Fungus

The Fungi (singular: fungus) are a large group of            Fungi
organisms, usually ranked as a kingdom in
Linnaean taxonomy. They include important          Scientific classification

decomposers and many parasites. Parasitic fungi    Domain:  Eukaryota
infect animals, including humans, other mammals,
birds, and insects, with results varying from      Kingdom: Fungi

mild itching to death. Other parasitic fungi               Divisions
infect plants, causing diseases such as butt rot
and making trees more vulnerable to toppling. The  Chytridiomycota
vast majority of vascular plants are associated    Zygomycota
with mutualistic fungi, called mycorrhizae, which  Ascomycota
assist in nutrient and water absorption. Some      Basidiomycota
fungi are used as food, such as shiitake
mushrooms and Mexican corn smut.

Structure

Most fungi have vegetative bodies composed of one-cell-thick filaments
called hyphae (singular: hypha). These generally do not coalesce into a
visible object, but instead form a microscopic network within the substrate,
called the mycelium, through which food is absorbed. The more conspicuous
parts of fungi like mushrooms are fruiting bodies, which are reproductive
structures that produce spores.

Division of hyphae into cells is either incomplete, in which case they are
called septate and the dividing walls are called septa (singular: septum),
or absent, in which case they are called coenocytic. Fungi generally have
cell walls made from chitin and other materials. The hyphae may be modified
to produce highly specialized cellular-scale structures. For instance, fungi
that parasitize plants grow haustoria that pierce their cell walls and
digest them from the inside, while some soil-dwelling fungi trap roundworms
and other small animals.

Most chytrids, which are generally considered the most primitive group of
fungi, do not form hyphae and instead grow directly from spores into
multinucleate sporangia. A few other fungi have reverted from a mycelial to
a unicellular organization. These are the yeasts, which belong among the
ascomycetes, and the Microsporidia, a group of reduced parasites whose
relationships to the other fungi are uncertain.

Nutrition

Fungi are heterotrophic, that is they get their energy by breaking down
organic molecules, and cannot synthesize organic molecules from inorganic
substances like plants. They feed by secreting exoenzymes into the
surrounding substrate. Exoenzymes act like the digestive enzymes of animals,
breaking apart large organic molecules, but function outside the organism.
The smaller organic molecules are then absorbed.

Among the parasitic fungi are species which are insectivorous or
helminthivorous (worm-eating). The insectivorous species produce sticky
substances which trap insects, while the worm-eating fungi produce
substances which drug and immobilize worms, which are then consumed.

Reproduction

Sexual

Fungal mycelia are typically haploid. When mycelia of different mating types
meet, they produce two multinucleate ball-shaped cells, which form a mating
bridge. The result is that nuclei move from one mycelium into the other,
forming a heterokaryon (meaning different nuclei). This is called
plasmogamy. Actual fusion to form diploid nuclei is called karyogamy, and
may not occur until sporangia are formed.

In the Zygomycota, the heterokaryon produces multiple fruiting bodies, in
the form of minuscule stalks with sporangia at the end. Most ascomycetes
produce fruiting bodies called ascocarps, composed entirely of hyphae. These
are usually bowl- or cup-shaped, but some have sponge-like structures. On
the inside of the cup, each hypha terminates in an ascus, which produces
eight spores.

In the Basidiomycota, the heterokaryon produces a new mycelium which may
live for years without producing a fruiting body. The familiar mushrooms are
examples of these. They usually have a stalk, composed mainly of hyphae, and
a cap, under which there are sheetlike structures called gills. On the
surface of each gill there are numerous hyphal cells called basidia, with
several spores on the end of each.

Asexual

Fungi may also reproduce asexually, for instance through the production of
spores called conidia (Greek for dust), which form at the tips of
specialized hyphae called conidiophores. In some fungi sexual reproduction
has been lost, or is unknown. These were originally grouped as the form
division Deuteromycota, or the Fungi imperfecti, since the means of sexual
reproduction was the primary means of classifying fungi, but are now
classified with their ancestral group.

Except among the chytrids, where spores are propelled by a posterior
flagellum, all fungal spores are non-motile. They develop into new mycelia,
which invade some substrate and repeat the life cycle. These may become very
large, often several square metres in size; fairy circles are an example.

Ecology

Two main ecological niches occupied by fungi are that of the decomposer
(detritivore) or saprophyte, and that of the parasite. The only difference
between detritivorous and parasitic fungi is that the latter grow on living
organisms, while the former grow on dead organisms. Many decomposer fungi
live as mycorrhizae, in mutualistic relationships with plants. Some of the
detritivorous fungi are also considered "facultative parasites," growing on
weakened or dying organisms.

Some fungi, usually ascomycetes, live as lichens. A lichen is a very close
mutualistic relationship between a fungus and a photosynthetic
microorganism, usually a cyanobacterium or green alga. A lichen behaves in
so many ways like a single organism that lichens are actually given genus
and species names.

Pathology

Fungal diseases include:

   * Chestnut blight
   * Ergot
   * Brown rot

Fungal infections may also be a factor in causing specific replant disease.

A fungal infection of the human body is called a mycosis. Mycoses are
particularly severe in the case of immunodeficient patients, such as those
suffering from AIDS.

Related and similar groups

The water molds, of which potato blight is the best known example, show a
hyphal organization and were once considered fungi. However they, and the
closely related hypochytrids, are not actually related to the true fungi and
instead belong among a group called the stramenopiles, together with the
golden algae and their allies. The chytrids were also formerly excluded from
the fungi due to the presence of flagellated spores, but are of definite
relation to the others and so are now usually treated with them.

Slime molds were also originally placed here, because they produce fruiting
bodies, but are now recognized to be several distinct groups of amoeboids.

Fungi are generally believed to have evolved from the same group of
flagellates that gave rise to animals and choanoflagellates. Similarities
include the structure of motile cells, when present, and the common presence
of chitin in some groups.

Miscellaneous

Many orchids require fungus to germinate.

Fungi of the genus Penicillium produce penicillin, the first antibiotic
known to modern science. Many bacteria have since become resistant to
penicillin, but it is still used against Streptococcus and other very
dangerous germs.

Other fungi include:

   * Puff-ball
   * Yeast

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