Chytridiomycota

Species of Chytridiomycota have traditionally been delineated and classified based on development, morphology, substrate, and method of zoospore discharge.[7][4] However, single spore isolates (or isogenic lines) display a great amount of variation in many of these features; thus, these features cannot be used to reliably classify or identify a species.[7][4][8] Currently, taxonomy in Chytridiomycota is based on molecular data, zoospore ultrastructure and some aspects of thallus morphology and development.[7][8]

In an older and more restricted sense (not used here), the term "chytrids" referred just to those fungi in the class Chytridiomycetes. Here, the term "chytrid" refers to all members of Chytridiomycota.[2]

The chytrids have also been included among the Protoctista,[7] but are now regularly classed as fungi.

In older classifications, chytrids, except the recently established order Spizellomycetales, were placed in the class Phycomycetes under the subphylum Myxomycophyta of the kingdom Fungi. Previously, they were placed in the Mastigomycotina as the class Chytridiomycetes.[9] The other classes of the Mastigomycotina, the Hyphochytriomycetes and Oomycetes, were removed from the fungi to be classified as heterokont pseudofungi.[10]

The class Chytridiomycetes has over 750 chytrid species distributed among ten orders.[11][12][13] Additional classes include the Monoblepharidomycetes,[14] with two orders, and the Hyaloraphidiomycetes with a single order.[15]

Molecular phylogenetics, and other techniques such as ultrastructure analysis, has greatly increased the understanding of chytrid phylogeny, and led to the formation of several new zoosporic fungal phyla:

• The order Blastocladiales, originally within the Chytridiomycota, are now classified as a separate phylum, the Blastocladiomycota.[16]
• The Neocallimastigales, originally an order of anaerobic fungi of the class Chytridiomycetes, found in the digestive tracts of herbivores, was later raised to a separate phylum, the Neocallimastigomycota.[15]
• The Olpidiaceae, including the type genus Olpidium, formerly classified in the order Chytridiales, were raised to a separate phylum, the Olpidiomycota.[17]

Chytridiomycota are unusual among the Fungi in that they reproduce with zoospores.[4][18] For most members of Chytridiomycetes, sexual reproduction is not known. Asexual reproduction occurs through the release of zoospores (presumably) derived through mitosis.[4]

Where it has been described, sexual reproduction of Chytridomycetes occurs via a variety of methods. It is generally accepted that the resulting zygote forms a resting spore, which functions as a means of surviving adverse conditions.[4] In some members, sexual reproduction is achieved through the fusion of isogametes (gametes of the same size and shape). This group includes the notable plant pathogens Synchytrium. Some algal parasites practice oogamy: 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.[4] In the last group, 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.[2]

Sexual reproduction is common and well known among members of the Monoblepharidomycetes. Typically, these chytrids practice a version of oogamy: the male is motile and the female is stationary. This is the first occurrence of oogamy in kingdom Fungi.[3] 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,[4] which ultimately becomes a resting spore that will later germinate and give rise to new zoosporangia.[3]

• 
  Life cycle of Batrachochytrium dendrobatidis
• 
  Life cycle of Synchytrium endobioticum in potato
• 
  Synchytrium endobioticum on potatoes.

Upon release from the germinated resting spore, 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.[3]

• 
  Types of chytrid thalli
• 
  Zoospore discharge from Phylctochytrium sp.
• 
  Sporangium and zoospores of the chytrid fungus B. dendrobatidis, under SEM

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.[4]

Chytrids are aquatic fungi, though those that thrive in the capillary network around soil particles are typically considered terrestrial.[7][4] 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.[19]

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.[7][4] This has led to the belief that many chytrid species are ubiquitous and cosmopolitan.[7][4] However, recent taxonomic work has demonstrated that this ubiquitous and cosmopolitan morphospecies hide cryptic diversity at the genetic and ultrastructural levels.[20][21] It was first thought aquatic chytrids (and other zoosporic fungi) were primarily active in fall, winter, and spring.[4] However, recent molecular inventories of lakes during the summer indicate that chytrids are an active, diverse part of the eukaryotic microbial community.[22]

One of the least expected terrestrial environments the chytrid thrive in are periglacial soils.[23] The population of the Chytridiomycota species are able to be supported even though there is a lack of plant life in these frozen regions due to the large amounts of water in periglacial soil and pollen blowing up from below the timberline.

The earliest fossils of chytrids are from the Scottish Rhynie chert, a Devonian-age lagerstätte with anatomical preservation of plants and fungi. Among the microfossils are chytrids preserved as parasites on rhyniophytes. These fossils closely resemble the modern genus Allomyces.[35] Holocarpic chytrid remains were found in cherts from Combres in central France that date back to the late Visean. These remains were found along with eucarpic remains and are ambiguous in nature although they are thought to be of chytrids.[36] Other chytrid-like fossils were found in cherts from the upper Pennsylvanian in the Saint-Etienne Basin in France, dating between 300 and 350ma.[37]

The novel Tom Clancy's Splinter Cell: Fallout (2007) features a species of chytrid that feeds on petroleum and oil-based products. In the story the species is modified using nuclear radiation, to increase the rate at which it feeds on oil. 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.[38]

The webcomic Black Mudpuppy by Ethan Kocak features an arc with an evil, anthropomorphic chytrid developed as a Nazi experiment to infect Xolotl, a giant, talking black axolotl.[39]

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)