Basidiomycetes from Ernst Haeckel's 1904 Kunstformen der Natur
Basidiomycota is a major division (or phyla) of the kingdom Fungi, whose members typically are characterized by the presence of a basidium, a microscopic reproductive structure where sexual spores are produced. The presence of basidia ("little pedestal") is one of the main diagnostic features of the Basidiomycota and is the source of the group's name. However, Basidiomycota also includes single-celled forms (yeasts) and asexual species. Basidiomycota commonly are known as club fungi, given the club-shaped basidium. They also are known as basidiomycetes based on some older classifications that recognize Basidiomycetes as a class-level taxon (ITIS 2000).
About 30,000 described species are placed within Basidiomycota, or about 37 percent of all described species of fungi (Swann and Hibbett 2007). Basidiomycota is so varied that unique morphological features of the group, such as basidium, dikaryon, and clamp connections, are not found in all representatives (see description of Basidomycota). Well-known Basidiomycota include rusts, smuts, various yeasts, true mushrooms, jelly fungi, false truffles, puffballs, stinkhorns. bracket fungi, boletes, chanterelles, earth stars, bunts, mirror yeasts, and the human pathogenic yeast, Cryptococcus.
Basidiomycota are very important for the ecosystem and for humans. Ecologically, they are vital for decaying dead organic matter, including wood and leaf litter, and thus vital for the carbon cycle (Swann and Hibbett 2007). Some also form important symbiotic relationships, such as mycorrhizal associations with the roots of a plant, whereby the fungus receives carbohydrates from the plant’s photosynthesis and the plant gains the mycelium's very large surface area to absorb water and mineral nutrients from the soil. Some are cultivated by ants. For humans, some Basidiomycota are a source of food. On the other hand, they also will decompose living wood tissue and thus can damage the wood in homes and other buildings, and the rusts and smuts are parasitic on plants and cause diseases of important agricultural crops, such as wheat. Some cause human and animal diseases.
Basidomycota are one of the major groups of the fungi kingdom. Fungi are eukaryotic organisms that digest their food externally and absorb the nutrient molecules into its cells. Unlike plants, they do not make their own food through photosynthesis and unlike animals they are not mobile and absorb nutrition externally rather than internally.
Fungi may be single-celled or multicellular. Multicellular fungi are composed of networks of long hollow tubes called hyphae. The hyphae often gather in dense networks known as mycelium. The mycelium is the vegetative part of a fungus, and consists of the mass branching, thread-like hyphae. The mycelium grows through the medium on which the fungus feeds. Because most fungi are embedded in the medium in which they grow, most often soil, they are often not visible.
Although fungi lack true organs, the mycelia can become organized into more complex reproductive structures called fruiting bodies, or sporocarps, under the right conditions. The sporocarp or fruiting body is a multicellular structure in which spore-producing structures, such as basidia or asci, are borne. The fruiting body is part of the sexual phase of a fungal life cycle, the rest of the life cycle is characterized by its vegetative mycelial growth. The sporocarp of a basidiomycete is known as a basidiocarp, while the fruiting body of an ascomycete is known as an ascocarp. Mushroom is the common name given to the fruiting bodies of many fungal species. Although these typically above-ground structures are the most conspicuous to humans, they make up only a small portion of the entire fungal body.
The major divisions (phyla) of fungi are mainly classified based on their sexual reproductive structures. Currently, four or five main divisions are recognized, one of which is the Basidomycota or club fungi. Another major division is the Ascomycota, or sac fungi. Together, the Basidomycota and Ascomycota are commonly known as "higher fungi" (subkingdom Dikarya).
Although considered monophyletic (Swann and Hibbert), Basidiomycota is very varied, with unicellular and multicellular, sexual and asexual, and terrestrial and aquatic forms (Swann and Hibbert 2007). As a result, "it is impossible to identify any morphological characteristics that are both unique to the group and constant in the group" (Swann and Hibbert 2007).
The production of basidia is the most diagnostic feature. A basidium is a cell in which nuclear fusion and meiosis occur prior to developing the typically haploid basidiospores. However, basidia are limited to the sexual Basidiomycota. Another characteristic feature is a long-lived dikaryon, in which all the cells in the thallus contain two haploid nuclei as a result of a mating event (Swann and Hibbert 2007). Clamp connections, a kind of hyphal outgrowth, are another unique feature to Basidiomycota (Swann and Hibbert 2007).
Basically, Basidiomycota are filamentous fungi composed of hyphae (except for those forming yeasts), and reproducing sexually via the formation of the specialized club-shaped end cells (basidia) that normally bear external spores (usually four), which are the specialized meiospores known as basidiospores. Asexually reproducing Basidiomycota (discussed below) can be recognized as members of this phylum by gross similarity to others, by the formation of the distinctive anatomical feature known as the clamp connection, by components of the cell wall, and definitively by phylogenetic molecular analysis of DNA sequence data.
The most recent classification (Hibbett et al. 2007) adopted by a coalition of 67 mycologists recognizes three subphyla (Pucciniomycotina, Ustilaginomycotina, Agaricomycotina) and two other class level taxa (Wallemiomycetes, Entorrhizomycetes) outside of these, among the Basidiomycota. As now classified, the subphyla join and also cut across various obsolete taxonomic groups (see below) previously commonly used to describe various Basidiomycota.
The Basidiomycota had traditionally been divided into two obsolete classes, the Homobasidiomycetes (including true mushrooms); and the Heterobasidiomycetes (the Jelly, Rust, and Smut fungi). Previously, the entire Basidiomycota were called Basidiomycetes, an invalid class level name coined in 1959 as a counterpart to the Ascomycetes, when neither of these taxa were recognized as phyla. The terms basidiomycetes and ascomycetes are frequently used loosely to refer to Basidiomycota and Ascomycota. They are often abbreviated to "basidios" and "ascos" as mycological slang.
The Agaricomycotina includes what had previously been called the Hymenomycetes (an obsolete morphological based class of Basidiomycota that formed hymenial layers on their fruitbodies), the Gasteromycetes (another obsolete class that included species mostly lacking hymenia and mostly forming spores in enclosed fruitbodies), as well as most of the jelly fungi.
The Ustilaginomycotina are most (but not all) of the former smut fungi, along with the Exobasidiales.
The Pucciniomycotina includes the rust fungi, the insect parasitic/symbiotic genus Septobasidium, a former group of smut fungi (in the Microbotryomycetes, which includes mirror yeasts), and a mixture of odd, infrequently seen or seldom recognized fungi, often parasitic on plants.
Two classes, Wallemiomycetes and Entorrhizomycetes cannot at present be placed in a subphylum.
Unlike higher animals and plants, which have readily recognizable male and female counterparts, Basidiomycota (except for the Rust (Pucciniales)) tend to have mutually indistinguishable, compatible haploids, which are usually mycelia being composed of filamentous hyphae.
Typically, haploid Basidiomycota mycelia fuse via plasmogamy and then the compatible nuclei migrate into each other's mycelia and pair up with the resident nuclei. Karyogamy is delayed, so that the compatible nuclei remain in pairs, called a dikaryon. The hyphae are then said to be dikaryotic. Conversely, the haploid mycelia are called monokaryons. Often, the dikaryotic mycelium is more vigorous than the individual monokaryotic mycelia, and proceeds to take over the substrate in which they are growing. The dikaryons can be long-lived, lasting years, decades, or centuries.
The monokaryons are neither male nor female. They have either a bipolar (unifactorial) or a tetrapolar (bifactorial) mating system. This results in the fact that following meiosis, the resulting haploid basidiospores and resultant monokaryons have nuclei that are compatible with 50 percent (if bipolar) or 25 percent (if tetrapolar) of their sister basidiospores (and their resultant monokaryons) because the mating genes must differ for them to be compatible. However, there are many variations of these genes in the population, and therefore, over 90 percent of monokaryons are compatible with each other. It is as if there were multiple sexes.
The maintenance of the dikaryotic status in dikaryons in many Basidiomycota is facilitated by the formation of clamp connections that physically appear to help coordinate and re-establish pairs of compatible nuclei following synchronous mitotic nuclear divisions. Variations are frequent and multiple. In a typical Basidiomycota lifecycle, the long lasting dikaryons periodically (seasonally or occasionally) produce basidia, the specialized usually club-shaped end cells, in which a pair of compatible nuclei fuse (karyogamy) to form a diploid cell. Meiosis follows shortly with the production of 4 haploid nuclei that migrate into four external, usually apical basidiospores.
Variations occur, however. Typically the basidiospores are ballistic, hence they are sometimes also called ballistospores. In most species, the basidiospores disperse and each can start a new haploid mycelium, continuing the life cycle. Basidia are microscopic, but they are often produced on or in multicelled large fructifications called basidiocarps or basidiomes, or fruitbodies), variously called mushrooms, puffballs, and so forth. Ballistic basidiospores are formed on sterigmata, which are tapered spine-like projections on basidia, and are typically curved, like the horns of a bull. In some Basidiomycota, the spores are not ballistic, and the sterigmata may be straight, reduced to stubs, or absent. The basidiospores of these non-ballistosporic basidia may either bud off, or be released via dissolution or disintegration of the basidia.
In summary, meiosis takes place in a diploid basidium. Each one of the four haploid nuclei migrates into its own basidiospore. The basidiospores are ballistically discharged and start new haploid mycelia called monokaryons. There are no typical males or females, rather there are compatible thalli with multiple compatibility factors. Plasmogamy between compatible individuals leads to delayed karyogamy leading to establishment of a dikaryon. The dikaryon is long lasting but ultimately gives rise to either fruitbodies with basidia or directly to basidia without fruitbodies. The paired dikaryon in the basidium fuse (i.e karyogamy takes place). The diploid basidium begins the cycle again.
There are many variations in the Basidiomycota life cycle from the typical cycle. Some Basidiomycota are self compatible and spontaneously form dikaryons without a separate compatible thallus being involved. These fungi are said to be homothallic versus the normal heterothallic species with mating types. Others are secondarily homothallic, in that two compatible nuclei following meiosis migrate into each basidiospore, which is then dispersed as a pre-existing dikaryon. Often such species form only two spores per basidium, but that too varies. Following meiosis, mitotic divisions can occur in the basidium. Multiple numbers of basidiospores can result, including odd numbers via degeneration of nuclei, or pairing up of nuclei, or lack of migration of nuclei. For example, the chanterelle genus Craterellus often has 6-spored basidia, while some corticioid Sistotrema species can have 2-, 4-, 6-, or 8-spored basidia, and the cultivated button mushroom, Agaricus bisporus can have 1-, 2-, 3- or 4-spored basidia under some circumstances.
Occasionally monokaryons of some taxa can form morphologically fully formed basidiomes and anatomically correct basidia and ballistic basidiospores in the absence of dikaryon formation, diploid nuclei, and meiosis. A rare few number of taxa have extended diploid life cycles, but can be common species. Examples exist in the mushroom genera Armillaria and Xerula, both in the Physalacriaceae.
Occasionally, basidiospores are not formed and parts of the "basidia" act as the dispersal agents, such as with the peculiar mycoparasitic jelly fungus, Tetragoniomyces. In other cases, the entire "basidium" acts as a "spore," such as in some false puffballs (Scleroderma).
In the human pathogenic genus Filobasidiella, four nuclei following meiosis remain in the basidium but continually divide mitotically, each nucleus migrating into synchronously forming nonballistic basidiospores, which are then pushed upwards by another set forming below them, resulting in 4 parallel chains of dry "basidiospores."
Other variations occur, some as standard life cycles (which themselves have variations within variations) within specific orders.
Rusts are fungi of the order Uredinales (or Pucciniales). Many of these species are plant parasites. Many of the rusts have two or more hosts (heteroecious) and up to five spore stages. However, they most commonly reproduce via asexual spore production. Their spores are airborne and can travel great distances. They mostly cause foliar infections. The group received its common name from the fact that some species have a reddish spore stage, which resembles the corrosion process known as rust. Rust occurs on many species of plant, but in most cases any one species of rust can only infect one species of plant.
Rusts at their greatest complexity produce five different types of spores on two different hosts in two unrelated host families. Such rusts are heteroecious (requiring two hosts) and macrocyclic (producing all five spores types). Wheat stem rust is an example.
By convention, the stages and spore states are numbered by Roman numerals. Typically, basidiospores infect host one and the mycelium forms pycnidia, called spermagonia, which are miniature, flask-shaped, hollow, submicroscopic bodies embedded in host tissue (such as a leaf). This stage, numbered "0," produces single-celled, minute spores that ooze out in a sweet liquid and that act as nonmotile spermatia and also protruding receptive hyphae. Insects and probably other vectors, such as rain, carry the spermatia from spermagonia to spermagonia, cross inoculating the mating types. Neither thallus is male or female. Once crossed, the dikaryons are established and a second spore stage is formed, numbered "I" and called aecia, which form dikaryotic aeciospores in dry chains in inverted cup-shaped bodies embedded in host tissue. These aeciospores then infect the second host genus and cannot infect the host on which they are formed (in macrocyclic rusts). On the second host, a repeating spore stage is formed, numbered "II," the uredospores in dry pustules called uredinia. Urediospores are dikaryotic and can infect the same host that produced them. They repeatedly infect this host over the growing season. At the end of the season, a fourth spore type, the teliospore, is formed. It is thicker-walled and serves to overwinter or to survive other harsh conditions. It does not continue the infection process, rather it remains dormant for a period and then germinates to form basidia (stage "IV"), sometimes called a promycelium.
In the Pucciniales (or Uredinales), the basidia are cylindrical and become 3-septate after meiosis, with each of the four cells bearing one basidiospore each. The basidospores disperse and start the infection process on host one again. Autoecious rusts complete their life cycles on one host instead of two, and microcyclic rusts cut out one or more stages.
Smuts (mostly Ustilaginomycetes of the class Teliomycetae) cause plant disease, and commonly affect grasses, notably including cereal crops such as maize. They initially attack the plant's reproductive system, forming galls which darken and burst, releasing fungal spores which infect other plants nearby.
The characteristic part of the life cycle of smuts is the thick-walled, often darkly pigmented, ornate, teliospore, which serves to survive harsh conditions such as overwintering and also serves to help disperse the fungus as dry diaspores. The teliospores are initially dikaryotic but become diploid via karyogamy. Meiosis takes place at the time of germination. A promycelim is formed that consists to a short hypha (equated to a basidium).
In some smuts, such as Ustilago maydis, the nuclei migrate into the promycelium that becomes septate, and haploid yeast-like conidia/basidiospores (sometimes called sporidia) bud off laterally from each cell. In various smuts, the yeast phase may proliferate, or they may fuse, or they may infect plant tissue and become hyphal. In other smuts, such as Tilletia caries, the elongated haploid basidiospores form apically, often in compatible pairs that fuse centrally resulting in "H"-shaped diaspores, which are by then dikaryotic. Dikaryotic conidia may then form. Eventually, the host is infected by infectious hyphae. Teliospores form in host tissue. Many variations on these general themes occur.
Smuts with both a yeast phase and an infectious hyphal stage are examples of dimorphic Basidiomycota. In plant parasitic taxa, the saprotrophic phase is normally the yeast, while the infectious stage is hyphal. However, there are examples of animal and human parasites where the species are dimorphic but it is the yeast-like state that is infectious. The genus Filobasidiella forms basidia on hyphae, but the main infectious stage is more commonly known by the anamorphic yeast name Cryptococcus (e.g., Cryptococcus neoformans and Cryptococcus gattii).
The dimorphic Basidiomycota with yeast stages and the pleiomorphic rusts are examples of fungi with anamorphs, which are the asexual stages. Some Basidiomycota are only known as anamorphs. Many are yeasts, collectively called basidiomycetous yeasts to differentiate them from ascomycetous yeasts in the Ascomycota. Aside from yeast anamorphs, and uredinia, aecia, and pycnidia, some Basidiomycota form other distinctive anamorphs as parts of their life cycles. Examples are Collybia tuberosa, with its apple-seed-shaped and colored sclerotium; Dendrocollybia racemosa, with its sclerotium and its Tilachlidiopsis racemosa conidia; Armillaria, with their rhizomorphs; Hohenbuehelia, with their Nematoctonus nematode infectious, state; and the coffee leaf parasite, Mycena citricolor and its Decapitatus flavidus propagules called gemmae.
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