Hibbett et al. (2007)
Genera incertae sedis
Chytridiomycota is a division of the kingdom Fungi. The name is derived from the Greek chytridion, meaning "little pot", describing the structure containing unreleased zoospores. In older classifications, chytrids (except the recently established order Spizellomycetales) were placed in the class Phycomycetes under the subdivision Myxomycophyta of the kingdom Fungi. At another time, they were placed in the Mastigomycotina as the class Chytridiomycetes. Also, in an older and more restricted sense (not used here), the term "chytrids" referred just to those fungi in the order Chytridiales. Here, the term “chytrid” will refer only to members of Chytridiomycota. The chytrids have also been included among the Protista, but are now regularly classed as fungi. Chytrids are one of the early diverging fungal lineages and are saprobic, degrading refractory materials such as chitin and keratin, or acting as parasites. Their membership in kingdom Fungi is demonstrated with chitin cell walls, a posterior whiplash flagellum, absorptive nutrition, use of glycogen as an energy storage compound, and synthesis of lysine by the α-amino adipic acid (AAA) pathway. There are approximately over 750 chytrid species distributed among 7 orders.
There has been a significant increase in the research of chytrids since the discovery of ‘’Batrachochytrium dendrobatidis’’, the casual agent of chytridiomycosis. This disease has also captured the public’s attention. The public’s awareness may be helped by or resulted in Tom Clancy's Splinter Cell: Fallout (2007), a novel by Tom Clancy that features a species of the chytridiomycota fungus that feeds on petroleum and oil-based products. The species is enhanced using nuclear radiation, increasing its survivability and rate of feeding. 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.
Life Cycle & Body Plan
Chytridiomycota is unique among the Fungi in that it reproduces by zoospores; there are few other fungi that reproduce with this method. For most members of Chytridiomycetes, sexual reproduction is not known. Asexual reproduction occurs through the release of zoospores (presumably) derived through mitosis. For those members of Chytridiomycetes that do undergo sexual reproduction, it is generally accepted that the resulting zygote forms a resting stage termed a resting spore, which functions as a means of surviving adverse conditions , and the methods of sexual reproduction are varied. In some members, sexual reproduction is achieved through the fusion of isogametes (that is, gametes of the same size and shape). This group includes the notable plant pathogens Synchytrium. In some algal parasites, 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, reproduction occurs through the rhizoids. 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. Sexual reproduction is common and well known among members of the Monblepharidomycetes. Typically, these chytrid practice a version of oogamy, wherein the male is motile and the female is stationary. This is the first occurrence of oogamy in kingdom Fungi.  Briefly, the monoblephs form oogonia, which give rise to eggs, and antheridia, which give rise to male gametes. Once fertilized, the zygote either becomes an encysted or motile oospore. which ultimately becomes a resting spore that will later germinate and give rise to new zoosporangia. Upon release, 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 produce a new thallus. Thalli are coenocytic and usually form no true mycelium (having rhizoids instead). Chytrids have several different growth patterns. Some are holocarpic, which means they only produce a zoosporangium and zoospores. Others are eucarpic, meaning they produce other structures, such as rhizoids, 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. Growth continues until a new batch of zoospores are 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.
Brief Taxonomic History
Species of Chytridiomycota have traditionally been delineated and classified based on the development, morphology, substrate, and method of zoospore discharge. However, several in the field showed that there is a great amount of variation in many these features within single spore isolates (or isogenic lines) and so these features cannot be used to reliably classify or identify a species. Today, systematics in Chytridiomycota is based on molecular data, zoospore ultrastructure and some aspects of thallus morphology and development.
Due to their reliance on a flagellum, all chytrids are aquatic, though those that thrive in the capillary network around soil particles are typically considered terrestrial. However, the zoospore is primarily a means of thoroughly exploring a small volume of water for a suitable substrate rather than a means of long range dispersal. 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, arctic and Antarctic soils. This has lead to the belief that many chytrid species are ubiquitous and cosmopolitan. However, recent taxonomic work has demonstrated that this ubiquitous and cosmopolitan morphospecies hide cryptic diversity at the genetic and ultrastructural levels. One of the least expected terrestrial environments the chytrid thrive in are periglacial soils. 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. It was first thought aquatic chytrids (and other zoosporic fungi) were primarily active in fall, winter, and spring. However, recent molecular inventories of lakes during the summer indicate that chytrids are an active, diverse part of the eukaryotic microbial community.
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. 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.
Chytrids mainly infect algae and other eukaryotic and prokaryotic microbes. The infection can be so severe as to control primary production within the lake. It has been suggested that parasitic chytrids have a large effect on lake and pond food webs. Chytrids may also infect plant species; in particular, Synchytrium endobioticum is an important potato pathogen.
Arguable, the most important ecological function performed by chytrids is decomposition. These ubiquitous and cosmopolitan organisms are responsible for decomposition of refractory materials, such as pollen, cellulose,chitin, and keratin. There are also chytrids that live and grow on pollen by attaching threadlike structures, the called rhizoids, onto the pollen grains. 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 accumulate pollen falling from trees and plants.
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. 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. 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.
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|Wikimedia Commons has media related to Chytridiomycota.|
- Chytrid Fungi Online: by the University of Alabama
- Longcore Lab from University of Maine--Including links on how to isolate and culture chytrids
- 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.
- Chytridiomycota at the US National Library of Medicine Medical Subject Headings (MeSH)