Ascomycota


The Ascomycota are a phylum in the kingdom Fungi that, together with the Basidiomycota, form the subkingdom Dikarya. Members of Ascomycota are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defining feature of this fungal group is the ascus, a microscopic sexual structure in which nonmotile spores, called ascospores, are formed. However, some species of Ascomycota are asexual and thus do not form asci or ascospores. Familiar examples of sac fungi include morels, truffles, brewers' and bakers' yeast, dead man's fingers, and cup fungi. The fungal symbionts in the majority of lichens such as Cladonia belong to the Ascomycota.
Ascomycota are a monophyletic group. Previously placed in the Basidiomycota along with asexual species from other fungal taxa, asexual ascomycetes are now identified and classified based on morphological or physiological similarities to ascus-bearing taxa, and by phylogenetic analyses of DNA sequences.
Ascomycetes are of particular use to humans as sources of medicinally important compounds such as antibiotics, as well as for fermenting bread, alcoholic beverages, and cheese. Examples of ascomycetes include Penicillium species on cheeses and those producing antibiotics for treating bacterial infectious diseases.
Many ascomycetes are pathogens, both of animals, including humans, and of plants. Examples of ascomycetes that can cause infections in humans include Candida albicans, Aspergillus niger and several tens of species that cause skin infections. The many plant-pathogenic ascomycetes include apple scab, rice blast, the ergot fungi, black knot, and the powdery mildews. The members of the genus Cordyceps are entomopathogenic fungi, meaning that they parasitise and kill insects. Other entomopathogenic ascomycetes have been used successfully in biological pest control, such as Beauveria.
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.

Reproduction in ascomycetes

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, Ascomycota also reproduce asexually through budding.

Conidia formation

Asexual reproduction may occur through vegetative reproductive spores, the conidia. The asexual, non-motile haploid spores of a fungus, which are named after the Greek word for dust, are hence also known as. The conidiospores commonly contain one nucleus and are products of mitotic cell divisions and thus are sometimes called, 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 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, from which the spores are produced. Not all of these asexual structures are a single hypha. In some groups, the conidiophores 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. 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. This is a cushion of conidiophores created from a pseudoparenchymatous stroma in plant tissue. The is a globose to flask-shaped parenchymatous structure, lined on its inner wall with conidiophores. The is a flat saucer shaped bed of conidiophores produced under a plant cuticle, which eventually erupt through the cuticle for dispersal.

Budding

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

  • Ascomycota are morphologically diverse. The group includes organisms from unicellular yeasts to complex cup fungi.
  • 98% of lichens have an Ascomycota as the fungal part of the lichen.
  • 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, 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 and apple blights.
  • 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 antibiotic penicillin.
  • Almost half of all members of the phylum Ascomycota form associations with algae to form lichens.
  • Others, such as morels, form important relationships with plants, thereby providing enhanced water and nutrient uptake and, in some cases, protection from insects.
  • Most ascomycetes are terrestrial or parasitic. However, some have adapted to marine or freshwater environments. As of 2015, there were 805 marine fungi in the Ascomycota, distributed among 352 genera.
  • 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 Ascomycota is the presence of Woronin bodies on each side of the septa separating the hyphal segments which control the septal pores. If an adjoining hypha is ruptured, the Woronin bodies block the pores to prevent loss of cytoplasm into the ruptured compartment. The Woronin bodies are spherical, hexagonal, or rectangular membrane bound structures with a crystalline protein matrix.

    Modern classification

There are three subphyla that are described and accepted:
  • The Pezizomycotina are the largest subphylum and contains all ascomycetes that produce ascocarps, except for one genus, Neolecta, in the Taphrinomycotina. It is roughly equivalent to the previous taxon, Euascomycetes. The Pezizomycotina includes most macroscopic "ascos" such as truffles, ergot, ascolichens, cup fungi, pyrenomycetes, lorchels, and caterpillar fungus. It also contains microscopic fungi such as powdery mildews, dermatophytic fungi, and Laboulbeniales.
  • The Saccharomycotina comprise most of the "true" yeasts, such as baker's yeast and Candida, which are single-celled fungi, which reproduce vegetatively by budding. Most of these species were previously classified in a taxon called Hemiascomycetes.
  • The Taphrinomycotina include a disparate and basal group within the Ascomycota that was recognized following molecular analyses. The taxon was originally named Archiascomycetes. It includes hyphal fungi, fission yeasts, and the mammalian lung parasite Pneumocystis.

    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 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 into the Saccharomycotina or Taphrinomycotina, while the Euascomycetes included the remaining species of the Ascomycota, which are now in the Pezizomycotina, and the Neolecta, which are in the Taphrinomycotina.
Some ascomycetes do not reproduce sexually or are not known to produce asci and are therefore anamorphic species. Those anamorphs that produce conidia were previously described as mitosporic Ascomycota. Some taxonomists placed this group into a separate artificial phylum, the Deuteromycota. 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.
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 Hyphomycetes were those species where the conidiophores are free or loosely organized. They are mostly isolated but sometimes also appear as bundles of cells aligned in parallel or as cushion-shaped masses.