Phylogeny

Fungi arose about 1 billion years ago along with plants (including green algae), animals plus choanoflagellates, red algae, and stramenopiles (heterokonts). Genetic comparisons indicate that the closest relatives to the fungi are the animals plus choanoflagellates. See also: Choanoflagellida; Fungal genetics; Fungal genomics

Most species of true fungi (often collectively referred to as the Eumycota or Eumycetes) belong to two crown groups, Ascomycota (sac-fungi) and Basidiomycota (mushrooms and relatives), which comprise the subkingdom Dikarya. Members of the Ascomycota (ascomycetes) are the most numerous fungi (75% of all described fungal species) and include lichen-forming symbionts. Traditionally, the group has been divided into unicellular yeasts and allies with naked asci, and hyphal forms with protected asci. However, gene sequences indicate that some traditional yeasts and allied forms diverged early, at about the time that ascomycetes were diverging from members of the Basidiomycota (basidiomycetes). See also: Fungal phylogenetic classification; Lichens

General characteristics

Organisms in the kingdom Fungi are mostly haploid, use chitin as a structural cell-wall polysaccharide, and synthesize lysine by the alpha-aminoadipic acid pathway. In addition, the fungal body is made of branching filaments (hyphae). The mycelium, generally the vegetative body of fungi, is extremely variable. Unicellular forms, which are 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, although 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

Asexual reproduction, that is, 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 (Fig. 2).

Fig. 2 Spore types (upper illustrations) and structures of parent mycelia (lower illustrations) in fungi.

Sporangiospores are borne in unicellular sacs termed sporangia (Fig. 3). Sporangia originate either by differentiation of vegetative cells or as more specialized, newly formed structures, frequently at the end of elongated stalks or sporangiophores. Sporangiospores are of two types: motile zoospores, equipped with flagella, and nonmotile aplanospores. The number of spores per sporangium varies from a few to thousands.

Fig. 3 Sporangia of saprophytic Rhizopus. (Credit: Richard Gross; copyright © McGraw-Hill Education)

Other important spore types are the conidia, oidia, and chlamydospores. Conidia resemble nonmotile sporangiospores in shape, size, and structure, but they are produced externally upon a conidiophore (which may be simple or quite elaborate). Oidia are small, thin-walled cells that usually have flat ends; they are produced by the autofragmentation of the vegetative hyphae. Chlamydospores are thick-walled, nondeciduous spores interposed along vegetative hyphae.

Numerous other spore forms represent variants of the aforementioned basic types. Various pigments may be deposited in spore walls and account for mold colors in most cases. Asexual spores typically germinate by germ tubes that develop directly into vegetative mycelia.

Sexual reproduction

Sexual reproduction occurs in a majority of fungal species of all classes. Juxtaposition and fusion of compatible sexual cells are achieved by four distinct sexual mechanisms (gametic copulation, gametangial copulation, gamete-gametangial copulation, and somatic copulation), involving various combinations of differentiated sexual cells (gametes), undifferentiated sexual cells (gametangia), and undifferentiated vegetative cells (Fig. 4). Gametic copulation is the fusion in pairs of differentiated, uninucleate sexual cells or gametes formed in specialized sporangialike gametangia [panel (a) of Fig. 4]. In isogamy, the two members of the fusion pair are alike; in anisogamy, they are morphologically different. Gametangial copulation is the direct fusion of gametangia without actual differentation of the gametes themselves [panel (b) of Fig. 4]. Gamete-gametangial copulation is the fusion of a differentiated gamete of one sex with a gametangium of the other sex [panel (c) of Fig. 4]; the differentiated gamete may be either female or male. Somatic copulation is the sexual fusion of undifferentiated vegetative cells [panel (d) of Fig. 4].

Fig. 4 Sexual mechanisms in fungi: (a) gametic copulation; (b) gametangial copulation; (c) gamete-gametangial copulation; and (d) somatic copulation.

Physiology

Fungi need to obtain organic substances (food) from their environment because fungi do not contain chlorophyll and are unable to manufacture their own food via photosynthetic activities. Fungi are able to digest food externally by releasing enzymes into their environment, and these enzymes have the capacity to break down large molecules into smaller ones. 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. See also: Metabolism

Fungi are multicellular organisms that obtain their food by absorption through cell walls. This characteristic distinguishes them from animals, which ingest their food, and from plants, which manufacture their food. Most fungi use nonliving plant material for food, but some use nonliving animal material and therefore are called saprophytic organisms. In nature, the decomposition of dead plant material is a significant function of fungi because the process releases nutrients back into the surrounding ecosystem, where the nutrients can be reused by other organisms, including humans.

Decomposition

Plant material contains high percentages of celluloses, lignins, and hemicelluloses, which must be broken down to release the carbon, hydrogen, and oxygen. Thus, the fungal decomposition of plant material is very important in nature. Fungi biodegrade the substrate, and as much as 70% of the dry organic matter is released as carbon dioxide and water through the process of respiration. Ecologically, fungi are highly involved in the carbon cycle of nature and influence the rate at which it proceeds. Fungi are the most effective decomposers of plant components, and evidence indicates that lignin decomposition is completed almost entirely by fungi. See also: Biodegradation; Cellulose; Lignin; Lignin-degrading fungi

Pathogens

Some fungi have the physiological capability to grow on living plants and may cause diseases (for example, wheat rust or corn smut) on economically important plants. Some fungi can grow on grains and may produce substances known as aflatoxins, which can be detrimental to animals or humans. Other species of fungi have the ability to grow on living animals, including cats, horses, and humans. For example, the disease known as ringworm may result, which is caused by an expanding circular growth of a fungus that has the physiological capability to use the components of skin or hair as food sources. See also: Aflatoxin; Medical mycology; Pathogen; Plant pathology

The most frequently encountered fungal disease in humans is candidiasis, which is caused by Candida albicans (a fungus that is normally found associated with humans). This organism does not ordinarily become a problem until the physiology of the body is changed or compromised in some way. See also: Candida; Fungal allergies; Fungal infections

Industrial applications

Certain species of fungi, including yeasts (Fig. 5), have been used by humans since early times in the preparation of various foods (for example, leavened bread, cheeses, and beverages). Additional by-products of fungal physiology are used in numerous industrial applications, including the manufacture of antibiotics, solvents, and pharmaceuticals. See also: Food fermentation; Fungal agribiotechnology; Fungal biotechnology; Industrial microbiology; Mushroom pharmacy; Mycorestoration

Fig. 5 Colorized scanning electron micrograph of brewer's yeast cells (Saccharomyces cerevisiae). This fungus consists of single vegetative cells, and small daughter cells can be seen budding off from the larger cells. Brewer's yeast is used in the fermentative production of beer, wine, and bread. (Credit: Steve Gschmeissner/Science Photo Library/Alamy Stock Photo)

Physiological associations

Certain species of fungi produce chemicals that permit them to gain entrance to internal plant locations, where they may grow and gain food from the host plant. Many of these fungi may become pathogenic and harm the host plant. In contrast, some 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 and survive under soil conditions that would be otherwise detrimental to the plant (for example, soil having an excess of acid or soil that lacks certain nutrients). See also: Ectomycorrhizal symbiosis; Mycorrhizae

John W. Taylor

Marvin C. Williams