Occurring worldwide, most fungi are largely invisible to the naked eye, living for the most part in soil, dead matter, and as symbionts of plants, animals, or other fungi. They perform an essential role in all ecosystems in decomposing organic matter and are indispensable in nutrient cycling and exchange. Some fungi become noticeable when fruiting, either as mushrooms or molds. Many fungal species have long been used as a direct source of food, such as mushrooms and truffles and in fermentation of various food products, such as wine, beer, and soy sauce. More recently, fungi are being used as sources for antibiotics used in medicine and various enzymes, such as cellulases, pectinases, and proteases, important for industrial use or as active ingredients of detergents. Many fungi produce bioactive compounds called mycotoxins, such as alkaloids and polyketides that are toxic to animals including humans. Some fungi are used recreationally or in traditional ceremonies as a source of psychotropic compounds. Several species of the fungi are significant pathogens of humans and other animals, and losses due to diseases of crops (e.g., rice blast disease) or food spoilage caused by fungi can have a large impact on human food supply and local economies.
Human use of fungi for food preparation or preservation and other purposes is extensive and has a long history: yeasts are required for fermentation of beer, wine and bread, some other fungal species are used in the production of soy sauce and tempeh. Mushroom farming and mushroom gathering are large industries in many countries. Many fungi are producers of antibiotics, including β-lactam antibiotics such as penicillin and cephalosporin. Widespread use of these antibiotics for the treatment of bacterial diseases, such as tuberculosis, syphilis, leprosy, and many others began in the early 20th century and continues to play a major part in anti-bacterial chemotherapy. The study of the historical uses and sociological impact of fungi is known as ethnomycology.
Mycotoxins belong to the group of secondary metabolites (or natural products). Originally, this group of compounds had been thought to be mere byproducts of primary metabolism, hence the name "secondary" metabolites. However, recent research has shown the existence of biochemical pathways solely for the purpose of producing mycotoxins and other natural products in fungi. Mycotoxins provide a number of fitness benefits to the fungi that produce them in terms of physiological adaptation, competition with other microbes and fungi, and protection from fungivory. These fitness benefits and the existence of dedicated biosynthetic pathways for mycotoxin production suggest that the mycotoxins are important for fungal persistence and survival.
Some of the best known types of fungi are the edible and the poisonous mushrooms. Many species are commercially raised, but others must be harvested from the wild. Agaricus bisporus, sold as button mushrooms when small or Portobello mushrooms when larger, are the most commonly eaten species, used in salads, soups, and many other dishes. Many Asian fungi are commercially grown and have gained in popularity in the West. They are often available fresh in grocery stores and markets, including straw mushrooms (Volvariella volvacea), oyster mushrooms (Pleurotus ostreatus), shiitakes (Lentinula edodes), and enokitake (Flammulina spp.).
There are many more mushroom species that are harvested from the wild for personal consumption or commercial sale. Milk mushrooms, morels, chanterelles, truffles, black trumpets, and porcini mushrooms (Boletus edulis) (also known as king boletes) all demand a high price on the market. They are often used in gourmet dishes.
For certain types of cheeses, it is also a common practice to inoculate milk curds with fungal spores to foment the growth of specific species of mold that impart a unique flavor and texture to the cheese. This accounts for the blue colour in cheeses such as Stilton or Roquefort which is created using Penicillium roqueforti spores. Molds used in cheese production are usually non-toxic and are thus safe for human consumption; however, mycotoxins (e.g., aflatoxins, roquefortine C, patulin, or others) may accumulate due to fungal spoilage during cheese ripening or storage.
Many mushroom species are toxic to humans, with toxicities ranging from slight digestive problems or allergic reactions as well as hallucinations to severe organ failures and death. Some of the most deadly mushrooms belong to the genera Inocybe, Cortinarius, and most infamously, Amanita. The latter genus includes the destroying angel (A. virosa) and the death cap (A. phalloides), the most common cause of deadly mushroom poisoning. The false morel (Gyromitra esculenta) is considered a delicacy by some when cooked, yet can be highly toxic when eaten raw. Tricholoma equestre was considered edible until being implicated in some serious poisonings causing rhabdomyolysis.
Fly agaric mushrooms (A. muscaria) also cause occasional poisonings, mostly as a result of ingestion for use as a recreational drug for its hallucinogenic properties. Historically Fly agaric was used by Celtic Druids in Northern Europe and the Koryak people of north-eastern Siberia for religious or shamanic purposes. It is difficult to identify a safe mushroom without proper training and knowledge, thus it is often advised to assume that a mushroom in the wild is poisonous and not to consume it.
Although often inconspicuous, fungi occur in every environment on Earth and play very important roles in most ecosystems. Along with bacteria, fungi are the major decomposers in most terrestrial (and some aquatic) ecosystems, and therefore play a critical role in biogeochemical cycles and in many food webs. As decomposers, they play an indispensable role in nutrient cycling, especially as saprotrophs and symbionts, degrading organic matter to inorganic molecules, which can then re-enter anabolic metabolic pathways in plants or other organisms.
Mycorrhizal symbiosis between plants and fungi is one of the most well-known plant-fungus associations and is of significant importance for plant growth and persistence in many ecosystems; over 90% of all plant species engage in some kind of mycorrhizal relationship with fungi and are dependent upon this relationship for survival. The mycorrhizal symbiosis is ancient, dating to at least 400 million years ago. It often increases the plant's uptake of inorganic compounds, such as nitrate and phosphate from soils having low concentrations of these key plant nutrients. In some mycorrhizal associations, the fungal partners may mediate plant-to-plant transfer of carbohydrates and other nutrients. Such mycorrhizal communities are called "common mycorrhizal networks".
Lichens are formed by a symbiotic relationship between algae or cyanobacteria (referred to in lichens as "photobionts") and fungi (mostly various species of ascomycetes and a few basidiomycetes), in which individual photobiont cells are embedded in a tissue formed by the fungus. As in mycorrhizas, the photobiont provides sugars and other carbohydrates, while the fungus provides minerals and water. The functions of both symbiotic organisms are so closely intertwined that they function almost as a single organism.
Many insects also engage in mutualistic relationships with various types of fungi. Several groups of ants cultivate fungi in the order Agaricales as their primary food source, while ambrosia beetles cultivate various species of fungi in the bark of trees that they infest. Termites on the African Savannah are also known to cultivate fungi.
However, many fungi are parasites on plants, animals (including humans), and other fungi. Serious fungal pathogens of many cultivated plants causing extensive damage and losses to agriculture and forestry include the rice blast fungus Magnaporthe oryzae, tree pathogens such as Ophiostoma ulmi and Ophiostoma novo-ulmi causing Dutch elm disease, and Cryphonectria parasitica responsible for chestnut blight, and plant-pathogenic fungi in the genera Fusarium, Ustilago, Alternaria, and Cochliobolus; fungi with the potential to cause serious human diseases, especially in persons with immuno-deficiencies, are in the genera Aspergillus, Candida, Cryptoccocus, Histoplasma, and Pneumocystis. Several pathogenic fungi are also responsible for relatively minor human diseases, such as athlete’s foot and ringworm. Some fungi are predators of nematodes, which they capture using an array of specialized structures, such as constricting rings or adhesive nets.
Growth of fungi as hyphae on or in solid substrates or single cells in aquatic environments is adapted to efficient extraction of nutrients from these environments, because these growth forms have high surface area to volume ratios. These adaptations in morphology are complemented by hydrolytic enzymes secreted into the environment for digestion of large organic molecules, such as polysaccharides, proteins, lipids, and other organic substrates into smaller molecules. These molecules are then absorbed as nutrients into the fungal cells.
Traditionally, the fungi are considered heterotrophs, organisms that rely solely on carbon fixed by other organisms for metabolism. Fungi have evolved a remarkable metabolic versatility that allows many of them to use a large variety of organic substrates for growth, including simple compounds as nitrate, ammonia, acetate, or ethanol. Recent research raises the possibility that some fungi utilize the pigment melanin to extract energy from ionizing radiation, such as gamma radiation for "radiotrophic" growth. It has been proposed that this process might bear some similarity to photosynthesis in plants, but detailed biochemical data supporting the existence of this hypothetical pathway are presently lacking.
Though fungi are part of the opisthokont clade, all phyla except for the chytrids have lost their posterior flagella. Fungi are unusual among the eukaryotes in having a cell wall that, besides glucans (e.g., β-1,3-glucan) and other typical components, contains the biopolymer chitin.
Many fungi grow as thread-like filamentous microscopic structures called hyphae, and an assemblage of intertwined and interconnected hyphae is called a mycelium. Hyphae can be septate, i.e., divided into hyphal compartments separated by a septum, each compartment containing one or more nuclei or can be coenocytic, i.e., lacking hyphal compartmentalization. However, septa have pores, such as the doliporus in the basidiomycetes that allow cytoplasm, organelles, and sometimes nuclei to pass through. Coenocytic hyphae are essentially multinucleate supercells. In some cases, fungi have developed specialized structures for nutrient uptake from living hosts; examples include haustoria in plant-parasitic fungi of nearly all divisions, and arbuscules of several mycorrhizal fungi, which penetrate into the host cells for nutrient uptake by the fungus.
Fungal mycelia can become visible macroscopically, for example, as concentric rings on various surfaces, such as damp walls, and on other substrates, such as spoilt food (see figure), and are commonly and generically called mould (American spelling, mold); fungal mycelia grown on solid agar media in laboratory petri dishes are usually referred to as colonies, with many species exhibiting characteristic macroscopic growth morphologies and colours, due to spores or pigmentation.
Specialized fungal structures important in sexual reproduction are the apothecia, perithecia, and cleistothecia in the ascomycetes, and the fruiting bodies of the basidiomycetes, and a few ascomycetes. These reproductive structures can sometimes grow very large, and are well known as mushrooms.
Reproduction of fungi is complex, reflecting the heterogeneity in lifestyles and genetic make up within this group of organisms. Many fungi reproduce either sexually or asexually, depending on conditions in the environment. These conditions trigger genetically determined developmental programs leading to the expression of specialized structures for sexual or asexual reproduction. These structures aid both reproduction and efficient dissemination of spores or spore-containing propagules.
Asexual reproduction via vegetative spores or through mycelial fragmentation is common in many fungal species and allows more rapid dispersal than sexual reproduction. In the case of the "Fungi imperfecti" or Deuteromycota, which lack a sexual cycle, it is the only means of propagation. Asexual spores, upon germination, may found a population that is clonal to the population from which the spore originated, and thus colonize new environments.
Most fungi have both a haploid and diploid stage in their life cycles. In all sexually reproducing fungi, compatible individuals combine by cell fusion of vegetative hyphae by anastomosis, required for the initiation of the sexual cycle. Ascomycetes and basidiomycetes go through a dikaryotic stage, in which the nuclei inherited from the two parents do not fuse immediately after cell fusion, but remain separate in the hyphal cells (see heterokaryosis).
In ascomycetes, dikaryotic hyphae of the hymenium form a characteristic hook at the hyphal septum. During cell division formation of the hook ensures proper distribution of the newly divided nuclei into the apical and basal hyphal compartments. An ascus (plural asci) is then formed, in which karyogamy (nuclear fusion) occurs. These asci are embedded in an ascocarp, or fruiting body, of the fungus. Karyogamy in the asci is followed immediately by meiosis and the production of ascospores. The ascospores are disseminated and germinate and may form a new haploid mycelium.
Sexual reproduction in basidiomycetes is similar to that of the ascomycetes. Compatible haploid hyphae fuse to produce a dikaryotic mycelium. However, the dikaryotic phase is more extensive in the basidiomycetes, in many cases also present in the vegetatively growing mycelium. A specialized anatomical structure, called a clamp connection, is formed at each hyphal septum. As with the structurally similar hook in the ascomycetes, formation of the clamp connection in the basidiomycetes is required for controlled transfer of nuclei during cell division, to maintain the dikaryotic stage with two genetically different nuclei in each hyphal compartment. A basidiocarp is formed in which club-like structures known as basidia generate haploid basidiospores after karyogamy and meiosis. The most commonly known basidiocarps are mushrooms, but they may also take many other forms (see Morphology section).
In zygomycetes, haploid hyphae of two individuals fuse, forming a zygote, which develops into a zygospore. When the zygospore germinates, it quickly undergoes meiosis, generating new haploid hyphae, which in turn may form asexual sporangiospores. These sporangiospores are means of rapid dispersal of the fungus and germinate into new genetically identical haploid fungal colonies, able to mate and undergo another sexual cycle followed by the generation of new zygospores, thus completing the lifecycle.
Both asexual and sexual spores or sporangiospores of many fungal species are actively dispersed by forcible ejection from their reproductive structures. This ejection ensures exit of the spores from the reproductive structures as well as travelling through the air over long distances. Many fungi thereby possess specialized mechanical and physiological mechanisms as well as spore-surface structures, such as hydrophobins, for spore ejection. These mechanisms include, for example, forcible discharge of ascospores enabled by the structure of the ascus and accumulation of osmolytes in the fluids of the ascus that lead to explosive discharge of the ascospores into the air. The forcible discharge of single spores termed ballistospores involves formation of a small drop of water (Buller's drop), which upon contact with the spore leads to its projectile release with an initial acceleration of more than 10,000 g. Other fungi rely on alternative mechanisms for spore release, such as external mechanical forces, exemplified by puffballs. Attracting insects, such as flies, to fruiting structures, by virtue of their having lively colours and a putrid odour, for dispersal of fungal spores is yet another strategy, most prominently used by the stinkhorns.
Besides regular sexual reproduction with meiosis, some fungal species may exchange genetic material via parasexual processes, initiated by anastomosis between hyphae and plasmogamy of fungal cells. The frequency and relative importance of parasexual events is unclear and may be lower than other sexual processes. However, it is known to play a role in intraspecific hybridization and is also likely required for hybridization between fungal species, which has been associated with major events in fungal evolution.
For a long time taxonomists considered fungi to be members of the Plant Kingdom. This early classification was based mainly on similarities in lifestyle: both fungi and plant are mainly sessile, have similarities in general morphology and growth habitat (like plants, fungi often grow in soil, in the case of mushrooms forming conspicuous fruiting bodies, which sometimes bear resemblance to plants such as mosses). Moreover, both groups possess a cell wall, which is absent in the Animal Kingdom. However, the fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged approximately one billion years ago. Many studies have identified several distinct morphological, biochemical, and genetic features in the Fungi, clearly delineating this group from the other kingdoms. For these reasons, the fungi are placed in their own kingdom.
Similar to animals and unlike most plants, fungi lack the capacity to synthesize organic carbon by chlorophyll-based photosynthesis; whereas plants store the reduced carbon as starch, fungi, like animals and some bacteria, use glycogen for storage of carbohydrates. A major component of the cell wall in many fungal species is the nitrogen-containing carbohydrate, chitin, also present in some animals, such as the insects and crustaceans, while the plant cell wall consists chiefly of the carbohydrate cellulose. The defining and unique characteristics of fungal cells include growth as hyphae, which are microscopic filaments of between 2-10 microns in diameter and up to several centimetres in length, and which combined form the fungal mycelium. Some fungi, such as yeasts, grow as single ovoid cells, similar to unicellular algae and the protists.
Unlike many plants, most fungi lack an efficient vascular system, such as xylem or phloem for long-distance transport of water and nutrients; as an example for convergent evolution, some fungi, such as Armillaria, form rhizomorphs or mycelial cords, resembling and functionally related to, but morphologically distinct from, plant roots.
Some characteristics shared between plants and fungi include the presence of vacuoles in the cell, and a similar pathway in the biosynthesis of terpenes using mevalonic acid and pyrophosphate as biochemical precursors; plants however use an additional terpene biosynthesis pathway in the chloroplasts that is apparently absent in fungi. Ancestral traits shared among members of the fungi include chitinous cell walls and heterotrophy by absorption. A further characteristic of the fungi that is absent from other eukaryotes, and shared only with some bacteria, is the biosynthesis of the amino acid, L-lysine, via the α-aminoadipate pathway.
Similar to plants, fungi produce a plethora of secondary metabolites functioning as defensive compounds or for niche adaptation; however, biochemical pathways for the synthesis of similar or even identical compounds often differ markedly between fungi and plants.
Even though traditionally included in many botany curricula and textbooks, fungi are now thought to be more closely related to animals than to plants and are placed with the animals in the monophyletic group of opisthokonts. For much of the Paleozoic Era, the fungi appear to have been aquatic, and consisted of organisms similar to the extant Chytrids in having flagellum-bearing spores. The early fossil record of the fungi is fragmentary, to say the least. The fungi probably colonized the land during the Cambrian, long before land plants. All modern classes of fungi were present in the Late Carboniferous (Pennsylvanian Epoch). For some time after the Permian-Triassic extinction event, a fungal spike, originally thought to be an extraordinary abundance of fungal spores in sediments formed shortly after this event, suggested that they were the dominant life form during this period—nearly 100% of the fossil record available from this period. However, the relative proportion of fungal spores relative to spores formed by algal species is difficult to assess, the spike did not appear world-wide, and in many places it did not fall on the Permian-Triassic boundary.
Analyses using molecular phylogenetics support a monophyletic origin of the Fungi. The taxonomy of the Fungi is in a state of constant flux, especially due to recent research based on DNA comparisons. These current phylogenetic analyses often overturn classifications based on older and sometimes less discriminative methods based on morphological features and biological species concepts obtained from experimental matings.
There is no unique generally accepted system at the higher taxonomic levels and there are constant name changes at every level, from species upwards. However, efforts among fungal researchers are now underway to establish and encourage usage of a unified and more consistent nomenclature. Fungal species can also have multiple scientific names depending on its life cycle and mode (sexual or asexual) of reproduction. Web sites such as Index Fungorum and ITIS define preferred up-to-date names (with cross-references to older synonyms), but do not always agree with each other.
Because of some similarities in morphology and lifestyle, the slime molds (myxomycetes) and water molds (oomycetes) were formerly classified in the kingdom Fungi. Unlike true fungi, however, the cell walls of these organisms contain cellulose and lack chitin. Slime molds are unikonts like fungi, but are grouped in the Amoebozoa. Water molds are diploid bikonts, grouped in the Chromalveolate kingdom. Neither water molds nor slime molds are closely related to the true fungi, and, therefore, taxonomists no longer group them in the kingdom Fungi. Nonetheless, studies of the oomycetes and myxomycetes are still often included in mycology textbooks and primary research literature.
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