Chytridiomycota
Chytridiomycota commonly referred to as Chytrid(s), is a phylum of zoosporic organisms within the fungi kingdom. Thought to be the oldest and most basal group of fungi,[6] Chytrids can be identified in substrate from the late Precambrian period over 500 MYA.[1] Though among the most ancient known fungi, Chytrids were first recorded in 1886 as vesicular structures which use prominent root-like appendages to anchor onto their desired substrate.[11] There is considerable variation in the morphological and ecological make-up of the phylum; this is exemplified by the approximately 1000 combined described species which inhabit both marine and, to a lesser extent, terrestrial substrates.[7]
Taxonomy
Chytridiomycota, one of the five total phyla of true fungi, make up the entirety of their own phylum.[1] Though Chytrids are the simplest of the true fungi, the phylum resolves into five distinct orders which include the Blastocladiles, Chytridialis, Monoblepharidales, Neocallimastigales, and Spizellomy cetales.[6]
Characteristics
Chytridiomycota reside in a wide range of aquatic habitats, however, a few species are considered to be terrestrial.[1] Like all fungi, Chytridiomycota contain cell walls made of chitin, however, the hyphochytrid subgroup represents the common exception as their cell walls utilize cellulose.[1][6] Chytrids can further be delineated by their cellularity, with most being unicellular while a select few can be considered multicellular due to their production of hyphae.[1] Though technically hyphae, the chytrid’s structures do not contain the typical cell divisions created by internal walls called septa (singular septum), which allow for the transfer of organelles across membranes. The most prominent morphological trait of adult Chytridiomycota is the sac-like structure of the sporangia; in which internal divisions of the protoplasm result in zoospore production. Fungi within the phylum display an additional defining characteristic unseen in all other known fungi of any life stage, a flagellum which is used by their zoospores.[1][3][7]
Life cycle
Chytridiomycota reproduction is generally asexual, though notable exceptions occur between species and their relative ecological niches. Asexual reproduction takes the form of mitotic divisions and the subsequent production of their motile, water-dependent zoospore. Sexual reproduction utilizes pheromone signaling to attract variably sized and colored gametes for conjugation. Further chytrid development relies upon the occupation of their desired environmental niche, upon which the individual will encyst and begin creating its cell wall. Development after the process of encysting is dependent upon the individual species, however, a few generalizations can be made. Growth will stimulate the formation of a thallus (the nondifferentiated cell and its wall) and either the hyphae-like anchoring appendage called a rhizoid or permeable true hyphae.[10] Parasitic Chytridiomycota will produce and contain microsporidia, despite the loss of their zoospore stage. [8][10]
Ecology
Chytridiomycota are either saprobic or parasitic depending on their substrate, aquatic or terrestrial respectively.[1] The parasitic relationships formed by Chytrids are highlighted by Batrachochytrium dendrobatidis, also known as Bd, which is the causative agent of chytridiomycosis in amphibians. Chytridiomycosis results in malformed skin which inhibits respiration and increases mortality which 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 and algae, as they have been known to cause serious damage which can expose the host to opportunistic infections.[2][7] Chytrids have been found to play an important role in the gut of many mammals, forming a proportionally rare mutualistic relationship.[2][11]
Role in Soil
Aquatic Chytrid species are saprobic, serving as decomposers in their environments. Terrestrial Chytrids are primarily thought to be obligate parasites in hosts of vascular plants, or more rarely, algae.[7] Due to the aquatic nature of the majority of Chytrids, the phylum has traditionally been considered to play little role in soil processes. There is increasing evidence that in periglacial soils chytrid fungi can make up 70% of fungal diversity and 30% of eukaryotic diversity; though this has shown to only be true in unvegetated areas at high altitudes. Chytrids in these high elevation areas make up the majority of decomposers for the photosynthetic microbial food chain, further relying upon snowmelt for dispersal and proliferation.[5]
Chytridiomycosis
Batrachochytrium dendrobatidis (Bd) was previously the only known instance of parasitic chytrid fungi within vertebrates.[7]. Batrachochytrium dendrobatidis is known to infect over 350 species of amphibians, though frogs appear to be among the most severely impacted. Chytridiomycosis occurs when Zoospores infect the keratin layer of the skin, resulting in excessive skin shedding. The earliest symptoms are anorexia and lethargy, though prolonged Infection may eventually lead to secondary infections and result in death of the host.[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.