Fungi

Fungi

Fungi (fŭnˈjī), kingdom of heterotrophic single-celled, multinucleated, or multicellular organisms, including yeasts, molds, and mushrooms. The organisms live as parasites, symbionts, or saprobes (see saprophyte). Previously classified in the plant kingdom, fungi are nonmotile, like plants, but lack the vascular tissues (phloem and xylem) that form the true roots, stems, and leaves of plants. Most coenocytic (multinucleated) or multicelluar fungi are composed of multiple filaments, called hyphae, grouped together into a discrete organism called a mycelium. The cell walls of most fungi are of chitin compounds instead of cellulose; a group fungi known as cryptomycota lack chitinous cell walls. In many ways fungi are more closely related to animals than to plants, and they have been thought to share a common protist ancestor with animals. A recent classification system suggested by nucleic acid (genetic material) comparisons places the fungi with the animals and the plants in an overarching taxonomic group called the eukarya.

Most fungi are capable of asexual and sexual reproduction. Asexual reproduction is by fragmentation or spore formation. Those that reproduce sexually produce gametes in specialized areas of the hyphae called gametangia. The gametes may be released to fuse into spores elsewhere, or the gametangia themselves may fuse. In some cases dikaryons [di = two, karyo = nucleus], which are found only among fungi, result when unspecialized hyphae fuse but their nuclei remain distinct for part of the life cycle.

Unlike algae or plants, fungi lack the chlorophyll necessary for photosynthesis and must therefore live as parasites or saprobes (see parasite). Typically they release digestive enzymes onto a food source, partially dissolving it to make the necessary organic or inorganic nutrients available. Some parasitic types obtain their food directly from the cells of a living food source. Some types of fungi are involved in symbiotic relationships, for example, lichens (a combination of a fungus, a green alga or a cyanobacterium, and sometimes a basidiomycete yeast) and the mycorrhizae (symbiosis between a fungus and the roots of a vascular plant).

Some fungi are pathogenic to humans and other animals. Such diseases are called mycoses or fungal infections. Some molds, in particular, release toxic chemicals called mycotoxins that can result in poisoning or death. Various fungi can also cause serious damage to fruit harvests and other crops (see diseases of plants).

Types of Fungi

The 100,000 identified species of organisms commonly classed together as fungi are customarily divided into four phyla, or divisions: Zygomycota, Ascomycota, Basidiomycota, and Deuteromycota.

Zygomycota includes black bread mold and molds, such as those of the genus Glomus, that form important symbiotic relationships with plants. Most are soil-living saprobes that feed on dead animal or plant remains. Some are parasitic of plants or insects. They reproduce sexually and form tough zygospores from the fusion of neighboring gametangia. There is no distinguishable male or female.

Ascomycota includes yeasts, the powdery mildews, the black and blue-green molds, edible types such as the morel and the truffle, and species that cause such diseases of plants as Dutch elm disease, chestnut blight, apple scab, and ergot. There are over 50,000 species, about 25,000 of which occur only in lichens. In ascomycetes, the hyphae are subdivided by porous walls through which the cytoplasm and the nuclei can pass. Their life cycle is a complex combination of sexual and asexual reproduction.

Basidiomycota includes the gill fungi (most mushrooms), the pore fungi (e.g., the bracket fungi, which grow shelflike on trees, and an edible type called tuckahoe), and the puffballs. It also includes the fungi that cause smut and rust in plants. Like ascomycetes, the hyphae are subdivided by porous walls. In basidiomycetes, two hyphae fuse to form a dikaryotic mycelium (a mycelium in which both nuclei remain distinct). These mycelia differentiate into reproductive structures called basidia that make up the basidiocarp (the body popularly known as the mushroom cap). The nuclei then fuse and undergo meiosis, creating spores with one nucleus each. When these spores germinate, they produce hyphae, and the process begins again.

Deuteromycota comprises a miscellaneous assortment of fungi that do not not fit neatly in other divisions; they have in common an apparent lack of sexual reproductive features. Also called Fungi Imperfecti, the group includes species that help create Roquefort and Camembert cheeses, that cause diseases of plants and of animals (e.g., athlete's foot and ringworm), and that produce penicillin. A number of the fungi classified as deuteromycetes have been found to be asexual stages of species in other groups, and some classification schemes consider the deuteromycetes a class under Ascomycota.

Usefulness of Fungi

Bibliography

See C. M. Christensen, The Molds and Man (3d. rev. ed. 1965); J. Webster, Introduction to Fungi (1980); B. Kendrick, The Fifth Kingdom (1985); A. Chandra, Elsevier's Dictionary of Edible Mushrooms (1989); C. T. Ingold and H. J. Hudson, The Biology of Fungi (6th ed. 1993); G. W. Hudler, Magical Mushrooms, Mischievous Molds (1998); P. Roberts and S. Evans, The Book of Fungi (2011).

Fungi

Nucleated, usually filamentous, sporebearing organisms devoid of chlorophyll; typically reproducing both sexually and asexually; living as parasites in plants, animals, or other fungi, or as saprobes on plant or animal remains, in aquatic, marine, terrestrial, or subaerial habitats. Yeasts, mildews, rusts, mushrooms, and truffles are examples of fungi.

Some fungal classifications were constructed to facilitate identification, whereas others emphasize phylogeny. The more widely used classifications reflect a series of compromises between identification and phylogeny, and tend to conserve the vocabulary and nomenclature familiar to broad groups of users. The following is a conventional classification, in which all organisms are treated as members of the kingdom Fungi:

• Division: Eumycota
• Subdivision: Mastigomycotina
• Class: Chytridiomycetes
• Class: Hyphochytriomycetes
• Oomycetes
• Subdivision: Zygomycotina
• Class: Zygomycetes
• Trichomycetes
• Subdivision: Ascomycotina
• Class: Hemiascomycetes
• Plectomycetes
• Pyrenomycetes
• Discomycetes
• Loculoascomycetes
• Subdivision: Basidiomycotina
• Class: Hymenomycetes
• Gasteromycetes
• Urediniomycetes
• Ustilaginomycetes
• Subdivision: Deuteromycotina
• Class: Blastomycetes
• Hyphomycetes
• Coelomycetes
• Agonomycetes
• Division: Myxomycota

Organisms in the kingdom Fungi are mostly haploid, use chitin as a structural cell-wall polysaccharide, and synthesize lysine by the alpha amino adipic acid pathway; and their body is made of branching filaments (hyphae). The fungi arose about 1 billion years ago along with plants (including green algae), animals plus choanoflagellates, red algae, and stramenopiles. Ribosomal comparison indicates that the closest relatives to the fungi are the animals plus choanoflagellates. See Choanoflagellida

Ascomycetes are the most numerous fungi (75% of all described species), and include lichen-forming symbionts. The group has traditionally been divided into unicellular yeasts and allies with naked asci, and hyphal forms with protected asci. However, ribosomal gene sequences indicate that some traditional yeasts and allied forms diverged early (early ascomycetes), at about the time ascomycetes were diverging from basidiomycetes. Hyphal ascomycetes protect their asci with a variety of fruiting bodies; the earliest fruiting bodies may have been open cups (Discomycetes), while in more recent groups they are flask shaped (Pyrenomycetes and Loculoascomycetes) or are completely closed (Plectomycetes). Ascomycetes lacking sexual structures have been classified in the Fungi Imperfecti, but molecular comparisons now allow their integration with the ascomycetes. See Ascomycota, Deuteromycotina

The mycelium, generally the vegetative body of fungi, is extremely variable. Unicellular forms, thought to be primitive or derived, grade into restricted mycelial forms; in most species, however, the mycelium is extensive and capable of indefinite growth. Some are typically perennial though most are ephemeral. The mycelium may be nonseptate, that is, coenocytic, with myriad scattered nuclei lying in a common cytoplasm, or septate, with each cell containing one to a very few nuclei or an indefinite number of nuclei. Septa may be either perforate or solid. Cell walls are composed largely of chitinlike materials except in one group of aquatic forms that have cellulose walls. Most mycelia are white, but a wide variety of pigments can be synthesized by specific forms and may be secreted into the medium or deposited in cell walls and protoplasm. Mycelial consistency varies from loose, soft wefts of hyphae to compact, hardened masses that resemble leather. Each cell is usually able to regenerate the entire mycelium, and vegetative propagation commonly results from mechanical fragmentation of the mycelium.

Asexual reproduction, propagation by specialized elements that originate without sexual fusion, occurs in most species and is extremely diverse. The most common and important means of asexual reproduction are unicellular or multicellular spores of various types that swim, fall, blow, or are forcibly discharged from the parent mycelium.

Sexual reproduction occurs in a majority of species of all classes. Juxtaposition and fusion of compatible sexual cells are achieved by four distinct sexual mechanisms, involving various combinations of differentiated sexual cells (gametes), undifferentiated sexual cells (gametangia), and undifferentiated vegetative cells.

Fungi obtain organic substances (food) from their environment which have been produced through the (photosynthetic) activities of green plants, since fungi do not contain chlorophyll and are unable to manufacture their own food. Fungi are able to digest food externally by releasing enzymes into their environment. These smaller molecules can be absorbed into the fungal body and transported to various locations where they can be used for energy or converted into different chemicals to make new cells or to serve other purposes. Some of the by-products of fungal metabolism may be useful to humans. Most fungi use nonliving plant material for food, but a few use nonliving animal material and therefore are called saprophytic organisms. In nature the decomposition of dead plant material is an important function of fungi, as the process releases nutrients back into the surrounding ecosystem where they can be reused by other organisms, including humans. See Biodegradation, Fungal ecology

A few fungi have the physiological capability to grow on living plants and may cause diseases such as wheat rust or corn smut on these economically important plants. Some fungi can grow on grains and may produce substances known as aflatoxins which can be detrimental to animals or humans. A few species of fungi have the ability to grow and acquire their food from skin or hair on living animals such as cats, horses, and humans. The disease known as ringworm may result. It is not caused by a worm but by an expanding circular growth of a fungus which has the physiological capability to use the components of skin or hair as the food source. The most frequently encountered fungal disease in humans is candidiasis, which is caused by one of the few fungi that is normally found associated with humans (Candida albicans). See Aflatoxin, Medical mycology, Plant pathology, Yeast infection

A number of fungal species are able to enter plant roots and develop an association that may be beneficial to the plant under natural field conditions. This association of a higher plant root and a fungus that does not produce a disease is called a mycorrhiza. This fungal association with the plant root may permit the plant to live under soil conditions where it may not otherwise survive because of an excess of acid in the soil or a lack or excess of certain nutrients. See Mycorrhizae

Certain species of fungi have been used by humans since early times in the preparation of foods such as leavened bread, cheeses, and beverages. Additional by-products of fungal physiology are used in industrial applications such as antibiotics, solvents, and pharmaceuticals. See Fungal biotechnology, Industrial microbiology, Yeast

Fungi

fungi