Laura Smith
Mushrooms are the reproductive structures of fungi, often referred to as fruiting bodies. They produce basidiospores for reproduction. The life cycle of mushrooms is predominantly haploid (1n) and involves asexual and sexual reproduction. In asexual reproduction, haploid (1n) mycelia produce spores through mitosis. In sexual reproduction, compatible mycelia fuse to form a dikaryotic (n+n) mycelium, which then develops into a secondary mycelium. Under the right conditions, a basidiocarp (mature mushroom) forms, and the cycle repeats. Therefore, mushrooms are both n and 2n at different stages of their life cycle.
| Characteristics | Values |
|---|---|
| Vegetative cells of many fungi | Single haploid (n) nuclei |
| Vegetative cells of higher fungi | Pairs of haploid nuclei (n + n) |
| Basidial meiosis | Produces 4 haploid spores (n) |
| Fusion of compatible primary mycelia (n) | Dikaryons (n + n) |
| Nuclear fusion | Diploid nuclei (2n) |
| Monokaryons | Persistent in vegetative stages (2n) |
| Vegetative stage hyphae | Haploid (n) |
| Vegetative heterozygous diploidy | Widely accepted model |
| Vegetative stages | Can possess recombinant, haploid nuclei |
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What You'll Learn
Vegetative-stage haploidization in A. gallica
Mushrooms are the reproductive structures of fungi, often referred to as "fruiting bodies". Members of the genus Armillaria are unique among basidiomycetes due to their persistent vegetative diploid phase. The dikaryon is the persistent vegetative phase of most basidiomycetes and arises from the fusion of two haploids with distinct mating-type alleles.
In the case of A. gallica, after the compatible primary mycelia (n) fuse to reestablish dikaryons (n + n), nuclear fusion produces diploid nuclei (2n). Interestingly, these diploid nuclei then undergo a second (vegetative-stage) haploidization at some point prior to mushroom formation. This results in haploid nuclei (n) that persist in the vegetative stages, such as soil mycelia and rhizomorphs. This process is proposed to contribute to the genetic variability observed in Armillaria populations.
The haploidization of the diploid nucleus in A. gallica has been studied by several researchers. Baumgartner et al. (2010) and Korhonen (1983) found that matings between diploids and haploids result in haploidization of the diploid nucleus. Carvalho et al. (1995) further supported this by analyzing 2N + N dikaryons of A. gallica using restriction fragment length polymorphisms. They observed that the diploid nucleus can be replaced by the haploid nucleus or undergo recombination, leading to haploidization.
The mechanism triggering haploidization and the signals that initiate it remain unknown. However, it is suggested that haploidization contributes to the genetic diversity within Armillaria populations. This is evident as somatic cells of A. gallica fruit bodies exhibit distinct genotypes for molecular markers and mating-type loci, resulting in varying growth rates and phenotypic plasticity.
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Vegetative cells of fungi
Fungi are a monophyletic group of eukaryotic heterotrophs that are closely related to animals. As such, their cells contain membrane-bound nuclei with chromosomes that contain DNA with noncoding regions called introns and coding regions called exons. Fungi, like animals, are heterotrophs; they acquire their food by absorbing dissolved molecules, typically by secreting digestive enzymes into their environment. They do not photosynthesize.
The vegetative (non-reproductive) part of a fungus is made up of a mass of branching, thread-like structures called hyphae, which together are called a mycelium. The hyphae of most fungi are composed of individual cells that are connected end-to-end in a long, branching thread. Depending on the species, the hyphae can grow on a surface, in the soil, or decaying matter, in a liquid, or even on living tissue.
The mycelium (the fungal body composed of hyphae) is haploid and reproduces asexually. Many fungi occur not as hyphae but as unicellular forms called yeasts, which reproduce vegetatively by budding. Candida is an example of a dimorphic fungus, meaning it can undergo rapid transformation from the yeast to the hyphal phase in vivo.
Mushrooms are the reproductive structures of fungi and are often referred to as "fruiting bodies". They are the fleshy, spore-bearing fruiting bodies of a fungus, typically produced above ground on soil or another food source. The gills of mushrooms produce microscopic spores, which help the fungus spread across the ground or its occupant surface.
Fungi are capable of sexual and asexual reproduction. In sexual reproduction, spores are produced from karyogamy, which are a product of meiotic cell division of a diploid cell. In asexual reproduction, spores are dispersed by wind or via an animal to a new environment, where they can germinate into a new mycelium and enter into a new cycle of vegetative growth.
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Higher fungi that produce mushrooms
Mushrooms are the fleshy, spore-bearing fruiting bodies of fungi, typically produced above ground on soil or another food source. They are genetically more closely related to animals than plants. The discipline of biology devoted to the study of fungi is known as mycology.
The term "mushroom" is most often applied to those fungi (Basidiomycota, Agaricomycetes) that have a stem (stipe), a cap (pileus), and gills (lamellae) on the underside of the cap. The gills produce microscopic spores, which help the fungus spread across the ground or its occupant surface. Mushrooms are also used as a source of dye for wool and other natural fibers. They produce strong and vivid colors, and all colors of the spectrum can be achieved with mushroom dyes.
Mushrooms are commonly used in cooking and are often treated as vegetables. They are also a great meat substitute due to their high protein content. However, it is important to accurately identify mushrooms intended for eating, as poisoning by wild mushrooms is common and may be fatal.
The standard for the name "mushroom" is the cultivated white button mushroom, Agaricus bisporus. However, the term "mushroom" also describes a variety of other gilled fungi, with or without stems. Forms deviating from the standard morphology usually have more specific names, such as "bolete", "truffle", "puffball", "stinkhorn", and "morel".
Mushrooms are also used in the development of new biological remediation techniques, such as using mycorrhizae to spur plant growth, and filtration technologies, such as using fungi to lower bacterial levels in contaminated water.
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Haploid and dikaryotic fungi
Mushrooms are the reproductive structures of fungi. They are often referred to as "fruiting bodies", as they are the spore-bearing part of the fungus. The spores are called basidiospores and are produced on the gills of the mushroom.
Most fungi are haploid through most of their life cycles. However, the basidiomycetes, a group of fungi that includes many common mushrooms, produce both haploid and dikaryotic mycelia, with the dikaryotic phase being dominant. In the basidiomycetes, sexual spores (basidiospores) are more common than asexual spores.
The dikaryotic phase is technically not diploid, as the nuclei remain unfused until shortly before spore production. However, the dikaryotic life cycle is genetically analogous to diploidy. Dikaryotic fungi, particularly mushroom fungi with dikaryotic phases dominating the life cycle, may have the advantages of both diploidy and haploidy. The advantages of haploidy include the increased promiscuity and mating potential demonstrated by haploid nuclei.
In the life cycle of a sexually reproducing fungus, a haploid phase alternates with a diploid phase. The haploid phase ends with nuclear fusion, and the diploid phase begins with the formation of the zygote. Meiosis (reduction division) restores the haploid number of chromosomes and initiates the haploid phase, which produces the gametes. In the majority of fungi, all structures are haploid except the zygote.
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Haploid basidiospores
Mushrooms are the fleshy, spore-bearing fruiting bodies of fungi, typically produced above ground on soil or another food source. The spores are called basidiospores, which are produced on the gills and fall in a fine rain of powder from under the caps. Basidiospores are shot off basidia, which are reproductive structures found on the gills, spines, tubes, or surfaces of basidiomycetes.
Basidiospores are generally haploid, carrying a single haploid (n) nucleus in each cell. They are formed through the karyogamy of the two haploid nuclei into one diploid nucleus in the terminal cell of a fungus. Following karyogamy, the nuclei in the basidia go through meiosis and migrate into typically four buds attached to the basidia by stalks called sterigmata. These buds then balloon as they are filled with cytoplasm from the basidia and differentiate into basidiospores.
Basidiospores can be dispersed actively (through a fungus's own mechanisms) or passively (through reliance on another organism or abiotic factor). Actively dispersed basidiospores are also called ballistospores. When basidiospores encounter a favourable substrate, they may germinate, typically by forming hyphae. These hyphae grow outward from the original spore, forming an expanding circle of mycelium. The circular shape of a fungal colony explains the formation of fairy rings.
The colour of the powdery print, called a spore print, is useful in both classifying and identifying mushrooms. Spore print colours include white (most common), brown, black, purple-brown, pink, yellow, and creamy, but almost never blue, green, or red. While modern identification of mushrooms is quickly becoming molecular, the standard methods for identification are still used by most and have developed into a fine art.
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Frequently asked questions
Yes, mushrooms are haploid (1n). Haploid spores germinate, form a monokaryon, then must fuse with another monokaryon to form a dikaryon (n+n).
A dikaryon, or dikaryotic hyphal cell, is formed when two primary hyphal cells between compatible monokaryons fuse together.
A monokaryon is formed when haploid spores germinate.
