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Chytrid

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Chytrid

Scientific classification
Kingdom: Fungi
Division: Chytridiomycota
Doweld (2001)
Classes/Orders

Chytridiomycota is a division of the kingdom Fungi. The name is derived from the Greek chytridion, meaning "little pot", describing the structure containing unreleased spores. In older classifications, chytrids (except the recently established order Spizellomycetales) were placed in the class Phycomycetes under the subdivision Myxomycophyta of the kingdom Fungi. Also, in an older and more restricted sense (not used here), the term "chytrids" referred just to those fungi in the order Chytridiales. The chytrids have also been included among the Protista, but are now regularly classed as fungi.

The chytrids are one of the early diverging lineages of the fungi and are mostly saprobic, degrading refractory materials such as chitin and keratin. The thalli are coenocytic and usually form no true mycelium (having rhizoids instead). Some species are unicellular. As with other fungi, the cell wall in chytrids is composed of chitin. There are approximately over 700 chytrid species, in 105 genera and 14 families, distributed among 4 orders.[1]

Habitats

Many chytrid species commonly thrive in aquatic environments because of their ability to get around in the water. During one of the phases of their life cycle, they go through asexual sporulation and transform into zoospores inside the sporangium during the motile stage of their life cycle and develop flagella which allow them to freely swim and move around in water. There are also chytrid that live and grow on pollen by attaching threadlike structures called rhizoids onto the pollen grains.[2] This mostly occurs during asexual reproduction because the zoospores that become attached to the pollen continuously reproduce and form new chytrids that will attach to other pollen grains for nutrients. This colonization of pollen happens during the spring time when bodies of water accumulates pollen falling from trees and plants. Even though chytrid are most commonly found in the water, there also prominent populations found in different kinds of soils. One of the least expected terrestrial environments the chytrid thrive in are periglacial soils.[3] Scientific research shows that because of the large amounts of water in the soil that becomes saturated and stored under the snow and specific carbon sources that is necessary for the growth of chydrids, the population of the Chytridiomycota species are able to be supported even though there is a lack of plant life in these frozen regions.

Reproduction

The reproductive nature of Chytridiomycota is unique in that it reproduces by zoospores (which are motile spores); this is rare because chytrids are the only fungi that reproduce with this method.[4] Both zoospores and gametes of the chytrids are mobile by their flagella, one whiplash per individual.

In Allomyces, the thallus (body) is attached by rhizoids, and has an erect trunk on which reproductive organs are formed at the end of branches. The life cycle has the ability to change from haploid and diploid generations. The haploid thallus forms male and female gametangia from which flagellated gametes are released and merge to form a Zygote. Gametes and female gametangia attract the opposite sex by producing pheromones. The germinated zygote produces a diploid thallus with two sorts of sporangia; thin-walled zoosporangia which release diploid zoospores resulting in a diploid thalli and thick-walled sporangia which after meiosis release haploid zoospores which form haploid thalli.

As a parasite

The chytrid Batrachochytrium dendrobatidis is responsible for a recently discovered disease of amphibians, chytridiomycosis. Discovered in 1998 in Australia and Panama this disease is known to kill amphibians in large numbers, and has been suggested as a principal cause for the worldwide amphibian decline. In one example an outbreak of the fungus was found responsible for killing much of the Kihansi Spray Toad population in its native habitat of Tanzania.[5] Another example is that it's also responsible for the extinction of the golden toad in 1989. The process leading to frog mortality is thought to be the loss of essential ions through pores made in the epidermal cells by the chytrid during its replication.[6]

Chytrids may also infect plant species; in particular, maize-attacking and alfalfa-attacking species have been described.[7] Synchytrium endobioticum is an important potato pathogen.[8]

Fossil record

The earliest fossils of chytrids are from the Scottish Rhynie chert, a Devonian-age locality with anatomical preservation of plants and fungi. Among the microfossils are chytrids preserved as parasites on rhyniophytes. These fossils closely resemble the genus Allomyces.[9] Holocarpic chytrid remains were found in cherts from Combres in central France that dates back to the late Visean. These remains were found along eucarpic remains and are ambiguous in nature though are thought to be chytrid remains.[10] Other chytrid-like fossils were found in cherts in Western Europe, especially France, dating between 300 and 350ma. Another such example is the chert from the upper Pennsylvanian in the Saint-Etienne Basin in France.[11] While none of the fossils found before the Permian system are specifically chytrids, their chytrid-like qualities give impressive insight into the evolutionary path of Fungi.

Spread of Chytrid Fungus Batrachochytrium dendrobatidis

It has been suggested that the amphibian killing chytrid fungus Batrachochytrium dendrobatidis was spread all over the world by the South African "Clawed Toad" Xenopus laevis. This fungus grows on the back of the Xenopus laevis[12] An article written in The British Medical Journal from 1939 outlines clearly how the "Clawed Toad" may be to blame. The article outline a comparison between rabbits, mice and the "Clawed Toad" (later classified as a frog) in use for pregnancy testing. Testing for pregnancy required exposure of the preferred animal to a woman's urine. If the urine contains human chorionic gonadotropin, or hCG which is the pregnancy hormone, the animal would display a specific reaction. In the case of mice the results may not be known for up to 5 days. The Friedman test was considered a "rapid" test using rabbits that would give results in 24 to 48 hours. The Hogben test using Xenopus laevis would yield test results in as little as 6 to 15 hours. "Impressed by the reports of Bellerby (1934) and of Shapiro and Zwarenstein (1934) concerning the usefulness of the "Clawed Toad" of South African (Xenopus laevis Daud.)as an experimental animal in biological tests for the diagnosis of pregnancy, I imported 1,500 well-grown females in 1937..."[13] "Crew encouraged New York gynaecologist Abner Weisman’s interest in importing Xenopus for pregnancy testing into the United States, and Weisman proved a tireless campaigner for the frog (Weisman and Coates, 1944). Other enthusiasts ensured that by the end of the War (World War II) there were Xenopus colonies in laboratories and clinics all over the world."[14]

References

External links

  • University of Alabama
  • Introduction to the Chytridiomycota: at the UC Museum of Paleontology
  • Impact of chytrid fungus on frogs (Foundation for National Parks & Wildlife)
  • 'Amphibian Ark' aims to save frogs from fungus
  • The Aquarium Wiki Encyclopaedia on Chytrid Fungus and how it impacts amphibians kept as domestic pets.
  • NIH
  • Medical Subject Headings (MeSH)
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