About Ascomycota
Ascomycota is a morphologically diverse phylum of fungi, ranging from unicellular yeasts to complex cup fungi. There are 2,000 identified genera and 30,000 described species of Ascomycota. 98% of all lichens include an ascomycete as the fungal partner of the symbiosis, and almost half of all members of this phylum form symbiotic associations with algae to create lichens. The unifying characteristic that connects all these diverse organisms is the presence of a specialized reproductive structure called the ascus, though the ascus has a reduced role in the life cycle of some species. Many ascomycetes are commercially important. Some have beneficial uses: yeasts are used for baking, brewing, and wine fermentation; truffles and morels are valued as gourmet delicacies; and the mold Penicillium is used to produce the antibiotic penicillin. Morels, a highly prized edible group of ascomycetes, also form important mycorrhizal relationships with plants that improve the plant’s water and nutrient uptake, and sometimes provide protection from insect pests. Many ascomycetes are plant and animal pathogens. A number of common plant diseases are caused by pathogenic ascomycetes, including Dutch elm disease, apple blights, apple scab, rice blast, ergot, black knot, and powdery mildews. Pneumocystis species can colonize lung cavities to cause pneumonia, excessive growth of Candida species causes thrush (candidiasis) in the mouth or vagina, and Ascosphaera causes chalkbrood mummification in honey bee larvae and pupae. Many ascomycetes also cause food spoilage, forming visible moldy pellicles or layers on items like jams and juices. Most ascomycetes are terrestrial or parasitic, but some have adapted to live in marine or freshwater environments; as of 2015, 805 species of marine ascomycetes distributed across 352 genera have been recorded. Ascomycota occur in all land ecosystems worldwide, found on every continent including Antarctica. Spores and hyphal fragments are dispersed through the atmosphere, freshwater environments, ocean beaches, and tidal zones. While some ascomycete species are found across all continents, others have very restricted ranges; for example, the white truffle Tuber magnatum only grows in isolated locations in Italy and Eastern Europe, and the genus Cyttaria is only found on Nothofagus (Southern Beech) trees in the Southern Hemisphere, matching the restricted distribution of its host. Like Basidiomycota, ascomycete hyphal cell walls are composed of chitin and β-glucans. In ascomycetes, these fibers are embedded in a glycoprotein matrix that contains the sugars galactose and mannose. The body (thallus) of most ascomycetes is a mycelium made of thread-like septate hyphae divided by internal septal walls, though there is no fixed number of nuclei per hyphal compartment. Septal walls have pores that maintain cytoplasmic continuity throughout the hypha, and nuclei can also move between compartments through these pores under appropriate conditions. Worononin bodies are a distinctive feature of Ascomycota (though they are not present in all species), found on both sides of the septa separating hyphal segments where they regulate septal pores. If an adjacent hypha is ruptured, Woronin bodies block the pore to stop cytoplasm from leaking into the damaged compartment. These structures are spherical, hexagonal, or rectangular membrane-bound structures with a crystalline protein matrix. Most ascomycete species grow either as filamentous microscopic hyphae, or as single budding cells (yeasts). Many interconnected hyphae form a mycelium; when visible to the naked eye, this growth is commonly called mold. During sexual reproduction, most ascomycetes produce large numbers of asci, which are often held within a multicellular, sometimes visible fruiting structure called an ascocarp (or ascoma). Ascocarps are extremely diverse in shape, texture, and color. They can be cup-shaped, club-shaped, potato-like, spongy, seed-like, oozing pimple-like, coral-like, nit-like, golf-ball-shaped, perforated tennis ball-like, cushion-shaped, miniature plated and feathered (in Laboulbeniales), microscopic Greek shield-shaped, stalked, or sessile. They can grow solitary or clustered, and their texture ranges from fleshy, carbonaceous (charcoal-like), leathery, rubbery, gelatinous, slimy, powdery, to cob-web-like. Ascocarp colors include red, orange, yellow, brown, black, and more rarely green or blue. Some single-celled yeasts like Saccharomyces cerevisiae develop an ascus during sexual reproduction but do not form fruiting bodies. In lichenized ascomycete species, the fungal thallus determines the shape of the whole symbiotic lichen colony. Some dimorphic species, such as Candida albicans, can switch between growing as single cells and growing as filamentous multicellular hyphae. Other species are pleomorphic, with both asexual (anamorphic) and sexual (teleomorphic) growth forms. Except for lichens, the non-reproductive vegetative mycelium of most ascomycetes is usually not visible, because it grows embedded in substrates like soil, or on or inside living hosts; only the fruiting ascoma is typically visible when the fungus reproduces. Pigmentation such as melanin in hyphal walls, plus prolific surface growth, can produce visible mold colonies; for example, Cladosporium species form characteristic black spots on bathroom caulking and other moist surfaces. Sooty molds that grow on plants, especially in the tropics, are the thalli of many different ascomycete species. Large macroscopic structures can also form from masses of yeast cells, asci, or conidia. Vegetative hyphae of most ascomycetes are uninucleate, containing one nucleus per cell, but multinucleate cells are common, especially in the apical growing regions of hyphae. Both the cell wall and internal septa provide hyphae with stability and rigidity, and can prevent cytoplasm loss if the cell wall or membrane sustains local damage. Ascomycota are commonly called "spore shooters," named for the asci that produce their sexual spores. Asexual reproduction is the dominant form of propagation in Ascomycota, and allows these fungi to spread rapidly into new areas. Their asexual reproduction is structurally and functionally diverse. The most common form of asexual reproduction produces spores called conidia, though chlamydospores are also frequently produced, and yeasts reproduce asexually through budding. Conidia are mitospores (products of mitotic division), usually contain a single nucleus, are genetically identical to the parent mycelium, and form at the tips of specialized hyphae called conidiophores. Conidia can be dispersed by wind, water, or animals depending on the species. Sexual reproduction produces the defining ascus structure that distinguishes Ascomycota from other fungal phyla. An ascus is a tube-shaped meiosporangium that holds sexual spores called ascospores produced by meiosis. With a few exceptions such as Candida albicans, most ascomycetes are haploid, with one set of chromosomes per nucleus. The diploid phase formed during sexual reproduction is typically very short, and meiosis restores the haploid state. A review by Wallen and Perlin of the adaptive reason Ascomycota maintain sexual reproduction concluded that the most plausible benefit is DNA damage repair via recombination during meiosis; DNA damage is often caused by stresses such as nutrient limitation. Ascomycota play a central role in most land-based ecosystems. They are key decomposers that break down dead organic materials including dead leaves and animal remains, helping detritivores access nutrients. Along with other fungi, ascomycetes can break down large molecules such as cellulose and lignin, making them critical to global nutrient cycles including the carbon cycle. Ascomycete fruiting bodies also serve as food for a wide range of animals, from insects and gastropods to rodents and larger mammals like deer and wild boars. Many ascomycetes additionally form symbiotic relationships with plants and animals.
