Fungi

• Categories: Fungi; taxonomic groups

Although some species of fungi are unicellular, most fungi are characterized by a mycelial growth form; that is, they generally are made up of a mass of hyphae (tubular filaments). All fungi live in their food and have an absorptive mode of nutrition by which they secrete digestive enzymes and absorb the breakdown products. They are therefore heterotrophs. Fungi are also characterized by possession of cell walls made of chitin, synthesis of the amino acid lysine via the amino adipic acid (AAA) pathway, and possession of a ribosomal DNA sequence that classifies them more closely with other fungi than with any other group of organisms. Fungi produce spores by either asexual or sexual means. The way they produce their spores constitutes one of the main taxonomic criteria for classifying them within the kingdom Fungi.

Ecology and Habitats

Because of their absorptive, heterotrophic mode of acquiring nutrients, fungi are important members of the decomposer community in ecosystems. They are saprobes that break down dead organic matter and simultaneously release nutrients that are taken up by other members of their ecosystem. Fungi exist in terrestrial and aquatic habitats. Because the saprotrophic fungi grow in a radial fashion from a point of origin, the mushroom types of fungi sometimes form so-called fairy rings, in which mushroom fruiting bodies appear in the shape of an irregular ring at the advancing edge of the mycelia. One mass of Armillaria gallica mycelia, for example, became so large that it occupied 37 acres (15 hectares, or 0.05 square miles) in the Upper Peninsula of Michigan and became known as the Humongous Fungus. More recently, another mass of mycelia, this one of the species Armillaria ostoyae, was also dubbed the Humongous Fungus when it was discovered to cover an area of approximately 3.7 square miles (9.6 square kilometers) in Malheur National Forest in Oregon. The Oregon Humongous Fungus is estimated to be between 1,900 and 8,650 years old and is the largest known living organism on Earth.

Fungi are major parasites, living on live plants, animals, and other fungi. Fungal plant pathogens have evolved a variety of mechanisms to enable the fungus to penetrate the host plant and overcome the host’s defenses. Next, the pathogen absorbs food from the host by establishing structures called haustoria (singular form: haustorium), which form a highly specialized absorbing system. These structures do not actually penetrate the host plasma membrane, but rather reside within pockets of the host cell, where they secrete extrahyphal enzymes and absorb the soluble result. Consequently, the host plant develops a series of symptoms characteristic of the infection. Farmers can suffer severe loss of crops if an infestation is left unchecked. Plant breeders attempt to breed disease resistance into crop plants as a means of combating fungal diseases.

In addition to plant fungal pathogens, some fungi grow within plants and do not cause disease symptoms. These fungi are called endophytes. They appear to protect host plants from herbivores and from certain pathogenic microbes.

Interactive Associations

Mycorrhizae are mutualistic associations between fungi and vascular plants whereby both members of the association benefit. A fungus that grows on plant roots facilitates nutrient and water uptake for the plant, while the plant provides organic nutrients to the fungus. About 90 percent of higher plants have mycorrhizal associates. The fungus will form one of two types of associations with the plant. Endotrophic mycorrhizae penetrate cortical root cells with specialized hyphae that are finely branched. Other fungi grow around the root and between the cortical cells but never penetrate the cells. More than 80 percent of fungal genera that form mychorrizae are in the phylum Basidiomycota.

Lichens are an association that appears to be a controlled parasitism of an alga partner by a fungal partner. Ninety percent of lichens have species from one of three genera as the photosynthetic partner: Trebouxia, Trentepohlia, and Nostoc. The first two are genera of Chlorophyta (green algae), and the latter is a genus of cyanobacteria (a type of bacteria that undergoes oxygen-producing photosynthesis, formerly and erroneously called blue-green algae). Of all fungal partners, 98 percent are members of the phylum Ascomycota. In this controlled parasitism, the fungus obtains minerals and water and develops a physical structure to house the algal or bacterial partner, which in turn provides photosynthetic products to the fungal partner. These organisms can live in extreme environments such as deserts or Arctic regions. Because they have no way to eliminate toxic materials, they are extremely sensitive to air pollution. Indeed, the condition of lichens is sometimes used as an indicator of air pollution.

Some fungi have trapping mechanisms that allow them to prey on invertebrate organisms such as nematodes, rotifers, and copepods. These trapping mechanisms involve networks of adhesive hyphae, adhesive branches, adhesive nets, adhesive knobs, nonconstricting rings, and constricting rings.

Chytrids

The type of sexually produced spore determines the taxonomy of fungal phyla. Chytridiomycota, or chytrids, reproduce sexually by a variety of means, including isogamy, anisogamy, oogamy, contact between gametangia (structures that contain the gametes), and contact between vegetative structures. However, the chytrids are better defined by their asexually produced, motile, uniflagellated zoospores; in fact, the chytrids are the only phylum of fungi that produce such spores. There are more than 700 species in this phylum.

Zygomycota

Zygomycota, or zygomycetes, can produce spores either sexually or asexually. These fungi have a mycelial growth form that forms crosswalls only at reproductive structures. Sexual reproduction leads to the production of a zygospore within a zygosporangium.

Mycelial cultures of a single strain may mate (in homothallic species) or may require a different mycelial strain (in heterothallic species) in order to mate. In both cases, copulation of two multinucleated gametangia occurs. Two lateral branches grow toward each other, sometimes attracted by the hormone trisporic acid. When these branches touch, a wall is laid down near the tip of each branch, creating the multinucleated gametangia. The wall separating the two gametangia at the point of contact breaks down, the protoplasmic contents mix, and the nuclei pair up and fuse. As the nuclei pair, a wall surrounding the fused gametangia develops into the zygosporangium. This thick-walled structure is resistant to environmental abuse. At or just prior to germination, the nuclei within the zygospore undergo meiosis. A stalk grows out of the zygospore bearing a sporangium. Among this phylum of about 1,065 species are Rhizopus, which includes Rhizopus stolonifer, the black bread mold; Pilobolus, which includes Pilobolus crystallinus, the “hat thrower” fungus; and Entomophthora, which includes Entomophthora muscae, the fly fungus.

Ascomycota

Ascomycota, or ascomycetes, reproduce sexually by producing ascospores within an ascus. Ascomycetes have a mycelial growth form with incomplete crosswalls. The septal hole permits the migration of cellular contents from cell to cell. Yeasts are single-celled ascomycetes that normally do not produce mycelial forms. Asexually, the Ascomycota reproduce by budding, fission, or production of chlamydospores or conidiospores. The unicellular yeasts reproduce using fission and budding methods. The majority of ascomycetes reproduce asexually, using conidiospores. A cell that is going to generate a conidium grows directly from hyphae or at the end of a stalk. The conidiogenous cell produces a swelling at the tip, into which protoplasm and a nucleus migrate. The nucleus is produced by mitotic means and is haploid. After swelling is complete and migration finishes, a wall is formed, separating the conidiospore from the conidiogenous cell.

Sexual reproduction occurs by gametangial contact between the female structure (the ascogonium) and the male structure (the antheridium). Usually, opposite mating types are required for sexual reproduction to occur. When contact between the gametangia occurs, nuclei from the antheridium migrate into the ascogonium and pair up with nuclei in the ascogonium. Alternatively, conidia or microconidia may land on the ascogonium and fuse with it. In both cases, fusion of paired nuclei is delayed until development of the ascus. Instead, special hyphae grow out from the ascogonium and will eventually generate the ascus. These ascogenous hyphae possess two nuclei, one of each mating type, in each cell.

Frequently, the development of the ascogenous hyphae and ascus is associated with the development of a fruiting body. When the fruiting body is at the appropriate developmental stage, a hooklike structure develops at the end of the ascogenous hyphae. At this time the nuclei fuse, producing the only diploid cell in the life cycle of these organisms. As the saclike ascus elongates, the diploid nucleus undergoes meiosis, producing four haploid nuclei. One mitotic division then usually occurs, resulting in eight nuclei in the elongated sac. Eventually, walls are formed around each nucleus, creating the ascospore. As noted above, many Ascomycota—but not the yeasts—produce a fruiting body, called the ascoma. The ascoma may take a variety of shapes, from a closed, ball-like structure (cleistothecium) to a pear-shaped structure with an opening at the top (perithecium) to a cup-shaped open structure (apothecium) or a stromatic structure containing cavities (ascostroma). In all cases, the asci with the ascospores are contained within these structures. The type of ascoma, the number of walls in the ascus, and the presence or absence of a fertile layer from which the asci arise are criteria used to distinguish among the more than 64,000 species of Ascomycota.

Basidiomycota

Basidiomycota, or basidiomycetes, reproduce sexually by producing basidiospores on a basidium. Like the Ascomycota, the Basidiomycota fungi also have a mycelial growth form with incomplete crosswalls. However, the structure of these crosswalls is different in that there is a swelling surrounding the pore where the wall is incomplete. There is also a curved membrane on each side of the hole, together looking much like parentheses. This septal structure is called the dolipore septum and is a secondary characteristic of the Basidiomycota.

The mycelia of most basidiomycetes pass through three stages. The primary mycelium has cells with a single nucleus, all derived from the germinated basidiospore; it is said to be homokaryotic. Later in development, fusion of hyphae of opposite mating types occurs, establishing the secondary mycelium, in which each cell has two nuclei; the secondary mycelium is therefore dikaryotic. Clamp connections occur on the secondary mycelium to facilitate division of the two nuclei in limited space. Clamp connections are another secondary characteristic of basidiomycetes. Tertiary mycelium develops in the specialized organized tissues of the fruiting bodies, the basidioma.

Asexual reproduction takes place by means of budding, fragmentation of mycelium, production of chlamydospores, or conidia. Chlamydospores are fragmented sections of mycelia that have rounded up and formed thick walls. Sexual reproduction occurs primarily by fusion of genetically compatible hyphae, thus establishing the secondary mycelium. Fusion of these nuclei is delayed until the production of the basidium. Nuclei of the dikaryotic cell that is to become the basidium fuse to produce one diploid nucleus that immediately undergoes meiosis, resulting in four haploid nuclei. Depending on the type of basidiomycete fungus, the haploid nucleus and associated cytoplasm migrate into a swelling that develops at the tip of the basidium. When full-sized, a wall separates the basidiospore from the basidium. Eventually, the basidiospore falls off or is shot off the basidium. Classification of the approximately 31,500 species of Basidiomycota depends on the presence or absence of a fruiting body, septation, and the number of cells in the basidium.

Deuteromycota

Deuteromycota, or deuteromycetes, are fungi that reproduce only asexually, by production of conidiospores. The majority of these fungi are thought to be derived from the Ascomycota by evolutionary loss of sexual stages; either that, or the sexual stages have yet to be discovered. Deuteromycetes are also called anamorphic fungi because the sexual stages (also called the “perfect” stages) have been lost, “imperfect fungi,” or “fungi imperfecti.” About fifteen thousand form-species are grouped into larger form-genera and form-classes, based on the morphological characteristics of their asexual reproductive structures. Because this is an artificial classification (not based on evolutionary relationships), no basis for conclusions about relatedness within groups can be implied or inferred.

Evolutionary History

Because of a lack of sufficient fossil evidence, phylogenetic relationships have been inferred based on morphological features associated with cell structure and sexually produced structures. With the advent of sequencing analysis of proteins and nucleic acids, observations of some relationships have been confirmed, and new relationships are being discovered.

Small subunit rDNA sequence analysis shows that the fungi are derived from a flagellated animal ancestor. These data show fungi to be a monophyletic group, with the Chytridiomycota and Zygomycota as the earliest branches within the group. The facts that all fungi utilize the AAA synthetic pathway for lysine and possess cell walls of chitin support this monophyletic view of all fungi. Sequence analysis supports a relationship between the Ascomycota and Basidiomycota, perhaps both being derived from yeasts. The evidence for the monophyletic evolution of the Basidiomycota is strong, based on sequence analysis and morphological features such as ballistospores, basidia, and clamp connections. Sequence analysis, however, appears to contradict morphologically based phylogeny groupings, which use structure and number of cells in the basidium and presence or absence of a fruiting body as key features. Within the Ascomycota, phylogeny appears to be monophyletic, but the evidence is not strong.

GroupSpeciesCharacteristicsAscomycota32,000+Members of this phylum produce sexual spores in a specialized cell called an ascus. Includes Neospora (powder mildew), Morchella (morels), and Tuber (truffles). Causes mildews, fruit rots, , .Basidiomycota22,000+Members of this phylum produce sexual spores on a specialized cell called a basidium. Includes three classes: (mushrooms, stinkhorns, puffballs, bird's nest fungi), Teliomycetes (rusts), and (smuts). Causes rusts and smuts; includes poisonous varieties of mushrooms such as Amanita.Chytridiomycota800+Members of this phylum are mainly aquatic organisms with motile spores with a whiplike flagellum. Includes gut chytrids, which live in the rumina of herbivores. Includes Allomyces, Batrachochytrium (parasite of frogs), and Coelomomyces (parasite of various insect hosts). Causes spots, crown wart in alfalfa, black wart in potatoes.Zygomycota750+Members of this phylum are recognized by their rapidly growing hyphae. Includes bread molds and fruit rots (Rhizopus stolonifer); Glomus versiforme is often involved in mycorrhizae ("fungus roots").15,000+Also called "fungi imperfecti," an artificial group based on the members' purely asexual form of reproduction. Includes fungi in the genus Penicillium, involved in cheese making and in the antibiotic penicillin. Also cause rots, molds, diseases of grains, vegetables, and fruits. Aspergillus flavus produces the carcinogenic toxin found on peanuts, aflatoxin.Lichens17,000+Symbiotic relationships of fungal (mycobiont) and algal (photobiont) partners, familiar on substrates such as rocks, wood, tree trunks. Most of the mycobionts are members of Ascomycota or Basidiomycota. Some species live more than a century. Important as environmental indicators, food for animals, sources of dyes, potential medicinal properties.Yeasts600+Not a formal taxon but an artificial group of unicellular fungi that grow by budding. Yeasts in genus Saccharomyces are used in food manufacturing (bread dough, alcohol fermentation); also cause disease, such as fungal infections (Candida albicans) and pneumonia (Pneumocystis carini). Most are members of Ascomycota; a quarter are from Basidiomycota.

Bibliography

Alexopoulos, Constantine J., et al. Introductory Mycology. 4th ed., John Wiley & Sons, 1996.

"Benefits of Fungi for the Environment and Humans." United Nations Decade on Ecosystem Restoration, www.decadeonrestoration.org/stories/benefits-fungi-environment-and-humans. Accessed 2 Dec. 2024.

Fleming, Nic. "The Largest Living Thing on Earth Is a Humongous Fungus." BBC Earth, BBC, 19 Nov. 2014, www.bbc.com/earth/story/20141114-the-biggest-organism-in-the-world. Accessed 5 Apr. 2017.

Hudler, George W. Magical Mushrooms, Mischievous Molds. Princeton UP, 1998.

Kendrick, Bryce. The Fifth Kingdom. 3rd ed., Focus Publishing, 2000.

Kirk, Paul M., et al., editors. Ainsworth & Bisby's Dictionary of the Fungi. 10th ed., CABI, 2008.

Stephenson, Steven L. The Kingdom Fungi: The Biology of Mushrooms, Molds, and Lichens. Timber Press, 2010.

Watkinson, Sarah C., et al. The Fungi. 3rd ed., Academic Press, 2016.