Chytridiomycota are a division of zoosporic organisms in the kingdom Fungi, informally known as chytrids. The name is derived from the Ancient Greek χυτρίδιον ( ), meaning "little pot", describing the structure containing unreleased . Chytrids are one of the earliest diverging fungal lineages, and their membership in kingdom Fungi is demonstrated with chitin , a posterior whiplash flagellum, absorptive nutrition, use of glycogen as an energy storage compound, and synthesis of lysine by the -amino adipic acid (AAA) pathway.[
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Chytrids are saprobic, degrading refractory materials such as chitin and keratin, and sometimes act as parasites.[ There has been a significant increase in the research of chytrids since the discovery of Batrachochytrium dendrobatidis, the causal agent of chytridiomycosis.]
Classification
Species of Chytridiomycota have traditionally been delineated and classified based on development, morphology, substrate, and method of zoospore discharge.[ However, single spore isolates (or isogenic lines) display a great amount of variation in many of these features; thus, these features cannot be used to classify or identify a species reliably.][
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In an older and more restricted sense (not used here), the term "chytrids" referred just to those fungi in the class Chytridiomycetes. Here, the term "chytrid" refers to all members of Chytridiomycota.
The chytrids have also been included among the Protoctista,[ but are now regularly classed as fungi.
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In older classifications, chytrids, except the recently established order Spizellomycetales, were placed in the class Phycomycetes under the subphylum Myxomycophyta of the kingdom Fungi. Previously, they were placed in the Mastigomycotina as the class Chytridiomycetes.
The class Chytridiomycetes has over 750 chytrid species distributed among ten orders.[ ]
Molecular phylogenetics and other techniques such as ultrastructure analysis have greatly increased the understanding of chytrid phylogeny and led to the formation of several new zoosporic fungal phyla:
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The order Blastocladiales, originally within the Chytridiomycota, are now classified as a separate phylum, the Blastocladiomycota.
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The Neocallimastigales, originally an order of anaerobic fungus of the class Chytridiomycetes, found in the digestive tracts of herbivores, was later raised to a separate phylum, the Neocallimastigomycota.
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The Olpidiaceae, including the type genus Olpidium, formerly classified in the order Chytridiales, were raised to a separate phylum, the Olpidiomycota.
Morphology
Life cycle
Chytridiomycota are unusual among the Fungi in that they reproduce with zoospores.[
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Where it has been described, the sexual reproduction of chytrids occurs via a variety of methods. It is generally accepted that the resulting zygote forms a resting spore, which functions as a means of surviving adverse conditions.[ In some members, sexual reproduction is achieved through the fusion of Isogamy (gametes of the same size and shape). This group includes the notable plant pathogens Synchytrium. Some algal parasites practice oogamy: A motile male gamete attaches itself to a nonmotile structure containing the female gamete. In another group, two thalli produce tubes that fuse and allow the gametes to meet and fuse.][ In the last group, rhizoids of compatible strains meet and fuse. Both nuclei migrate out of the zoosporangium and into the conjoined rhizoids, where they fuse. The resulting zygote germinates into a resting spore.][
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Sexual reproduction is common and well-known among members of the Monoblepharidomycetes. Typically, these chytrids practice a version of oogamy: The male is motile, and the female is stationary. This is the first occurrence of oogamy in the kingdom Fungi.[ Briefly, the monoblephs form oogonia, which give rise to eggs, and antheridia, which give rise to male gametes. Once fertilized, the zygote becomes an encysted or motile oospore,][ which ultimately becomes a resting spore that will later germinate and give rise to new zoosporangia.][
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File:06 12 life cycle, asexual, Batrachochytrium dendrobatidis, Chytridiomycota (M. Piepenbring).png|alt=A diagram displaying the life cycle of Batrachochytrium dendrobatidis. Starting at the top and moving clockwise: A zoospore seeks out an amphibian and encysts on the skin. It grows and develops rhizoids to anchor it and absorb nutrients. The thallus divides numerous times to form a colony of zoosporia, which develop discharge tubes when mature. When ready, the discharge tubes open, and the zoospores are released.|Life cycle of Batrachochytrium dendrobatidis
File:06 11 life cycle, Synchytrium endobioticum on potato, Chytridiomycota (M. Piepenbring).png|alt=Diagram of the asexual and sexual parts of the Synchytrium endobioticum life cycle.|Life cycle of Synchytrium endobioticum in potato
File: Synchytridium endobioticum.jpg| Synchytrium endobioticum on potatoes.
Upon release from the germinated resting spore, zoospores seek out a suitable substrate for growth using chemotaxis or phototaxis.. Some species encyst and germinate directly upon the substrate; others encyst and germinate a short distance away. Once germinated, enzymes released from the zoospore begin to break down the substrate and utilize it to produce a new thallus. Thalli are coenocytic and usually form no true mycelium (having instead).
Chytrids have several different growth patterns. Some are holocarpic, which means they only produce a Sporangium and . Others are eucarpic, meaning they produce other structures, such as , in addition to the zoosporangium and zoospores. Some chytrids are monocentric, meaning a single zoospore gives rise to a single zoosporangium. Others are polycentric, meaning one zoospore gives rise to many zoosporangium connected by a rhizomycelium. Rhizoids do not have nuclei, while a rhizomycelium can.[
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File:06 10 types of thalli, Chytridiomycota (M. Piepenbring).png|alt=Line drawing diagram of the five major classifications of chytrid thalli displayed in two columns. At the top of the left column is a diagram of a eucarpic, monocentric chytrid thallus before and after zoospore discharge. Below is a diagram of an epibiotic chytrid, followed by a diagram of an endobiotic chytrid. At the top of the right column is a diagram of a holocarpic chytrid thallus. Below that is a diagram of a eucarpic polycentric chytrid thallus.|Types of chytrid thalli
File:8-30-11 Crowned Duckweed Chytrid (2).jpg|alt=A zoosporangium of Phlyctochytrium sp. on a duckweed frond. The zoosporangium is empty, and the spines that had crowned it are folded back. Suspended above the empty zoosporangium is a mass of zoospores.|Zoospore discharge from Phylctochytrium sp.
File: CSIRO ScienceImage 1392 Scanning Electron Micrograph of Chytrid Fungus.jpg|alt=Scanning electron image of a sporangium of Batrachochytrium dendrobatidis. The sproangium is spherical with four discharge papillae spaced equidistant around the circumference of the sphere.|Sporangium and of the chytrid fungus B. dendrobatidis, under a scanning electron microscope
File:Chytrid under the microscope.webm|alt=A video of zoospores being released from a chytrid thallus.|Chytrid sporangium and under the microscope
Growth continues until a new batch of zoospores is ready for release. Chytrids have a diverse set of release mechanisms that can be grouped into the broad categories of operculate or inoperculate. Operculate discharge involves the complete or incomplete detachment of a lid-like structure, called an operculum, allowing the zoospores out of the sporangium. Inoperculate chytrids release their zoospores through pores, slits, or papillae.[
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Habitats
Chytrids are aquatic fungi, though those that thrive in the capillary network around soil particles are typically considered terrestrial.
Chytrids have been isolated from a variety of aquatic habitats, including peats, bogs, rivers, ponds, springs, and ditches, and terrestrial habitats, such as acidic soils, alkaline soils, temperate forest soils, rainforest soils, and Arctic and Antarctic soils.[ This has led to the belief that many chytrid species are ubiquitous and cosmopolitan.][ However, recent taxonomic work has demonstrated that this ubiquitous and cosmopolitan morphospecies hides cryptic diversity at the genetic and ultrastructural levels.][ However, recent molecular inventories of lakes during the summer indicate that chytrids are an active, diverse part of the eukaryotic microbial community.]
One of the least expected terrestrial environments the chytrid thrive in is periglacial soils.
Ecological functions
Batrachochytrium dendrobatidis
The chytrid Batrachochytrium dendrobatidis is responsible for chytridiomycosis, a disease of amphibians. Discovered in 1998 in Australia and Panama, this disease is known to kill in large numbers and has been suggested as a principal cause for the worldwide amphibian decline. Outbreaks of the fungus were found responsible for killing much of the Kihansi Spray Toad population in its native habitat of Tanzania,
Recent research has revealed that elevating salt levels slightly may be able to cure chytridiomycosis in some Australian frog species,
Other parasites
Chytrids mainly infect algae and other eukaryote and prokaryote microbes. The infection can be so severe as to control primary production within the lake.[
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Saprobes
Arguably, the most important ecological function chytrids perform is decomposition.[ These ubiquitous and cosmopolitan organisms are responsible for the decomposition of refractory materials, such as pollen, cellulose, chitin, and keratin.][
Some chytrids live and grow on pollen by attaching threadlike structures, called rhizoids, onto the pollen grains.][
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Fossil record
The earliest of chytrids are from the Scotland Rhynie chert, a Devonian-age lagerstätte with anatomical preservation of and fungi. Among the microfossils are chytrids preserved as on . These fossils closely resemble the modern genus Allomyces.
In fictional media
The novel (2007) features a species of chytrid that feeds on petroleum and oil-based products. In the story, the species is modified using nuclear radiation to increase the rate at which it feeds on oil. It is then used by Islamic extremists in an attempt to destroy the world's oil supplies, thereby taking away the technological advantage of the United States.
External links
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— Includes links on how to isolate and culture chytrids.
