants, plasmogamy is not immediately followed by the merging of the nuclei (called
karyogamyKaryogamy is the fusion of pronuclei of two cells, as part of syngamy. It is one of the two major modes of reproduction in fungi. It is also the fusion of the pronuclei of two cells, as occurs in fertilization or true conjugation....
). 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
recombinationGenetic recombination is the process by which a strand of genetic material is broken and then joined to a different DNA molecule. In eukaryotes recombination occurs in mitosis as a common mechanism of DNA repair and in meiosis as a way of facilitating chromosomal crossover...
and is followed by
meiosisIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
. A similar sexual cycle is present in the red algae (Rhodophyta).
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 grows into fruiting body, the
ascocarpAn ascocarp, or ascoma , is the fruiting body of an ascomycete fungus. It consists of very tightly interwoven hyphae and may contain millions of asci, each of which contains typically eight ascospores...
, which may contain millions of fertile hyphae.
The sexual structures are formed in the fruiting layer of the ascocarp, the
hymeniumThe hymenium is the tissue layer on the hymenophore of a fungal fruiting body where the cells develop into basidia or asci, which produce spores. In some species all of the cells of the hymenium develop into basidia or asci, while in others some cells develop into sterile cells called cystidia or...
. 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 spindleIn cell biology, the spindle apparatus is the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...
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 fromation 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.
Fusion of the nuclei (karyogamy) takes place in the U-shaped cells in the hymenium, and results in the formation of a diploid
zygoteA zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development...
. The zygote grows into the ascus, an elongated tube-shaped or cylinder-shaped capsule.
MeiosisIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
then gives rise to four 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 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
asciAn ascus is the sexual spore-bearing cell produced in ascomycete fungi. On average, asci normally contain 8 ascospores, produced by a meiotic cell division followed, in most species, by a mitotic cell division. However, asci in some genera or species can number 1 , 2, 4, or multiples of four...
for further details.
Ecology
The Ascomycota fulfil a central role in most land-based
ecosystemAn ecosystem is a system of interdependent organisms which share the same habitat, in an area functioning together with all of the physical factors of the environment. Ecosystems can be permanent or temporary. Ecosystems usually form a number of food webs...
s. They are important
decomposerDecomposers are organisms that eat the dead or decaying organisms, and in doing so carry out the natural process of decomposition. The herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and...
s which break down organic materials, such as dead leaves and animals, and help the
detritivoreDetritivores, also known as detritus feeders or saprophages, are heterotrophs that obtain nutrients by consuming detritus . By doing so, they contribute to decomposition and the nutrient cycles....
s (animals which feed on decomposing material) to obtain their nutrients. Ascomycetes along with other fungi can break down large
moleculesBiopolymers are polymers produced by living organisms. Cellulose and starch, proteins and peptides, and DNA and RNA are all examples of biopolymers, in which the monomeric units, respectively, are sugars, amino acids, and nucleotides....
such as
celluloseCellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
or
ligninLignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
, and thus have important roles in nutrient cycling such as the
carbon cycleThe carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
.
The fruiting bodies of the Ascomycota provide food for many animals ranging from
insectInsects are arthropods, having a hard exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae. They are the most diverse group of animals on the planet and include more than a million species that are already described. Insects represent more than half of all...
s and slugs and snails (
GastropodaThe class Gastropoda or gastropods form a major part of the phylum Mollusca. Gastropods are more commonly known as snails and slugs....
) to
rodentRodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s and larger mammals such as
deerDeer are the ruminant mammals forming the family Cervidae. They include for example Moose, Red Deer, Reindeer, Roe and Chital. Animals from related families within the order Artiodactyla are often also considered to be deer – these include muntjac and water deer...
and wild boars.
Many ascomycetes also form symbiotic relationships with other organisms, including plants and animals.
Lichens
Probably since early in their evolutionary history, the Ascomycota have formed symbiotic associations with
green algaeThe green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...
(
Chlorophyta), and other types of algae and
cyanobacteriaCyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria = blue)...
. 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, including the
ArcticThe Arctic is the region around the Earth's North Pole, opposite the Antarctic region around the South Pole. The Arctic includes the Arctic Ocean and parts of Canada, Greenland , Russia, the United States , Iceland, Norway, Sweden and Finland.The word Arctic comes from the Greek αρκτικός , "near...
, the
AntarcticThe Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence...
,
desertA desert is a landscape or region that receives almost no precipitation. Deserts are defined as areas with an average annual precipitation of less than per year, or as areas where more water is lost by evapotranspiration than falls as precipitation. In the Köppen climate classification system,...
s, and mountaintops. While the photoautotrophic algal partner generates metabolic energy through photosynthesis, the fungus offers a stable, supportive matrix and protects cells from radiation and dehydration. Around 42% of the Ascomycota (about 18,000 species) form lichens, and almost all the fungal partners of lichens belong to the Ascomycota.
Mycorrhizal fungi and endophytes
Members of the Ascomycota form two important types of relationship with plants: as
mycorrhizaA mycorrhiza is a symbiotic association between a fungus and the roots of a plant. In a mycorrhizal association, the fungus may colonize the roots of a host plant, either intracellularly or extracellularly...
l fungi and as
endophyteAn endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life without causing apparent disease. Endophytes are ubiquitous and have been found in all the species of plants studied to date; however, most of these endophyte/plant relationships...
s. Mycorrhiza are symbiotic associations of fungi with the root systems of the plants, which can be of vital importance for growth and persistence for the plant. The fine mycelial network of the fungus enables the increased uptake of mineral salts that occur at low levels in the soil. In return, the plant provides the fungus with metabolic energy in the form of photosynthetic products.
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
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
; in the case of
grass endophytesNeotyphodium is a form genus containing species of endophytic fungi. These endophytes are asexual, seed-borne symbionts of cool-season grasses, and grow intercellularly throughout the aerial tissues of their hosts, including shoot apical meristems, leaf sheaths and blades, inflorescences, seeds and...
the fungal symbiont produces poisonous
alkaloidAlkaloids are naturally occurring chemical compounds containing basic nitrogen atoms. The name derives from the word alkaline and was used to describe any nitrogen-containing base. Alkaloids are produced by a large variety of organisms, including bacteria, fungi, plants, and animals and are part of...
s, which can affect the health of plant-eating (herbivorous)
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s and deter or kill insect herbivores.
Symbiotic relationships with animals
Several ascomycetes of the genus
XylariaXylaria is a genus of ascomycetous fungi commonly found growing on dead wood.Two of the common species of the genus are Xylaria hypoxylon and Xylaria polymorpha ...
colonize the nests of
leafcutter antLeafcutter ants are social insects found in warmer regions of the Americas. These unique ants have evolved an advanced agricultural system based on ant-fungus mutualism...
s and other
fungus-growing antsThe fungus-growing ants comprises all the known fungus-growing ant species in the world participating in ant-fungus mutualism. Leafcutter ants make up 2 of the genera....
of the tribe Attini, and the fungal gardens of
termiteThe termites are a group of eusocial insects usually classified at the taxonomic rank of order Isoptera . Along with ants and some bees and wasps which are all placed in the separate order Hymenoptera, termites divide labour among gender lines, produce overlapping generations and take care of...
s (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
BasidiomycotaBasidiomycota is one of two large phyla that, together with the Ascomycota, comprise the subkingdom Dikarya within the Kingdom Fungi...
species, they may be cultivated.
Bark beetleA bark beetle is one of approximately 220 genera with 6,000 species of beetles in the subfamily Scolytinae. Traditionally this was considered a distinct family Scolytidae, but nowadays it is understood that bark beetles are in fact very specialized members of the "true weevil" family...
s (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 diseaseDutch elm disease is a fungal disease of elm trees which is spread by the elm bark beetle. Although believed to be originally native to Asia, the disease has been accidentally introduced into America and Europe, where it has devastated native populations of elms which had not had the opportunity...
, caused by
Ophiostoma ulmi, which is carried by the European elm bark beetle,
Scolytus multistriatus.
Importance for humans
Ascomycetes make many contributions to the good of humanity, and also have many ill effects.
Harmful interactions
One of their most harmful roles is as the agent of many plant diseases. For instance:
- Dutch Elm Disease
Dutch elm disease is a fungal disease of elm trees which is spread by the elm bark beetle. Although believed to be originally native to Asia, the disease has been accidentally introduced into America and Europe, where it has devastated native populations of elms which had not had the opportunity...
, caused by the closely related species Ophiostoma ulmi and Ophiostoma novo-ulmi, has led to the death of many elms in Europe and North America.
- The originally Asian Cryphonectria parasitica
The chestnut blight fungus, Cryphonectria parasitica , virtually eliminated the once-widespread American chestnut tree....
is responsible for attacking Sweet ChestnutCastanea sativa is a species of the flowering plant family Fagaceae, the tree and its edible seeds are referred to by several common names such Sweet Chestnut or Marron. Originally native to southeastern Europe and Asia Minor, it is now widely dispersed throughout Europe...
s (Castanea sativa), and virtually eliminated the once-widespread American ChestnutThe American Chestnut is a large, deciduous tree of the beech family native to eastern North America. Before the species was devastated by the chestnut blight, a fungal disease, it was one of the most important forest trees throughout its range...
(Castanea dentata),
- A disease of Maize
Maize , is a herbaceous plant domesticated in Mesoamerica and subsequently spread throughout the American continents...
(Zea mays), which is especially prevalent in North America, is brought about by Cochliobolus heterostrophus.
- Taphrina deformans causes leaf curl
Leaf curl is a plant disease caused by a fungus or virus and characterized by curling of leaves...
of peach.
- Uncinula necator
Uncinula necatorsyn. "Erysiphe necator" is a fungus that causes powdery mildew on grape. It is a common pathogen of grape . The fungus is believed to have originated in North America. European wine varieties of Vitis vinifera are more or less susceptible to this fungus...
is responsible for the disease Powdery mildewPowdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected plants display white powder-like spots on...
, which attacks grapevines.
- Species of Monilia cause brown rot of stone fruit such as peaches (Prunus persica) and sour cherries (Prunus ceranus).
- Members of the Ascomycota such as Stachybotrys chartarum
Stachybotrys chartarum is a greenish-black mold that is commonly found outdoors and sometimes found in damp or flooded homes. It was originally described from the wall of a house in Prague in 1837 by Czech mycologist August Carl Joseph Corda...
are responsible for fading of woollen textiles, which is a common problem especially in the tropics.
- Blue-green, red and brown moulds attack and spoil foodstuffs - for instance Penicillium
Penicillium is a genus of ascomycetous fungi of major importance in the environment, food and drug production...
italicum rots oranges.
- Cereals infected with Fusarium
Fusarium is a large genus of filamentous fungi widely distributed in soil and in association with plants. Most species are harmless saprobes and are relatively abundant members of the soil microbial community. Some species produce mycotoxins in cereal crops that can affect human and animal health...
graminearum contain mycotoxinA mycotoxin is a toxic secondary metabolite produced by an organism of the fungus kingdom, including mushrooms, molds, and yeasts. The term 'mycotoxin' is usually reserved for the toxic chemical products produced by fungi that readily colonize crops...
s like deoxynivalenol (DON), which can lead to skin and mucous membrane lesions when eaten by pigs.
- Ergot
Ergot refers to a group of fungi of the genus Claviceps . The most prominent member of this group is Claviceps purpurea. This fungus grows on rye and related plants, and can cause ergotism in humans and other mammals consuming seeds contaminated with the fruiting structure of this fungus, called an...
(Claviceps purpurea) is a direct menace to humans when it attacks wheat or rye and produces highly poisonous and carcinogenic alkaloidAlkaloids are naturally occurring chemical compounds containing basic nitrogen atoms. The name derives from the word alkaline and was used to describe any nitrogen-containing base. Alkaloids are produced by a large variety of organisms, including bacteria, fungi, plants, and animals and are part of...
s, causing ergotismErgotism is the effect of long-term ergot poisoning, traditionally due to the ingestion of the alkaloids produced by the Claviceps purpurea fungus which infects rye and other cereals, and more recently by the action of a number of ergoline-based drugs. It is also known as ergotoxicosis, ergot...
if consumed. Symptoms include hallucinations, stomach cramp, and a burning sensation in the limbs ("Saint Anthony's Fire").
- Aspergillus flavus
Aspergillus flavus is a fungus. It is a common mold in the environment, and can cause storage problems in stored grains. It can also be a human pathogen, associated with aspergillosis of the lungs and sometimes causing corneal, otomycotic, and nasoorbital infections. Many strains produce...
, which grows on peanuts and other hosts, generates aflatoxinAflatoxins are naturally occurring mycotoxins that are produced by many species of Aspergillus, a fungus, most notably Aspergillus flavus and Aspergillus parasiticus. Aflatoxins are toxic and among the most carcinogenic substances known...
, which damages the liver and is highly carcinogenic.
- Candida albicans
Candida albicans is a diploid fungus and a causal agent of opportunistic oral and genital infections in humans. Systemic fungal infections have emerged as important causes of morbidity and mortality in immunocompromised patients...
, a yeast which attacks the mucous membranes, can cause an infection of the mouth or vagina called thrush or candidiasisCandidiasis or thrush is a fungal infection of any of the Candida species, of which Candida albicans is the most common. Candidiasis encompasses infections that range from superficial, such as oral thrush and vaginitis, to systemic and potentially life-threatening diseases...
, and is also blamed for "yeast allergies".
- Fungi like Epidermophyton
Epidermophyton is a genus of fungus causing superficial and cutaneous mycoses, including E. floccosum, a cause of tinea corporis , tinea cruris , tinea pedis , and onychomycosis or tinea unguium, a fungal infection of the nail bed.-External links:**...
cause skin infections but are not very dangerous for people with healthy immune systems. However if the immune system is damaged they can be life-threatening; for instance, Pneumocystis jiroveci is responsible for severe lung infections which occur in AIDSAcquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus ....
patients.
Positive effects
On the other hand, ascus fungi have brought some important benefits to humanity.
- The most famous case may be that of the mould Penicillium chrysogenum (formerly Penicillium notatum), which, probably to attack competing bacteria, produces an antibiotic which, under the name of Penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. Penicillin antibiotics are historically significant because they are the first drugs that were effective against many previously serious diseases such as syphilis and Staphylococcus infections...
, triggered a revolution in the treatment of bacterial infectious diseases in the 20th century.
- The medical importance of Tolypocladium niveum as an immunosuppressor
Immunosuppression involves an act that reduces the activation or efficacy of the immune system. Some portions of the immune system itself have immuno-suppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reaction to treatment of other conditions...
can hardly be exaggerated. It excretes CiclosporinCiclosporin , cyclosporine , cyclosporin or cyclosporin A, is an immunosuppressant drug widely used in post-allogeneic organ transplant to reduce the activity of the patient's immune system and, so, the risk of organ rejection. It has been studied in transplants of skin, heart, kidney, liver,...
, which, as well as being given during organ transplantOrgan transplant is the moving of an organ from one body to another , for the purpose of replacing the recipient's damaged or failing organ with a working one from the donor site. Organ donors can be living or deceased...
s to prevent rejection, is also prescribed for auto-immune diseases such as multiple sclerosisMultiple sclerosis is an idiopathic disease of suspected autoimmune cause, in which the body's immune response attacks a person's central nervous system , leading to demyelination. Disease onset usually occurs in young adults, and it is more common in females...
, although there is some doubt over the long-term side-effects of the treatment.
- Some ascomycete fungi can be altered relatively easily through genetic engineering
Genetic engineering, recombinant DNA technology, genetic modification/manipulation and gene splicing are terms that apply to the direct manipulation of an organism's genes. Genetic engineering is different from traditional breeding, where the organism's genes are manipulated indirectly...
procedures. They can then produce useful proteins such as insulinInsulin is a hormone that has extensive effects on metabolism and other body functions, such as vascular compliance. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle, and stopping use of fat as an energy...
, human growth hormone, or TPa, which is employed to dissolve blood clots.
- The red bread mold Neurospora crassa
Neurospora crassa is a type of red bread mold of the phylum Ascomycota. The genus name, meaning "nerve spore" refers to the characteristic striations on the spores....
is an important model organismA model organism is a species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms...
in biology, of which the genomeIn modern molecular biology the genome refers to all of its hereditary information encoded in DNA .The genome includes both the genes and the non-coding sequences of the DNA. The term was adapted in 1920 by Hans Winkler, Professor of Botany at the University of Hamburg, Germany...
has now been fully sequenced.
- Baker's Yeast (Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of budding yeast. It is perhaps the most useful yeast owing to its use since ancient times in baking and brewing. It is believed that it was originally isolated from the skins of grapes...
) is used to make breadBread is a staple food prepared by cooking a dough of flour and water and possibly more ingredients. Doughs are usually baked in the Western world , but in some cuisines breads are steamed, fried, or baked on a hot skillet. It may be leavened or unleavened...
, beerBeer is the world's oldest and most widely consumed alcoholic beverage and the third most popular drink overall after water and tea. It is produced by the brewing and fermentation of starches, mainly derived from cereal grains—most commonly malted barley, although wheat, maize , and rice are widely...
and wineWine is an alcoholic beverage typically made of fermented grape juice. The natural chemical balance of grapes is such that they can ferment without the addition of sugars, acids, enzymes or other nutrients. Wine is produced by fermenting crushed grapes using various types of yeast. Yeast consumes...
, during which process sugars such as glucoseGlucose , a monosaccharide also known as - grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate...
or sucroseSucrose, commonly called table sugar, is a moosaccharide of glucose and fructose with the molecular formula C12H22O11. This white, odorless, crystalline powder has a pleasing, sweet taste. It is best known for its role in human nutrition...
are fermented to make alcoholIn chemistry, an alcohol is any organic compound in which a hydroxyl group is bound to a carbon atom of an alkyl or substituted alkyl group. An important group of acohols is formed by the simple acyclic alcohols, the general formula for which is C
nH
2n+1OH...
and carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
. In the case of bread-making, the alcohol evaporates and the carbon dioxide serves to make the dough rise.
- Enzymes of Penicillium camemberti
Penicillium camemberti is a species of fungus used in the production of Camembert and Brie cheeses, on which colonies of P. camemberti form a hard, white crust. It is responsible for giving these cheeses their distinctive taste.- Synonyms :...
play a role in the manufacture of the cheeses CamembertCamembert is a soft, creamy French cheese. It was first made in the late 18th century in Normandy in northern France.-Production:Camembert is made from unpasteurised cows' milk, and is ripened by the moulds Penicillium candidum and Penicillium camemberti for at least three weeks...
and BrieBrie is a soft cows' cheese named after Brie, the French province in which it originated . It is pale in colour with a slight greyish tinge under crusty white mould; very soft and savoury with a hint of ammonia...
, while those of Penicillium roquefortiPenicillium roqueforti is a common saprotrophic fungus from the family Trichocomaceae. Widespread in nature, it can be isolated from soil, decaying organic matter, and plants. The major industrial use of this fungus is the production of blue cheeses, flavoring agents, antifungals, polysaccharides,...
do the same for GorgonzolaGorgonzola is a veined Italian blue cheese, made from unskimmed cow's and/or goat's milk. It can be buttery or firm, crumbly and quite salty, with a 'bite' from its blue veining. It has been made since the early Middle Ages, but became marbled with greenish-blue mold only in the eleventh century. ...
, Roquefort and StiltonStilton is a type of English cheese, known for its characteristic strong smell. It is produced in two varieties: the well-known blue and the lesser-known white. Both have been granted the status of a protected designation of origin by the European Commission...
.
- In Asia Aspergillus oryzae
Aspergillus oryzae is a filamentous fungus . It is used in Chinese and Japanese cuisine to ferment soybeans. It is also used to saccharify rice, other grains, and potatoes in the making of alcoholic beverages such as huangjiu, sake, and shōchū...
is added to a pulp of soaked soya beans to make soy sauceSoy sauce , soya sauce or is produced by fermenting soybeans with the molds Aspergillus oryzae and Aspergillus soyae along with roasted grain, water, and salt. Soy sauce was invented in China, where it has been used as a condiment for close to 2,500 years...
.
- Finally, some members of the Ascomycota are eaten with relish; morel
Morchella, the true morels, is a genus of edible mushrooms closely related to anatomically simpler cup fungi. These distinctive mushrooms appear honeycomb-like in that the upper portion is composed of a network of ridges with pits between them....
s (Morchella) and truffles (Tuber) are some of the most sought-after fungus delicacies.
See also