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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
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).
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
- Subdivision: Zygomycotina
- Class: Zygomycetes
- Subdivision: Ascomycotina
- Class: Hemiascomycetes
- Subdivision: Basidiomycotina
- Class: Hymenomycetes
- Subdivision: Deuteromycotina
- Class: Blastomycetes
- 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
a group (division) of lower plants that lack chlorophyll.
Fungi need readily available organic substances in order to grow (that is, they are heterotrophic); they usually grow on plants, animals, or decayed matter. Depending on their nutrient substrate (living or nonliving), the fungi are divided into parasites and saprophytes; including forms that are edible or toxic for man and animals. There are more than 100,000 species of fungus. The science that studies them is called mycology.
Structure. In the majority of fungi, excluding some forms of intracellular parasites, the vegetative body is the mycelium, which consists of fine branching threads, or hyphae, which grow from their ends and are distributed over the surface of the nutrient substrate or penetrate into it. The developing mycelium often looks life a soft, downy, or gossamer bloom, thin films, or cottony clusters; this may vary, depending on growth conditions. Thus, for example, rather thick, dense mycelial films grow in the cracks of wood attacked by fungi of the Polyporaceae family. The variations of mycelium include mycelial threads (lacelike formations consisting of parallel and often longitudinally fused hyphae, such as in the house fungus), rhizomorphs, and sclerotia.
Reproduction. Fungi perpetuate themselves vegetatively and reproductively (asexually and sexually). The stage of sexual reproduction is called perfect; that of asexual reproduction is called imperfect. Vegetative reproduction is accomplished by fragments or particles of the mycelium, threads, or rhizomorphs, as well as by the sclerotia. These particles migrate and, arriving at a favorable environment, are the start of new mycelial growth. A more specialized method of this kind of reproduction is the separation of the mycelium into individual cells (spores)—oidia, gemmae, and chlamydospores. Oidia are round or elongated cells covered with a thin membrane and are unable to sustain themselves for long periods (they are found mainly in fungi of the Exoa-scales order); gemmae have a thicker, colored membrane, as a rule, and last a long time (they are found in Ascomycetes, Phycomycetes, Hyphomycetes, and some Ustilaginales, particularly in the fungi that cause oat smut); chlamydospores arise from the isolation and thickening of individual sections of the hyphae, which are covered with a thick, darkly colored membrane (they are characteristic of many smuts of the Ustilaginales order; they are also found in Ascomycetes and fusaria). One variety of vegetative reproduction is budding, which is characteristic of the yeasts.
Asexual reproduction is accomplished by spores, which form asexually on specially isolated sporiferous branches of the mycelium. Spores are either of endogenous or exogenous origin. Endogenous spores usually develop in large numbers within the globular, turgid ends at the hyphae called sporangia (bearing nonmotile spores) or zoosporangia (bearing motile zoospores with one or two flagella). Sporangia and zoosporangia are characteristic of Phycomycetes. Exogenous spores (conidia) develop singly, in groups, and frequently in chains, on specialized sporiferous branches of the mycelium called conidiophores. Conidial sporophores are characteristic of higher fungi, although they are also found among Phycomycetes. In some fungi the conidiophores are gathered into fascicles called coremia, or they grow in special conceptacles called the cupule or pycnidium. Coremia are found in some fungi of the order Hyphomycetales, cupules in fungi of the order Melanconiales, and pycnidia in fungi of the order Pycnidiales. Class Phycomycetes is characterized primarily by a rudimentary, or noncellular (lacking septa), mycelium; it reproduces asexually by means of sporangio-spores or zoospores. In the most highly organized representatives of the Phycomycetes (for example, in order Perono-sporales), the sporangia function like individual conidia. In the higher fungi (classes Ascomycetes, Basidiomycetes, and Deuteromycetes), which are characterized by a septate mycelium, true conidia develop during asexual reproduction.
The sexual reproduction of fungi is accomplished by spores that arise after the fusion of two nonidentical germ cells. In the very lowest fungi (subclass Archimycetidae) the germ cells, or planogametes, are identical and resemble the zoospores of asexual reproduction in external appearance. As a result of their fusion a diploid motile planozygote is formed, which embeds itself in the tissue of the host plant, loses its flagella, and develops a dense membrane, turning into a sporocyst. After a dormant period the cyst germinates into a sporangium, while the nucleus undergoes repeated division; the first of these divisions is meiotic. Zoospores are also formed, corresponding to the number of haploid nuclei formed. Among more highly organized Phycomycetes (for example, in subclass Zygomycetidae and order Mucorales), two sexually different hyphae form morphologically similar, contiguous processes; each breaks off a multinuclear cell, which then fuse to become a diploid zygospore, since the fusion of cell nuclei occurs in pairs. Such a sexual process is called zygogenesis. After a resting phase the zygospore germinates, forming a thread ending in an embryonic sporangium, in which, as a result of meiosis and subsequent ordinary nuclear divisions, there arises a large number of sexually differentiated haploid spores. Among other Phycomycetes, as in subclass Oomycetidae, there are morphologically different sexual organs. The female organs are large globose oogonia, which contain from one to several oocytes, and the male organs are elongated outgrowths (antheridia), which are not differentiated into gametes; fertilization of the ovum occurs through fusion with part of the contents of the an-theridium, which has a single nucleus (order Saprolegniales). The sexual process in Phycomycetes can also occur by the fusion of two motile gametes (order Blastocladiales), or of a motile gamete-antherozoid with a nonmotile ovum (order Monoblepharidales). The common trait of Phycomycetes is the predominance in their developmental cycle of a haploid phase (vegetative mycelium and reproductive organs); the diploid phase is represented only by the zygospore or oospore—that is, by the dormant stage of the fungus. In higher fungi the sexual process is accomplished by the formation of sacs or basidia (classes Ascomycetes and Basidiomycetes). A spore does not form immediately upon the fusion of two cells; a new mycelium (a dikaryon with paired approximate nuclei), fundamentally different from the vegetative mycelium, usually develops first. Among Ascomycetes a diploid nucleus forms in the developing sac; in Basidiomycetes it forms in the basidium.
The majority of higher fungi, like the sac or basidial (sexual) fungi, have various conidial (asexual) sporophores.
Geographic distribution. Fungi are found everywhere. Many are cosmopolitan; however, the species composition of fungal associations differs for individual floristic regions. There are numerous known cases of the contagion of pathogenic species of fungus from one continent to another. For example, Plasmopara viticola (which causes grape mildew) and Phytophtora infestans (which causes late blight of potatoes) were brought to Europe from America; and Cronartium ribicola (which causes rust in pine, cedar, and currant) was transmitted to America from Europe.
Life-span. A significant number of fungi are not long-lived in nature. The mycelium develops in several days and sporulation begins, after which growth ceases and the mycelium dies. There are fungi with perennial mycelia. These are the pathogenic and parasitic fungi from the groups Polyporaceae and Uredinales, as well as the saprophytes (specifically, many Hymenomycetes). Apart from perennial mycelia, fungi are preserved by the sclerotia and various resting spores. In dry conditions, many spores can retain their viability for decades.
Basic physiological characteristics. In order to develop, fungi must have free oxygen; they are all aerobic, although some (particularly yeasts) can survive with only a small amount of oxygen. Many fungi cause various types of fermentation, including alcohol and citric acid fermentation. Most fungi grow optimally at temperatures of 20°-25° C, and some can sustain growth between 2° C and 40° C. Most fungi prefer acid substrates, although some grow well in neutral or slightly basic substrates. Light is not an essential factor for the growth of the mycelium, and direct solar rays, as a rule, have an adverse effect upon growth and sporulation.
Role in nature and importance to man. Fungi living in the soil decompose and mineralize plant remains (in particular, cellulose and lignin, which are difficult to decompose). Polyporaceae are of major importance in the decomposition of lignified parts. Many fungi cause various fungal diseases in plants. Mycotoxicoses are known to be caused in man and animals by the ergot of bread and fodder grains (Claviceps purpurea), as well as by “temulent corn,” prepared from grains affected with fungi of the genus Fusarium. A number of diseases in man, including impetigo, ringworm (trichophytosis), candidiasis, histoplasmosis, and micro-sporosis, are caused by fungi. A large number of fungi are useful. Molds of the genera Penicillium and Aspergillus yield vitamins, antibiotics, citric acid, and steroids. Yeasts are used in making wine, bread, and beer. Fungi yield various enzymes used in the leather, textile, and other industries. In many countries fungi are used as food; there are more than 100 edible species, manv of which are nutritionally rather valuable, since they contain proteins, vitamins, and enzymes. Fungi spoil quickly, and for this reason they are preserved in dried, salted, or marinated form. The edible fungi include Boletus edulis, Russulafoetens. Lactarius piperatus, Boletus luridus, Cantharellus cibarius, Boletus scaber, Boletus rufus, and Lactarius deliciosus. Species of the inedible group are not used as food; these fungi include poisonous species that cause serious, and sometimes fatal, poisoning. The most poisonous mushroom growing in the USSR is the death cup (Amanita phalloides).
Origin. It is widely held that fungi are descended from algae, with which they have much in common. However, a number of botanists believe that algae and fungi merely had common ancestors from the group Flagellata. There is also divergent thought about whether fungi are monophyletic (having one common ancestor) or polyphyletic (derived from different groups).
Classification. There are four classes of fungi.
Class I—Phycomycetes: The mycelium is noncellular or has a small number of septa; the most primitive in the form of a naked lump of protoplasm (amoeboid) or a single cell with rhizoids.
Class II—Ascomycetes: The mycleium is multicellular and haploid; with conidial sporophores. There is characteristic formation of sacs with ascospores, the basic organs of reproduction.
Class III—Basidiomycetes: The mycelium is multicellular and, as a rule, dikaryotic; conidial (haploid) stages are rare. There is characteristic formation of basidia, which bear basidiospores on sterigmata. This class includes the overwhelming majority of fungi used by man as food, as well as poisonous forms and many parasites of grains and other cultivated and wild plants.
Class IV—Deuteromycetes, or Fungi Imperfecti: The mycelium is multicellular, and reproduction is asexual (conidial) only; neither sacs nor basidia are formed. One of the orders of this group of fungi (Mycelia Sterilia) is characterized by the complete absence of any type of spores, including conidia.
Until recently Archimycetidae, one of the subclasses of class I, was considered an independent class (Archimycetes).
REFERENCESIachevskii, A. A. Osnovy mikologii. Moscow-Leningrad, 1933.
Kursanov, L. I. Mikologiia, 2nd ed. Moscow, 1940.
Kursanov, L. I., and N. A. Komarnitskii. Kurs nizshikh rastenii, 3rd ed. Moscow, 1945.
Bessey, E. A. Morphology and Taxonomy of Fungi. Philadelphia-Toronto, 1950.
Cejp, K. Houby, vols. 1–2. Prague, 1957–58.
M. A. LITVINOV