Basidiomycota are filamentous fungi composed ofhyphae (except forbasidiomycota-yeast) and reproduce sexually via the formation of specialized club-shaped endcells calledbasidia that normally bear externalmeiospores (usually four). These specializedspores are calledbasidiospores.[5] However, some Basidiomycota are obligateasexual reproducers. Basidiomycota that reproduce asexually (discussed below) can typically be recognized as members of this division by gross similarity to others, by the formation of a distinctive anatomical feature (theclamp connection),cell wall components, and definitively byphylogenetic molecular analysis ofDNA sequence data.
TheAgaricomycotina include what had previously been called theHymenomycetes (an obsolete morphological based class of Basidiomycota that formedhymenial layers on theirfruitbodies), theGasteromycetes (another obsolete class that included species mostly lacking hymenia and mostly forming spores in enclosedfruitbodies), as well as most of thejelly fungi. This sub-phyla also includes the "classic" mushrooms, polypores, corals, chanterelles, crusts, puffballs and stinkhorns.[9] The three classes in the Agaricomycotina are theAgaricomycetes, theDacrymycetes, and theTremellomycetes.[10]
There are several genera classified in the Basidiomycota that are 1) poorly known, 2) have not been subjected to DNA analysis, or 3) if analysed phylogenetically do not group with as yet named or identified families, and have not been assigned to a specific family (i.e., they areincertae sedis with respect to familial placement). These include:
Sexual reproduction cycle of basidiomycetesBasidiomycota life cycleCell cycle of a Dikaryotic basidiomycete
Unlike animals and plants which have readily recognizable male and female counterparts, Basidiomycota (except for theRust (Pucciniales)) tend to have mutually indistinguishable, compatiblehaploids which are usuallymycelia being composed of filamentoushyphae. Typically haploid Basidiomycota mycelia fuse viaplasmogamy 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 adikaryon. The hyphae are then said to be dikaryotic. Conversely, the haploid mycelia are calledmonokaryons. 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 areneither male nor female. They have either abipolar (unifactorial) or atetrapolar (bifactorial) mating system. This results in the fact that followingmeiosis, the resulting haploidbasidiospores and resultant monokaryons, have nuclei that are compatible with 50% (if bipolar) or 25% (if tetrapolar) of their sister basidiospores (and their resultant monokaryons) because the mating genes must differ for them to be compatible. However, there are sometimes more than two possible alleles for a given locus, and in such species, depending on the specifics, over 90% of monokaryons could be compatible with each other.[citation needed]
The maintenance of the dikaryotic status in dikaryons in many Basidiomycota is facilitated by the formation ofclamp connections that physically appear to help coordinate and re-establish pairs of compatible nuclei following synchronousmitotic nuclear divisions. Variations are frequent and multiple. In a typical Basidiomycota lifecycle the long lasting dikaryons periodically (seasonally or occasionally) producebasidia, the specialized usually club-shaped end cells, in which a pair of compatible nuclei fuse (karyogamy) to form adiploid cell.Meiosis follows shortly with the production of 4 haploid nuclei that migrate into 4 external, usually apical basidiospores. Variations occur, however. Typically the basidiospores areballistic, hence they are sometimes also calledballistospores. In most species, the basidiospores disperse and each can start a new haploid mycelium, continuing the lifecycle. Basidia are microscopic but they are often produced on or in multicelled large fructifications calledbasidiocarps or basidiomes, orfruitbodies, variously called mushrooms,puffballs, etc. Ballistic basidiospores are formed onsterigmata 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.[citation needed]
Scheme of a typical basidiocarp, the dipoid reproductive structure of a basidiomycete, showing fruiting body,hymenium and 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 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.[citation needed]
Coprinopsis cinerea is a basidiomycete mushroom. It is particularly suited to the study ofmeiosis because meiosis progresses synchronously in about 10 million cells within the mushroom cap, and the meiotic prophase stage is prolonged. Burns et al.[15] studied the expression of genes involved in the 15-hour meiotic process, and found that the pattern of gene expression ofC. cinerea was similar to two other fungal species, the yeastsSaccharomyces cerevisiae andSchizosaccharomyces pombe. These similarities in the patterns of expression led to the conclusion that the core expression program of meiosis has been conserved in these fungi for over half a billion years of evolution since these species diverged.[15]
Cryptococcus neoformans andMycosarcoma maydis are examples of pathogenic basidiomycota. Such pathogens must be able to overcome the oxidative defenses of their respective hosts in order to produce a successful infection. The ability to undergo meiosis may provide a survival benefit for these fungi by promoting successful infection. A characteristic central feature of meiosis is recombination between homologous chromosomes. This process is associated with repair of DNA damage, particularlydouble-strand breaks. The ability ofC. neoformans andM. maydis to undergo meiosis may contribute to their virulence by repairing the oxidative DNA damage caused by their host's release ofreactive oxygen species.[16][17]
Many variations occur: some variations 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 genusCraterellus often has six-spored basidia, while some corticioidSistotrema species can have two-, four-, six-, or eight-spored basidia, and the cultivated button mushroom,Agaricus bisporus. can have one-, two-, three- or four-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 lifecycles, but can be common species. Examples exist in the mushroom generaArmillaria andXerula, both in thePhysalacriaceae. Occasionally, basidiospores are not formed and parts of the "basidia" act as the dispersal agents, e.g. the peculiar mycoparasitic jelly fungus,Tetragoniomyces or the entire "basidium" acts as a "spore", e.g. in some false puffballs (Scleroderma). In the human pathogenic genusCryptococcus, four nuclei following meiosis remain in the basidium, but continually divide mitotically, each nucleus migrating into synchronously forming nonballistic basidiospores that are then pushed upwards by another set forming below them, resulting in four parallel chains of dry "basidiospores".[citation needed]
Other variations occur: some as standard lifecycles (that themselves have variations within variations) within specific orders.[citation needed]
Rusts (Pucciniales, previously known asUredinales) at their greatest complexity, produce five different types of spores on two different host plants in two unrelated host families. Such rusts are heteroecious (requiring two hosts) and macrocyclic (producing all five spores types). Wheatstem rust is an example. By convention, the stages and spore states are numbered byRoman numerals. Typically, basidiospores infect host one, also known as the alternate or sexual host, and the mycelium formspycnidia, which are miniature, flask-shaped, hollow, submicroscopic bodies embedded in the host tissue (such as a leaf). This stage, numbered "0", produces single-celled spores that ooze out in a sweet liquid and that act as nonmotilespermatia, and also protrudingreceptive hyphae.Insects and probably othervectors such as rain carry the spermatia from spermagonium to spermagonium, 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 calledaecia, which form dikaryoticaeciospores in dry chains in inverted cup-shaped bodies embedded in host tissue. These aeciospores then infect the second host, known as the primary or asexual host (in macrocyclic rusts). On the primary host a repeating spore stage is formed, numbered "II", theurediospores in dry pustules calleduredinia. 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, theteliospore, 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 apromycelium. In the Pucciniales, the basidia are cylindrical and become 3-septate after meiosis, with each of the 4 cells bearing one basidiospore each. The basidiospores disperse and start the infection process on host 1 again.Autoecious rusts complete their life-cycles on one host instead of two, and microcyclic rusts cut out one or more stages.[citation needed]
The characteristic part of the life-cycle ofsmuts is the thick-walled, often darkly pigmented, ornate, teliospore that serves to survive harsh conditions such as overwintering and also serves to help disperse the fungus as drydiaspores. The teliospores are initially dikaryotic but become diploid via karyogamy. Meiosis takes place at the time of germination. A promycelium is formed that consists of a short hypha (equated to a basidium). In some smuts such asMycosarcoma maydis the nuclei migrate into the promycelium that becomes septate (i.e., divided into cellular compartments separated by cell walls calledsepta), 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 asTilletia caries, the elongated haploid basidiospores form apically, often in compatible pairs that fuse centrally resulting in H-shapeddiaspores 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.[citation needed]
Smuts with both a yeast phase and an infectious hyphal state are examples ofdimorphic Basidiomycota.[18] 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.[19] The genusFilobasidiella forms basidia on hyphae but the main infectious stage is more commonly known by theanamorphic yeast nameCryptococcus, e.g.Cryptococcus neoformans[20] andCryptococcus gattii.[19]
^Moore, R. T. (1980). "Taxonomic proposals for the classification of marine yeasts and other yeast-like fungi including the smuts".Botanica Marina.23 (6): 371.Bibcode:1980BoMar..23..361M.doi:10.1515/bot-1980-230605.
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