Chytridiomycota

Chytridiomycota is a phylum of zoosporic organisms within the kingdom fungi, they are also known as Chytrids. The first recognizable Chytrids appeared in the late Precambrian period, over 500 MYA.[1][11] They were first recorded in 1886 and were described as a simple vestical like structure which penetrates its host with a root-like appendage. They are thought to be the oldest and most basal group of fungi.[6] There is considerable variation in morphology and ecology within the phylum. There are approximately 1000 described species.[7]

Spizellomyces palustris, a monocentric, eucarpic chytrid. From Chen, et al. (2000).

Taxonomy

This Phylum is one of the five true phyla of Fungi, they are the simplest of the true fungi. [1] The phylum contains only one class, Chytridiomycetes.[1][6] Within that class are five different orders Blastocladiales, Chytridialis, Monoblepharidales, Neocallimastigales, and Spizellomycetales.[6]

Characteristics

Chytrids are primarily aquatic but there are species that are considered terrestrial.[1] Most Chytrids are unicellular but certain species form multicellular organisms or hyphae.[1] When hyphae are formed there are no septa between cells. Species are capable of both sexual and asexual reproduction. A defining characteristic of Chytrid fungi is their gamete cells have a flagellum, they are the only fungi known to have this in any life stage.[1][3][7] Like all fungi, their cell walls contain chitin, but one unique group contains both cellulose and chitin.[1][6] The sporangia is one of the most prominent morphological traits of adult Chytridiomycota. The sporangia is a sac-like structure in which internal divisions of the protoplasm result in the production of zoospores.[7]

Life cycle

The life cycle varies considerably between species and their ecology. Reproduction is generally asexual, but there are exceptions. Reproduction occurs through mitosis resulting in the production of one more zoospore. The zoospore is motile and requires water in this stage for survival and dispersal.[6] Sexual reproduction involves motile gametes of opposite sexes with different sizes and colorations that attract and swim towards each other through pheromone signaling. Once an appropriate niche is found the Chytrid will encyst, creating a fungal cell wall. The chytrid life cycle includes a thallus with a cell wall and hyphal-like feeding structure known as a rhizoid. Some species are capable of developing true hyphae.[10] Microsporidia is contained within Chytridiomycota, despite the loss of their zoospore stage, as a result of phylogenetic analysis.[8][10]

Ecology

Some Chytridiomycota are saprobic and others are parasitic, famously to amphibians.[1] Many of the most well-studied chytrids are parasitic, this became especially true after Batrachochytrium dendrobatidis was found to be the causative agent of chytridiomycosis in amphibians. This disease has led to a global decline in amphibian populations. [2][3] Chytrids are also commonly parasitic to the roots of plants. They are important vectors of viruses in plants as they have been known to cause serious damage to roots opening the plant up to opportunistic infections.[2][7] They are also a know algal parasite, an interaction that has been studied relatively little.[6] Chytrids have been found to play an important role in the gut of many mammals, forming a mutualism.[2][11]

Roll in Soil

Chytrid species are often saprobic, serving as decomposers in their environments. In most terrestrial environments Chytrids are thought to be primarily obligate parasites in a host of vascular plants.[7] Due to their aquatic nature Chytrids have traditionally been considered to play little role in soil processes. There is increasing evidence that in periglacial soils chytrid fungi make up 70% fungal diversity and 30% of eukaryotic diversity at high elevations. This is only true in areas unvegetated and at high altitudes. Chytrids in these areas rely on snowmelt for dispersal. They make up the main decomposers for the photosynthetic microbial food chains that exist in the highest altitudes.[5]

Chytridiomycosis

Batrachochytrium dendrobatidis was previously the only known instance of chytrid fungi being paracytic to vertebrates.[7] Currently, there are four strains of Batrachochytrium dendrobatidis with one being globally distributed. The East Asian strain was found to share the most genetic diversity with the global strain suggesting this as the origin of the parasite. Batrachochytrium dendrobatidis was later found to be endemic in the area further supporting this theory.[4]

Known to infect over 350 species of amphibians, though frogs seem to be most severely impacted. Zoospores infect the keratin layer of the skin, resulting in excessive skin shedding. The earliest symptoms are anorexia and lethargy. Infection may eventually lead to death or secondary infections. Treatment of wild frogs is currently impossible. [9]

References

[1] 24.3A: Chytridiomycota: The Chytrids. 2018, July 15. . https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/24%3A_Fungi/24.3%3A_Classifications_of_Fungi/24.3A%3A_Chytridiomycota%3A_The_Chytrids.

[2] Chytridiomycota. (n.d.). . http://website.nbm-mnb.ca/mycologywebpages/NaturalHistoryOfFungi/Chytridiomycota.html.

[3] Chytridiomycota | phylum of fungi. (n.d.). . https://www.britannica.com/science/Chytridiomycota.

[4] Fisher, M. C., and T. W. J. Garner. 2020. Chytrid fungi and global amphibian declines. Nature Reviews Microbiology 18:332–343.

[5] Freeman, K. R., A. P. Martin, D. Karki, R. C. Lynch, M. S. Mitter, A. F. Meyer, J. E. Longcore, D. R. Simmons, and S. K. Schmidt. 2009. Evidence that chytrids dominate fungal communities in high-elevation soils. Proceedings of the National Academy of Sciences 106:18315–18320.

[6] Ibelings, B. W., A. D. Bruin, M. Kagami, M. Rijkeboer, M. Brehm, and E. V. Donk. 2004. Host Parasite Interactions Between Freshwater Phytoplankton and Chytrid Fungi (chytridiomycota)1. Journal of Phycology 40:437–453..

[7] James, T. Y., P. M. Letcher, J. E. Longcore, S. E. Mozley-Standridge, D. Porter, M. J. Powell, G. W. Griffith, and R. Vilgalys. 2006b. A Molecular Phylogeny of the Flagellated Fungi (Chytridiomycota) and Description of a New Phylum (Blastocladiomycota). Mycologia 98:860–871.

[8] James, T. Y., A. Pelin, L. Bonen, S. Ahrendt, D. Sain, N. Corradi, and J. E. Stajich. 2013. Shared Signatures of Parasitism and Phylogenomics Unite Cryptomycota and Microsporidia. Current Biology 23:1548–1553.

[9] jlp342. 2018, March 21. Chytridiomycosis. Text. https://cwhl.vet.cornell.edu/disease/chytridiomycosis.

[10] Medina, E. M., and N. E. Buchler. 2020. Chytrid fungi. Current Biology 30:R516–R520.

[11] Taylor, Thomas N., et al. 2014. Fossil Fungi, Elsevier Science & Technology. ProQuest Ebook Central, https://ebookcentral-proquest-com.gate.lib.buffalo.edu/lib/buffalo/detail.action?docID=1774309.