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Chytridiomycota

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Chytridiomycota
Temporal range: Devonian–Recent
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Fungi
Division: Chytridiomycota
M.J. Powell 2007[1]
Type species
Chytridium spp.
A. Braun, 1851

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.

Many chytrids are aquatic (mostly found in fresh water). There are approximately 1,000 chytrid species, in 127 genera, distributed among 5 orders.

Evolutionary History of Chytrid Fungus Batrachochytrium dendrobatidis

Understanding the evolutionary history of Emerging Infectious Diseases (EID's) is critical to understanding the origin and spread of EID's and focusing disease mitigating efforts. Studies sequencing the genome of Batrachochytrium dendrobatidis(Bd) partially answer a lot of important questions. 29 isolates for this study combined with 49 global isolates used in another study and then rooted with a non-Bd chytrid (Homolaphlyctis polyrhiza, Hp)allowed the group to infer the branching evolutionary event. The tree topologies indicate far greater genetic and evolutionary diversity in Bd then previous recognized. the study concludes that Bd is composed of multiple divergent lineages and that the emergence of the globally distributed GPL occurred within the context of deeper ancestral diversity, much of which may yet be discovered. [2]

Two hypotheses have been suggested to describe this pathogen: novel or endemic. This study suggests that it may not be one or the other but both. Some of the isolates did share a common geography and were closely related suggesting that this is not ancient geographic structure or long-term association but more likely, invasion and subsequent evolution of clones in a particular region. However, there are lineages present that diverged before the GPL (global panzootic lineage). GPL is a term used to describe similar, geographically widespread lineages. The fact that there are lineages present that diverged before the GPL suggests that Bd is far more ancient than represented by the novel pathogen hypothesis which suggests that Bd is endemic in some parts of the world. So it is very possible that Bd is both novel and endemic at the same time. [2]

Origins of Chytrid Fungus Batrachochytrium dendrobatidis

Genome sequencing studies are unable to pinpoint an exact geographic location as the study uses a non-Bd chytrid as a root. However, A Brazilian isolate collected from Hylodes ornatus from São Paulo, and one isolate collected from a bullfrog in a US market that likely traces back to Brazil suggest that Brazil seems to contain the more ancient Bd variations. [2]

Dating analysis has been done to estimate an approximate date for the original emergence of Bd. The study applied a molecular clock to two key nodes (the base of the Bd tree and the common ancestor of the GPL) to determine the age. The study cautions however, that this dating, without the use of fossil record should not be used as an exact date but as an estimate. The study determined that the base of the Bd tree is approximately 104,700 y B.P., and the common ancestor to the GPL is approximately 26,400 y B.P.[2]

Reproduction

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 (itself commonly known as "Chytrid"[3]) 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.[4] 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. [5]

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

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.[6] 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. [7] 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. [8] 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. What is interesting is how this was done. 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..."[9]

Treatment

Experiments were done on a variety of different species with a variety of different approaches trying to discover a viable treatment option. The first experiment was conducted on metamorphosed Bufo bufo with antimicrobial peptides on five separate groups under different conditions. Though it was determined that the peptide treatment did not increase survival rate overall, one particular group did show some signs that the idea shouldn’t be abandoned. This group of three was treated with the peptide first and then exposed to Batrachochytrium dendrobatidis and upon retesting at the end of the study, all three tested negative for Bd. [10]

Itraconazol was used in two studies. The first experiment was conducted on Rana muscosa that were extracted from Sixty Lake Basin in the Sierra Nevada Mountains of California, USA. These frogs were all in advanced stages of infection and itraconazol was deemed ineffective. However, it was effective in another study combining Itraconazol with heat (35 degrees C). Lithobates pipiens showed success with the combination of therapies with four of four previously infected frogs in two separate groups tested completely negative and only one of seven uninfected frogs became infected after treatment.Though these tests did not develop an exact treatment, they are instrumental in developing treatment protocols and materials that should continue to be tested in the future. [11]

References

  1. ^ Hibbett, D.S.; et al. (2007). "A higher level phylogenetic classification of the Fungi". Mycol. Res. 111 (5): 509–547. doi:10.1016/j.mycres.2007.03.004. PMID 17572334. {{cite journal}}: Explicit use of et al. in: |author= (help); Unknown parameter |month= ignored (help)
  2. ^ a b c d Rosenblum, E. E. (n.d). Complex history of the amphibian-killing chytrid fungus revealed with genome resequencing data. 110(23), 9385-9391.
  3. ^ "Stories: Reporting from the William Allen White School of Journalism and Mass Communications". Reporting.journalism.ku.edu. Retrieved 2013-10-24.
  4. ^ Saving Tiny Toads Without a Home, by Cornelia Dean. The New York Times. February 1, 2010.
  5. ^ (Voyles, J., L. Berger, S. Young, et al. 2007. Electrolyte depletion and osmotic imbalance in amphibians with chytridiomycosis. Dis Aquat Organ. 77: 113-118.)
  6. ^ Taylor, T.N. (1994). "Allomyces in the Devonian". Nature. 367 (6464): 601. doi:10.1038/367601a0. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ Krings, Micheal (2009). "Microfungi from the upper Visean (Mississippian) of central France: Chytridiomycota and chytrid-like remains of uncertain affinity". Review of Palaeobotany and Palynology. 156 (3–4): 319–328. doi:10.1016/j.revpalbo.2009.03.011. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ Krings, Micheal (2009). "Chytrid-like microfungi in Biscalitheca cf. musata (Zygopteridales) from the Upper Pennsylvanian Grand-Croix cherts (Saint-Etienne Basin, France)". Review of Palaeobotany and Palynology. 157 (3–4): 309–316. doi:10.1016/j.revpalbo.2009.06.001. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  9. ^ Crew, F. (1939). Biological Pregnancy Diagnosis Tests: A Comparison Of The Rabbit, The Mouse, And The "Clawed Toad" (Xenopus Laevis) As the Experimental Animal. British Medical Journal, 766-770.
  10. ^ Woodhams, D.C. (2012). "Treatment of amphibians infected with chytrid fungus: Learning from failed trials with itraconazole, antimicrobial peptides, bacteria, and heat therapy". Diseases of Aguatic Organisms. 98 (1): 11–25. {{cite journal}}: |access-date= requires |url= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  11. ^ Woodhams, D. C., Geiger, C. C., Reinert, L. K., Rollins-Smith, L., Lam, B., Harris, R. N., . . . Voyles, J. (2012). Treatment of amphibians infected with chytrid fungus: Learning from failed trials with itraconazole, antimicrobial peptides, bacteria, and heat therapy. Diseases of Aquatic Organisms, 98(1), 11-25. doi:http://dx.doi.org/10.3354/dao02429
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