Fungus

A fungus (plural fungi) is a eukaryotic organism that digests its food externally and absorbs the nutrient molecules into its cells. Fungi are used extensively by humans: yeasts are responsible for fermentation of beer and bread, and mushroom farming is a large industry in many countries. Fungi are the primary decomposers of dead plant and animal matter in many ecosystems, and are commonly seen on old bread as mold. However, the complex biology of fungi extends beyond this common knowledge and experience of them.

Phylogeny of fungi

Fungi were originally classified as plants, however have since been seperated as they are heterotrophs. This means they do not fix their own carbon through photosynthesis, but use carbon fixed by other organisms for metabolism. Fungi are now thought to be more closely related to animals than to plants, and are placed with animals in the monophyletic group of opisthokonts. Fungi absorb their food while animals ingest it, and their cells have cell walls. For these reasons, these organisms are placed in their own kingdom, Fungi.

The Fungi are a monophyletic group, meaning all varieties of fungi come from a common ancestor. Mycologists believe they are monophyletic because they have chitin in their cell walls, they are absorbtive heterotrophs, along with other shared characteristics.

Overview

Although often inconspicuous, fungi occur in every environment on earth and play very important roles in most ecosystems. Many fungi are major decomposers of dead plant and animal matter in forests and many other environments. Some fungi are predators of nematodes, which they capture using an array of devices such as constricting rings or adhesive nets (see illustrations). Other types are parasites on plants and animals, including humans. They are responsible for numerous diseases, such as athlete’s foot and ringworm in humans and Dutch elm disease in plants. Other fungi are partners in symbiotic relationships with other organisms. For example, lichens are formed by a symbiotic relationship between algae or cyanobacteria and fungi. Most vascular plants benefit from a symbiosis between their roots and fungi.

Fungi have a long history of use by humans. Many types of mushrooms and other fungi are eaten, including button mushrooms, shiitake mushrooms, and oyster mushrooms. Of course, many species of mushrooms are poisonous and are responsible for numerous cases of sickness and death every year. A type of single-celled fungus called yeast is used in baking bread and fermenting alcoholic beverages. Fungi are also used to produce industrial chemicals like lactic acid, antibiotics and even to make stonewashed jeans. Some types of fungi are ingested for their psychedelic properties, both recreationally and religiously (see main article, Psychedelic mushroom).

Types of fungi

The major divisions (phyla) of fungi are mainly classified based on their sexual reproductive structures. Currently, five divisions are recognized:

The Chytridiomycota are commonly known as chytrids. These fungi produce zoospores that are capable of moving on their own through liquid menstrua by simple flagella.
The Zygomycota are known as zygomycetes and reproduce sexually with meiospores called zygospores. Black bread mold (Rhizopus stolonifer) is a common species that belongs to this group, another is Pilobolus, which shoots specialized structures through the air for several meters and was the source of the name for the modern dance troupe.
Members of the Glomeromycota are also known as the arbuscular mycorrhizal fungi. Only one species has been observed forming zygospores; all other species only reproduce asexually. This is an ancient association, with evidence dating to 350 million years ago.
The Ascomycota, commonly known as sac fungi or ascomycetes, form meiotic spores called ascospores, which are enclosed in a special sac-like structure called an ascus. This division includes morels, some mushrooms and truffles, as well as single-celled yeasts and many species that have only been observed undergoing asexual reproduction. Because the products of meiosis are retained within the sac-like ascus, several ascomyctes have been used for elucidating principles of genetics and heredity (e.g. Neurospora crassa).
Members of the Basidiomycota, commonly known as the club fungi or basidiomycetes, produce meiospores called basidiospores on club-like stalks called basidia. Most common mushrooms belong to this group, as well as rust (fungus) and smut fungi, which are major pathogens of grains.

Although the water molds and slime molds have traditionally been placed in kingdom Fungi and are still studied by mycologists, they are not true fungi. Unlike true fungi, the water molds and slime molds do not have cell walls made of chitin. In the 5-kingdom system, they are currently placed in kingdom Protista.

Structure

Fungi may be single-celled or multicellular. Multicellular fungi are composed of networks of long hollow tubes called hyphae. The hyphae often aggregate in a dense network known as mycelium. The mycelium grows through the medium on which the fungus feeds. Because fungi are embedded in the medium in which they grow, they are often not visible to the naked eye.

Although fungi lack true organs, the mycelia of ascomycetes and basidiomycetes may become organized into more complex reproductive structures called fruiting bodies, or sporocarps, when conditions are right. "Mushroom" is the common name given to the above-ground fruiting bodies of many fungal species. Although these above-ground structures are the most conspicuous to humans, they make up only a small portion of the entire fungal body. Some fungi form rhizoids, which are underground root-like structures that provide support and transport nutrients from the soil to the rest of the mycelium.

The largest organism in the world is purported to be a single Armillaria ostoyae individual growing in a forest in eastern Oregon, U.S.. The underground mycelial network may cover as much as 890 ha (2200 acres).

Reproduction

Fungi may reproduce sexually or asexually. In asexual reproduction, the offspring are genetically identical to the “parent” organism (they are clones). During sexual reproduction, a mixing of genetic material occurs so that the offspring exhibit traits of both parents. Many species can use both strategies at different times, while others are apparently strictly sexual or strictly asexual. Sexual reproduction has not been observed in some fungi of the Glomeromycota and Ascomycota. These are commonly referred to as Fungi imperfecti or Deuteromycota.

Yeasts and other unicellular fungi can reproduce simply by budding, or “pinching off” a new cell. Many multicellular species produce a variety of different asexual spores that are easily dispersed and resistant to harsh environmental conditions. When the conditions are right, these spores will germinate and colonize new habitats.

Sexual reproduction in fungi is somewhat different from that of animals or plants, and each fungal division reproduces using different strategies. Fungi that are known to reproduce sexually all have a haploid stage and a diploid stage in their life cycles. Ascomycetes and basidiomycetes also go through a dikaryotic stage, in which the nuclei inherited by the two parents do not fuse right away, but remain separate in the hyphal cells (see heterokaryosis).

In zygomycetes, the haploid hyphae of two compatible individuals fuse, forming a zygote, which becomes a resistant zygospore. When this zygospore germinates, it quickly undergoes meiosis, generating new haploid hyphae and asexual sporangiospores. These sporangiospores may then be distributed and germinate into new genetically-identical individuals, each producing their own haploid hyphae. When the hyphae of two compatible individuals come into contact with one another, they will fuse and generate new zygospores, thus completing the cycle.

In ascomycetes, when compatible haploid hyphae fuse with one another, their nuclei do not immediately fuse. The dikaryotic hyphae form structures called asci (sing. ascus), 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 to form new haploid mycelium. Asexual conidia may be produced by the haploid mycelium. Many ascomycetes appear to have lost the ability to reproduce sexually and reproduce only via conidia.

Sexual reproduction in basidiomycetes is similar to that of ascomycetes. Sexually compatible haploid hyphae fuse to produce a dikaryotic mycelium. This leads to the production of a basidiocarp. The most commonly-known basidiocarps are mushrooms, but they may also take many other forms. Club-like structures known as basidia generate haploid basidiospores following karyogamy and meiosis. These basidiospores then germinate to produce new haploid myceliumata.

Edible and poisonous fungi

Some of the most well-known types of fungi are the edible and poisonous mushrooms. Many species are commercially raised, but others must be harvested from the wild. Button mushrooms (Agaricus bisporus) are the most commonly eaten species, used in salads, soups, and many other dishes. Portobello mushrooms are also members of this species, but grow to a much larger size. Other commercially-grown mushrooms that have gained in popularity in the West and are often available fresh in grocery stores include oyster mushrooms, shiitakes, and enoki mushrooms.

There are many more mushroom species that are harvested from the wild for personal consumption or commercial sale. Morels, chanterelles, truffles, black trumpets, and porcini mushrooms (also known as king boletes) all command a high price on the market. They are often used in gourmet dishes.

It is also a common practice to permit the growth of specific species of mold in certain types of cheeses that give them their unique flavor. This mold is non-toxic and is safe for human consumption. This accounts for the blue colour in cheeses such as Roquefort or Stilton.

Hundreds of mushroom species are toxic to humans, causing anything from upset stomachs to hallucinations to death. Some of the most deadly belong to the genus Amanita, including A. virosa (the "Destroying Angel") and A. phalloides (the "Death Cap"). Stomach cramps, vomiting, and diarrhea usually occur within 6-24 hours after ingestion of these mushrooms, followed by a brief period of remission (usually 1-2 days). Patients often fail to present themselves for treatment at this time, assuming that they have recovered. However, within 2-4 weeks liver and kidney failure leads to death if untreated. There is no antidote for the toxins in these mushrooms, but kidney dialysis and administration of corticosteroids may help. In severe cases, a liver transplant may be necessary (Kaminstein 2002). 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 leave it alone.

Fly agaric mushrooms (A. muscaria) are also responsible for a large number of poisonings, but these cases rarely result in death. The most common symptoms are nausea and vomiting, drowsiness, and hallucinations. In fact, this species is used ritually and recreationally for its hallucinogenic properties. However, if it is taken in over a long period of time (regularly over more than six months), this species might cause a temporary loss of sight, which can last from several minutes to an hour.

Fungi in the biological control of pests

Many fungi compete with other organisms, or directly infect them. Some of these fungi are considered beneficial because they can restrict, and sometimes eliminate, the populations of noxious organisms like pest insects, mites, weeds, nematodes and other fungi, such as those that kill plants. There is much interest on the manipulation of these beneficial fungi for the biological control of pests. Some of these fungi can be used as biopesticides, like the ones that kill insects (entomopathogenic fungi). Specific examples of fungi that have been developed as bioinsecticides are Beauveria bassiana, Metarhizium anisopliae, Hirsutella, Paecilomyces fumosoroseus, and Verticillium lecanii (=Lecanicillium lecanii ).

Fungi size

A fungus of the species Armillaria ostoyae may be the largest organism on the planet. It was discovered in the Malheur National Forest in Oregon, and covers a area of 8.9 km² (2200 acres)The Humongous Fungus--Ten Years Later at the University of Wisconsin, Department of Botany. Accessed 20 August 2005.. Whether or not this is an actual individual organism is disputed: some tests have indicated that they have the same genetic makeup Beale, Bob. 10 April 2003. Humungous fungus: world's largest organism? at Environment & Nature News, ABC Online. Accessed 20 August 2005., but this does not exclude its being a clonal colony of numerous smaller individuals.

Notes

References

External links

Fungi

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