{fuhn'-jy} The fungi constitute a large and diverse group of organisms that share some characteristics with both lower plants (algae) and lower animals but are not closely related to either. They contain true mitochondria and membrane-enclosed nuclei, lack chlorophyll and chloroplasts, and reproduce by both asexual and sexual means. Most fungi grow as branched tubular systems, or mycelium, whose individual filaments or hyphae, are surrounded by rigid cell walls containing chitin, cellulose, or both, and other polysaccharides. All fungi are heterotrophic, that is, they lack photosynthetic ability and therefore require preformed organic compounds. They exist throughout the world. INDUSTRIAL USES The microfungi are most often used commercially because of their rapid growth. Brewer's yeast was used for brewing beer, fermenting grapes and other substances to produce wines, and starting mashes for distilled spirits long before the process of \Tfermentation\t was scientifically understood. Alcohol, the product of fermentation, also has chemical and medical uses. Baker's yeast is equally important in the baking industry. Camembert cheese derives its characteristic flavor from Penicillium camemberti and Roquefort from P. roqueforti. The production of a number of Asian foods also involves fermentation with molds; soy sauce, for example, is fermented with Aspergillus oryzae or A. soyae. \Tantibiotics\t were first produced using penicillin from P. notatum; the antibiotic activity of this fungus was described (1929) by the British scientist Alexander Fleming. Only through a joint effort of British and American scientists during World War II, however, was industrial-scale production achieved by using better-producing mutant strains of P. chrysogenum. A huge antibiotic industry has since developed. Only a few of the many antibiotics now available, however, are of fungal origin: the penicillins, the cephalosporins, and gresiofulvin, which is one of the few effective antifungal antibiotics. Various microfungi are used to produce a number of organic acids--gluconic, itaconic, and citric acids, for example--and in other chemical processes. Citric acid fermentation by Aspergillus niger yields about 99,000 tons each year. Fungi are also grown for the production of enzymes such as the acid proteases, which are used commercially for meat tenderizing and bread making. EVOLUTION OF FUNGI Prior to the development of the microscope in the 1600s, the only fungi described were the higher fungi that have large fruiting structures, such as \Lmorel\ls, \Tmushrooms\t, and \Lpuffball\ls. The first scientific description of fungi was given by Pier Antonio Micheli, an Italian botanist, in his work Nova Plantarum Genera (1729). In 1836 the study of fungi was termed \Tmycology\t. Because fungi and bacteria were for many years considered more similar to plants than to animals, mycology has traditionally been a branch of botany. Fossil records reveal that fungi occurred in the early periods; however, the record is sparse, and most phylogenetic speculations have been based on comparisons of living species. As a result, the evolutionary relationships are still not clear. It is generally accepted that fungi arose as more than one phylogenetic branch from flagellated protistan ancestors. Their evolution is most probably polyphyletic along at least three lines. Because of their unique characteristics a new kingdom, Fungi, was suggested for this group of organisms, which is considered equal to the plant and animal kingdoms. Groups of fungi are classified according to their method of sexual reproduction, appearance, and life cycle. CLASSIFICATION OF FUNGI Division Myxomycota The organisms in this division grow as multinucleate amoeboid plasmodia and produce motile uninucleate amoebae, as well as biflagellate cells. Although they are treated as fungi, their true evolutionary relations are still not known. Members of the class Plasmodiophoromycetes of this division parasitize the roots of plants, and some species may infect the hyphae of aquatic fungi. Plasmodiophora brassicae causes a disease of cabbages called clubroot. The class Myxomycetes comprises the true, free-living plasmodial \Lslime mold\ls, which range from microscopic species to those that produce very large and conspicuous plasmodia. Starvation induces the plasmodia to convert to sporangia, in which nonmotile spores are formed. They typically grow under moist conditions in or on decaying wood or other vegetation. Physarum polycephalum produces large, yellow plasmodia and has been used extensively for basic cell research. Division Eumycota The so-called true fungi are placed in this division, and all except the Oomycetes may be related as a single phyletic series. In the subdivision Mastigomycotina, which comprises two classes, the fungi produce motile spores, called zoospores, with one or two flagella. The class Chytridiomycetes (the chytrids, or water molds) includes three orders of fungi that produce asexual zoospores with a single posterior whiplash flagellum in a sporangium, or more specifically a zoosporangium. Most species are microscopic, and many grow as parasites within the cells of algae, other fungi, or higher plants. The cells are coenocytic (multinuclear) and enclosed in rigid walls containing chitin, except for some that grow as plasmodia within their host. The chytrids characteristically grow as sac-shaped cells with tapering, rootlike extensions, called rhizoids, that penetrate the substratum or its host. In asexual reproduction part or all of the cell body is converted into zoosporangia, or sporangia-producing zoospores. Sexual reproduction leads to a thick-walled, often dormant resting spore. The class Oomycetes, the second in the subdivision Mastigomycotina, is a group of fungi (water molds and fish molds) that typically occur in freshwater streams and ponds or as parasites of higher plants. They are economically significant in agriculture, because although many are saprobes, living off decayed matter, others cause damping-off or rotting of seedlings; downy mildews of many plants, such as potato blight; and fish diseases. Oomycetes reproduce asexually by motile biflagellate zoospores with one whiplash and one tinsel flagellum. Pathogenic species such as Phytophthora infestans (the cause of potato blight) and Plasmopara viticola (the cause of downy mildew of grape) produce zoospores on specialized branches (sporangiophores) of the mycelium. The zoosporangia break free and are carried by wind or water to new hosts, whereupon zoospores may be released to initiate new infections or, under dry conditions, the sporangia may directly produce a mycelium. The subdivision Zygomycotina has a single class, Zygomycetes. This class includes fungi (bread molds or pin molds) that typically produce an abundant and rapidly growing aerial, coenocytic mycelium and are common causes for the decay of foods and other rich sources of organic material. The cell walls contain chitin as a primary component. They reproduce asexually by means of nonmotile spores (sporangiospores) produced in sporangia formed on branches (sporangiophores) of the mycelium. In some species, such as Rhizopus nigricans, the sporangia arise in clusters with rhizoids at the base and hyphal strands (stolons) interconnecting the clusters. The spores are released by breakdown of the sporangial wall and dispersed by air or water currents. They "germinate" by direct outgrowth of a hyphal tube to produce a new mycelium. Sexual reproduction may occur between different parts of the same mycelium (homothallic mating) or between two self-sterile but cross-fertile strains of opposite mating type (heterothallic mating). The latter is regulated by a single pair of genes, or alleles. One gene is said to be of the plus mating type and the other of the minus mating type. The gamentangia, or sex organs, fuse to form a dormant, thick-walled, pigmented, and often sculptured zygote called the zygospore. The mature zygospores eventually germinate to produce a new haploid mycelium. The genetic regulation of sexual reproduction in fungi was first discovered (1904) in the Zygomycetes by Albert Francis Blakeslee, who coined the terms homothallism and heterothallism to describe the two types of mating strains. Both are now known to be common throughout the fungi. The subdivision Ascomycotina (formerly the class Ascomycetes) includes all true fungi in which sexual reproduction results in ascospores, produced within a specialized cell called an ascus. In many ascomycetes, male structures (antheridia) and female structures (ascogonia) are produced. The antheridia donate nuclei to the ascogonia by fusion with a receptive filament, the trichogyne. In others the same function may be accomplished by conidia (asexual spores that can also serve as fertilizing elements) or by hyphal fusion. The parental nuclei unite in the ascogonium and enter hyphal branches that grow out from it within a developing fruiting body, the ascocarp. The paired parental nuclei divide synchronously (conjugate division) in specialized hyphae with binucleate cells (ascogenous hyphae). The tip cells of the ascogneous hyphae form a hook in which the haploid parental nuclei fuse to produce a diploid zygote nucleus. The zygote nucleus immediately undergoes meiotic (reduction) divisions to produce four haploid nuclei in the enlarging cell, called the ascus at this stage of development. In most cases a mitotic nuclear division then doubles the number of nuclei per ascus, after which each nucleus is enclosed in a cell wall to form the ascospores. Other major features of the fungi of this subdivision are that the cell walls contain chitin; the hyphae have simple, washer-shaped septa with a central pore; and asexual reproduction occurs by formation of nonmotile spores (conidia, oidia, arthrospores, and others) that are usually produced on specialized branches called conidiophores. Several classes of ascomycetes exist. The class Hemiascomycetes includes the \Lyeast\ls; these may be unicellular or mycelial, but all lack ascogenous hyphae and fruits. Most yeasts are saprobic, commonly occurring on plant parts, in soil, and in other locations with adequate moisture and organic material. A small group is parasitic on the leaves, twigs, and branches of vascular plants, causing leaf curl and witches'-broom (tufts of branchlets resulting from repeated branching). It is unclear whether the yeasts are a primitive ascomycete type or whether they are derived from more complex forms. Another class of ascomycetes, Plectomycetes, includes several economically important fungi that form their asci in small, simple, closed, fruiting structures (cleistothecia). The powdery \Lmildew\ls--so named from the powdery appearance of infected leaves--are all obligate parasites of higher plants and are largely host specific. The fungus grows on the surface as a white, cottony mycelial mat and produces many simple conidiophores and ellipsoidal spores (conidia). The surface cells of the host are invaded by special extensions called haustoria. The conidia give rise to new sites of infections by germination on the surface and haustorium formation. The cleistothecia become brown or black at maturity, bear a number of characteristically shaped external appendages--hook-shaped or spearlike, for example--and overwinter on the fallen leaves. The ascospores are forcefully discharged in the spring and initiate new infections. Erisiphe graminis strains infect a number of grass species, including wheat and barley. A second major group of plectomycetes includes the commercially utilized genera Aspergillus and Penicillium, as well as important pathogens of plants and humans. Sexual reproduction is relatively rare among species of Aspergillus and Penicillium. Aspergillus produces chains of pigmented, asexual conidia on the surface of an inflated region of a branch, called a conidiophore. Conidium formation is similar in Penicillium, but the conidiophore is branched to form a brushlike structure (penicillus) instead of having an inflated vesicle. The conidia are connected in chains on the conidiophores but are readily dispersed by air currents. The green, black, yellow, and gray colors of the colonies of these common microfungi are the result of the color of the huge number of pigmented conidia produced on the surface. In addition to their roles in the decay of plant and animal residues and in food spoilage, these fungi are of great significance to humans in other ways. Aspergillus fumagatus, a common inhabitant of heated compost, can cause respiratory disease in humans, and a number of related species may produce aflatoxin, a tumor-inducing alkaloid, in poorly stored, moldy grain. Species of both Penicillium and Aspergillus are used extensively in commercial fermentations. This class also includes other species that cause disease in humans, animals, and plants; for example, the fungus Ceratocystis ulmi is responsible for \TDutch elm disease\t, other species cause a wilt disease in oaks, and still others reduce the quality of lumber. All fungi in the class Pyrenomycetes produce asci and ascospores as an organized hymenial layer in a fruiting body called a perithecium. The perithecium is a small, flask-shaped structure with a thin wall that surrounds a basal tuft of asci; the opening at the top is called an ostiole. The ascospores are typically discharged violently from the tips of the asci as they sequentially protrude through the ostiole. The perithecia may form as separate structures on the mycelium, or they may lie just below the surface of a larger mass of sterile hyphae called a stroma. This class includes a large number of fungi in several orders. Many are saprobes that grow on dung or cellulosic materials; others cause diseases of higher plants. Fungi of the genus Neurospora produce single, dark perithecia on the substratum and large numbers of salmon-colored, ellipsoidal conidia on the abundant aerial mycelium. These fungi are characterized by linearly arranged ascospores, minimal nutritional requirements, and a rapid growth rate--traits that make them ideal for laboratory experimentation. In the 1930s, George W. Beadle and Edward L. Tatum began experiments with N. crassa that led to the development of a new discipline, biochemical genetics. They demonstrated that each protein is a product of a single gene. Xylaria species produce upright stromatic fruits 4-8 cm (1.5-3 in) tall with a dark, hard outer rind. Just below the surface large numbers of perithecia are formed, their ostioles opening to the outside. Conidia are formed on the surface of the stroma before the perithecia mature and produce ascospores. The dark ascospores are discharged in such large numbers that the area around the fruits may appear black. Other genera produce dark, cushion-shaped fruits. Most species are saprobes on dead stumps, logs, and woody branches; a few are parasitic on living trees, for example, Daldinia concentrica on ash. Claviceps purpurea, which causes the disease \Tergot\t of rye, produces perithecia in the inflated end of small, purple, nonwoody stromata in the spring when susceptible grasses, such as rye, are in flower. The needlelike ascospores are discharged and, if they fall on a flower, produce a mycelium that grows and invades the ovary tissue, producing large numbers of small conidiophores on the surface. The conidia are exuded in droplets of sweet material that attract insects, who carry the conidia to other flowers and start new infections. Eventually the entire ovary is replaced by tightly packed mycelium enclosed in a hard, lavender rind. This dormant stage is called a sclerotium and may be harvested along with good seed in rye grown for consumption. The sclerotia overwinter on the straw or on the ground, and in the following season they produce new stromata and infect the new crop of rye or other grass. The \Lcup fung\li, earthtongues, \Lmorel\ls, and \Ltruffle\ls are ascomycetes in the class Discomycetes. The fertile hymenial layer of asci is exposed at maturity in all but the truffles, which are produced below ground. This layer lines various surfaces of the fruiting bodies, or apothecia; it occurs on the inner surface of cup fungi, within channels in the closed fruits of the truffle, on the outer surface in earthtongues, and on the surfaces lining the pits on the caps of morels. Parasitic species such as Monilinia fructicola, the cause of brown rot in peach, may produce numerous conidia, but many saprobic species do not. The mycelium of M. fructicola infects the leaves, twigs, and blossoms of the peach tree, causing twig and blossom blights. The morel, Morchella esculenta, produces rather large, tan brown, stalked fruits with a roughly conical cap that is lined with shallow pits separated by ridges. The fungi of this species grow in woodlands and fruit in the spring. They are considered excellent for eating, as are other, related species, but the false morel, Gyromitra, is poisonous. Some types that have cup-shaped fruits grow in similar habitats; others are commonly found growing on the dung of various herbivorous animals. Truffles, genus Tuber, are a popular delicacy in Europe. These fungi form dark, warty, potatolike fruits below the surface of the ground and are found in association with the roots of oak and beech trees. Several species have been found on the United States Pacific coast, but none are considered edible. The ascomycetes of the class Loculoascomycetes include those fungi whose asci lie within a cavity (locule) in a tightly knit mass of hyphae called a stroma. These differ from stromatic species in the class Pyrenomycetes in that the fertile cavities do not have their own distinct wall layers. Ascostromatic fungi also produce asci with a double wall. The group includes a number of fungi that are saprobes or pathogenic on plants. The fruits may be very small and may contain either a single locule or a number of locules with individually separated asci, some of which parasitize insects. The ascospores are discharged through a pore formed by lysis of stromatic cells between the cavity and the surface of the stroma. Venturia inaequalis, of this class, causes apple scab disease in various Malus species, such as the apple, crab apple, and hawthorn. It infects the leaves, twigs, and fruits. The true fungi also include the subdivision Basidiomycotina, the members of which produce haploid sexual spores (basidiospores) on a specialized cell called the basidium. Another important characteristic of the basidiomycetes is that they all produce a "primary" haploid mycelium (the monokaryon); also, as a result of crossing, a "secondary" mycelium (the dikaryon) results, which contains pairs of parental nuclei that replicate by conjugate division. The secondary hyphae usually bear tubular "clamp connections" around each cross-wall that separates the binucleate cells. This group includes the rust and smut fungi, as well as mushrooms, puffballs, and related forms. The rusts and smuts in the class Hemibasidiomycetes of basidiomycetes all produce basidia that are divided, or septate. The jelly fungi have rudimentary gelatinous fruiting bodies and are economically unimportant, mainly growing on decaying wood. Both the rusts and smuts produce a thick-walled spore (teliospore, or teleutospore) on the secondary mycelium, and it is this cell that produces the basidium and basidiospores. Both groups are parasites of many higher plants and do not produce fruiting bodies; they instead develop the teliospores in or on the tissues of the host plant. The rusts are obligate parasites of plants and may have very complex life cycles, occurring in up to five different stages on two unrelated plant species. Puccinia graminis, the cause of wheat rust, is one such long-cycled, heteroecious (two-host) fungus. Other species may lack some stages and may parasitize only a single host (autoecious). Smut fungi have simpler life cycles and can often be grown readily in culture, although they may not complete their life cycle in artificial media. The teliospores produce basidia that are either septate and form basidiospores as buds from each cell, or lack septa and develop several spores at the tip. The basidiospores of Ustilago maydis, the cause of corn smut, detach from the fruiting body and can be grown as a budding, yeastlike phase in culture. The subdivision Basidiomycotina also includes the classes Hymenomycetes and Gasteromycetes, which comprise the mushrooms, toadstools, puffballs, and related species. They are the most advanced groups of fungi and produce the largest fruiting bodies. In the Hymenomycetes the basidia occur in a hymenium that lines the surface of gills, pores, or spines and is exposed before the basidiospores are produced. In the Gasteromycetes (puffballs) there may or may not be a true hymenial layer, and the basidiocarps either remain closed or else open after the basidiospores have been produced. Both groups lack specialized structures for sexual mating, but they have complex genetic systems that regulate sexual compatibility by means of hyphal fusion between monokaryons. Many species are edible, but others, particularly the death angels, are deadly poisonous. The subdivision Deuteromycotina includes all fungi that lack known sexual reproductive structures and thus cannot be otherwise classified. Many soil fungi, plant pathogens, and industrially useful species are included in this group. James S. Lovett Bibliography: Ainsworth, G. C., Introduction to the History of Mycology (1976); Alexopoulas, C. J., Introductory Mycology, 3d ed. (1979); Emmons, C. W., Binford, J. P., Utz, J. P., and Kwon-Chung, Medical Mycology (1977); Garraway, M.O., and Evans, R.C., Fungal Nutrition and Physiology (1984); Gray, W. D., The Relation of Fungi to Human Affairs (1959); Gray, W. D., and Alexopoulos, C. J., Biology of the Myxomycetes (1968); Hawksworth, D.L., et al., Ainsworth and Bisby's Dictionary of the Fungi, 7th ed. (1983); Moore-Landecker, E., Fundamentals of the Fungi, 3d ed. (1990); Webster, J., Introduction to Fungi, 2d ed. (1980). See also: \Tparasitic diseases\t.