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Chytridiomycota[1]
Chytridiomycota[1]

Contents

[edit] Chytrids and their role in the food web

[edit] Introduction

Chytrids are a unique phylum within the kingdom Fungi; there are over 1000 species all of which are found in a wide range of locations around the world, anywhere from the arctic to the tropics (Moore et al., 2011). They are the only fungi phyla which produce motile spores called zoospores which have flagella (James et al., 2006). Because their zoospores are motile, the Chytrids are required to live in aquatic environments, commonly streams, ponds, estuaries and other marine systems, the majority of Chytrids live in moist terrestrial environments such as forests, agricultural soils and bogs (Moore et al., 2011). In the past, they were not considered to be true fungi because of their zoospores, but because of DNA analyses they have been accepted into the kingdom Fungi (Hibbett et al., 2007). They are classified into 5 orders based on their morphology, reproductive methods, habitats, life cycles and other defining characteristics (James et al., 2006). Those 5 orders are: Chytridiales, Blastocladiales, Monoblepharidales, Spizellomycetales and Neocallimastigales. It was previously thought (based on physical characteristics) that there was a single lineage for the Chytrids but because of recent molecular data, there are now thought to be 4 separate lineages, for this reason the Chytrids are not considered to be a monophyletic group. The Blastocladiales and Neocallimastigales which were previously thought to be orders were promoted to phyla status with recent scientific findings (Moore et al., 2011). Each of the Chytrid groupings has defining characteristics which distinguish them from the other groupings. The Chytridiales are the largest grouping, they reproduce using zygotic meiosis and they have an operculum on their zoospores (James et al., 2006). The Blastocladiales reproduce using sporic meiosis and exhibit alternation of sporophytic and gametophytic reproductive generations. The Monoblepharidales reproduce using oogamous sexual reproduction. Each genus of the Spizellomycetales has distinctive zoospore structures. The Neocallimastigales are anaerobic symbionts of various herbivores’ digestive tracts. Although a lot of discoveries concerning Chytrids have been made in the recent history, there is a lot more not known about them and they are proving to be an area within biology with a lot of potential with further research.

Chytrids play an important role in the food web, especially in fresh water ecosystems.
Chytrid Food Web:Generalized food web of a fresh water catchment containing chytrid zoospores and plankton. (Gleason et al. 2008)
Chytrid Food Web:Generalized food web of a fresh water catchment containing chytrid zoospores and plankton. (Gleason et al. 2008)
The five main roles of chytrids as described by Gleason et al. (2008) are as follows: chytrid zoospores are a good food source for zooplankton, chytrids decompose particulate organic matter, chytrids are parasites of aquatic plants, chytrids are parasites of aquatic animals, and chytrids convert inorganic compounds into organic compounds. These are important factors as each of these specific situations affect the ecosystem greatly. Chytrid zoospores are a great source of nutrition and are a good size (2-3 μm in diameter) for feeding zooplankton (Gleason et al. 2008). They also contain some organic compounds such as nitrogen, phosphorous, and sulphur, important for supplying vitamins to the zooplankton. Chytrids are also rich in polyunstaturated fatty acids and have a high concentration of cholesterol (Kagami et al 2007). As the chytrid zoospores are ingested, their nutrients are transferred up the food chain to higher trophic levels. Chytrids are able to decompose particulate organic matter such as chitin, cellulose, and protein found in snake and hair (Gleason et al. 2008). This is important because these molecules are hard to digest or virtually indigestible by other organisms. With the help of chytrids, these molecules can become dissolved organic matter and thus be taken up by organisms higher up the trophic levels. Chytrids can be parasites of both phytoplankton and vascular plants. They help control seasonal successions of phytoplankton species (Gleason et al. 2008). Chytrids are also parasites of aquatic invertebrates such as rotifers, nematodes, and mites. They help with the regulation of populations in these environments. The fact that chytrids can convert inorganic compounds into organic compounds is essential in the food chain because some of these inorganic compounds are not usable in their inorganic state. For example, there are insoluble states of phosphorous that need to be broken down by chytrids before they can be absorbed. Organic compounds made by chytrids are then able to be used by other organisms in the ecosystem. After introducing the five main roles of chytrids and their importance in the food web, the processes they are involved in, their variety of species, and their evolutionary history will be further discussed in more detail.
[edit] Chytrid Life Cycle
Considering the processes involved in chytrid formation and reproduction, it has been discovered that the Chytridiomycota are not monophyletic, but are polyphyletic, due to analyses of SSU rDNA (Nagahama et al 1995;Blackwell 2004). A typical chytrid posesses a flagellated cell (or zoospore/gametes) during their life cycle which is a plesiomorphic trait (James et al;2000). The overall life cycle lasts approximately two weeks, and grow typically best in moist, warm conditions. They produce asexually produce, then eventually release a single cellular zoospore, which travels through the use of its flagella. The zoospore will travel to find a food source in order to continually and cyclically begin another life cycle (Boynton, C. 2013).
Chytrid Life Cycle Diagram [2]
Chytrid Life Cycle Diagram [2]

[edit] Species involved

Chytrids contain 706 species in 105 genera. These species of chytrid can be found in aquatic environments such as streams, ponds and estuaries due to their inherited flagellated zoospores which are great for dispersal in water (Moore et al. 2011). Some species of chytrid are saprobes which feed on dead and rotting organic matter while others are parasites and live on plants or invertebrate animals. Batrachochytrium dendrobanditis, a relatively new species discovered in 1999, is the only known chytrid parasite of vertebrates. This parasite infects the skin of amphibians and causes chytridiomycosis, which is a fatal disease that has resulted in a major decline in the amphibian population.
Chytridomycosis; accountable factor in decline of frog population[3]
Chytridomycosis; accountable factor in decline of frog population[3]

[7] Batrachochytrium dendrobanditis is currently under study so as to help species of amphibian on the verge of extinction (Pessier, Allan. 2013). 060112_frog_climate_big.jpg

Another parasitic chytrid species is Olpidium brassicae. This species was one of the latest to diverge from the rest of the chytrids. Olpidium brassicae live within a host cell (endoparasitic) and affect plants such as melons of all kinds and gourds (Moore et al. 2011). No symptoms are left but these parasites serve as vectors in several soil-borne plant viruses. The largest genus of Chytridomycota are the Rhizophydium which are pathogens of plants. Upon germination some species of the Rhizophydium release zoospores from one or more pores, whereas other species release zoospores by having a large amount of the sporangial wall melt away.

Rozella[4]
Rozella[4]
Rozella is a diverse lineage of chytrid containing 22 different species. Like Oldipium this related species is endoparasitic. Unlike Oldipium, Rozella appears to be one of the very earliest lineages to diverge from the rest of the chytrids (James et al. 2006). Rozella was circumscribed by French mycologist Marie Maxime Cornu in 1872.

Blastocladiales diverged from the the core chytrids and has a life cycle with sporic meiosis whereas most core chytrids have zygotic meiosis. Blastocladiales carry out asexual reproduction by thick-walled resting spores that produce zoospores upon germination. Three of the five families within Blastocladiales are pathogenic and affect nematodes, mosquito larvae and many aquatic plants (James T.Y et al. 2006). One of the saprobic chytrid related species from the phylum Neocallimastigomycota includes Neocallimastix spp. which reside in the rumen of herbivores and are active in cellulose degradation.

Synchytrium endobioticum also known as potato wart disease; these warts reduce edibility and therefore crop yield (Surman, W. 2010). In the year 2000 Synchytrium endobioticum was the disease responsible for the quarantine of the P.E.I potato crop. Potato wart is a serious disease of cultivated potato that has been detected worldwide, but generally with limited distribution due to stringent quarantine and regulatory measures. In Canada, it has been found in Newfoundland in home gardens since the early 1900’s. It was also discovered in Prince Edward Island in 2000, where it remains under regulatory control by the Canadian Food Inspection Agency (CFIA). For more information on the Potato Wart Disease click on the following link. [8]

[edit] Processes involved

Various Forms of Reproduction in Chytrids - Asexually

Asexual reproduction in chytrids is typically done through (1) bud formation, (2) fragmentation and (3) sporulation.

Example of bud formation, fragmentation and sporulation in yeasts [5]
Example of bud formation, fragmentation and sporulation in yeasts [5]

(1) Bud Formation

Simply formed, asexual reproduction can be endured through the process of budding of binary fission. The initial stages of budding occur with the hardening of chitin around the cell wall, which is where the bud is going to appear, and eventually bud off the original cell. Turgor pressure acts as a release mechanism, so that the bud can eventually fall off grow onward from its initial formation state (MicrobiologyBytes, 2007).

(2) Fragmentation

Secondary option of asexual reproduction can be described as fragmentation. It is that any mycelium which undergoes a disruption of its ultimate growth size and initiates its own growth colony. The division of the hyphal fungal tip fragment, do not regenerate, however the branches can, with some form of negative damage (MicrobiologyBytes, 2007)

(3) Sporulation

This is the most crucial form of asexual reproduction which chytrids use. The asexual spores are formed on a phase of fungal life cycle which is mitosporic or anamorphic (MicrobiologyBytes, 2007).


Asexual Reproduction

Chytrids have a simple thallus and motile zoospores, making them distinct form other fungi. The structure is very simple and consists of a single cell, which consists of rhizoids that help to anchor it onto a substrate. Motile zoopsores are produced by the chytrids in order for asexual reproduction to occur. The zoospores have a single, posterior flagellum in sporangia. Chytrids are normally holocarpic meaning that the entire thallus functions as sporangium. A holocarpic chytrid has a thallus that consists of only one cell with rhizoids. They are usually parasitic on aquatic plants or fish. The fungus "feeds" from its substrate directly through the rhizoids. A holocarpic chytrid's entire cell content will convert to motile zoospores. (MicrobiologyBytes, 2007)


Sexual Reproduction of Chytridiomycota [6]
Sexual Reproduction of Chytridiomycota [6]

Sexual Reproduction

Diploid spore production after the fusion of two different mating types is how sexual reproduction may occur in some members of the chytrids. A diploid mycelium or haploid mycelium may be produced. The spore may germinate to produce a diploid vegetative mycelium, or a haploid mycelium may be produced through meitotic division. Another way for a haploid vegetative mycelium to be produced is through having resting sporangia undergo meiosis. Then, the haploid zoospres may germinate to produce haploid mycelium. (MicrobiologyBytes, 2007)


Thallus Production

There are three main types of thallus production and growth in chytrids which include: (1) endogenous-monocentric, (2) exogenous-monocentric, and exogenous-polycentric. (1) Endogenous-monocentric: is when the development occurs when the zoospore of the nucleus stays within the encysted wall of the zoospore, which then undergoes mitosis. (2) Exogenous-monocentric: is the development occuring when the zoospore nucleus migrates toward the germ tube, and eventually evolves into it. It then undergoes mitosis. (3) Exogenous-polycentric: is the development which occurs when the zoospore nucleus migrates toward and eventually into the germ tube, undergoes mitosis, and then spreads into various locations. (Infolinks, 2012)


Metabolic Pathway of Chytrid [Moore et al. (2011). 21st century guidebook to fungi. New York, NY.]
Metabolic Pathway of Chytrid [Moore et al. (2011). 21st century guidebook to fungi. New York, NY.]

The α-aminoadipate Pathway for Lysine Biosynthesis

Chytrids use the α-aminoadipate pathway for lysine biosynthesis. The AAA pathway is only available to higher fungi like the chytrids. Enzymes involved in this pathway are unique specifically to enzyme synthesis. The first half of the process occurs in the mitochondrion where acetyl CoA and α-ketoglutarate for the AAA. Homocitrate is a product formed by the combination of acetyl CoA and α-ketoglutarate and is what drives this reaction. Conversion of homocitrate is catalyzed by the Homoaconitase (HAc), which is an enzyme that was the first link between the AAA pathway in fungal organisms. (Jones, 1996). The second half of the pathway, the aminotransferase is present in both the mitochondrion and the cytoplasm. Phosphopantetheinyl transferase (PPT) allows enzymes to be activated in order for adenylation and reduction of the AAA reductase to occur for the semialdehyde to be formed. Semialdehyde is then oxidized to produce aminoadipate semialdehyde, which is then reduced to form L-lysine. (Johansson et al, 2000). NADPH and NADH are important in this pathway because they are used as coenzymes. A high NADPH-to-NADH ratio would allow the synthesis of lysine from aminoadipate semialdehyde, whereas a low ratio would inhibit the synthesis, therefore production of lysine would not occur. The two separate steps in this pathway occur because of the levels of NADPH and ATP (adenosine triphosphate). The two separate processes better regulate the overall whole pathway. The AAA pathway is important because it is used for detecting and controlling pathogenic molds and yeasts. (West, 2006).


Metabolic Pathway of certain Chytrids

Chytrids occur in aquatic environments such as streams, ponds, estuaries and marine systems, living as parasites of algae and planktonic organisms. The majority of chytrids occur in terrestrial forest, agricultural and desert soils, and in acidic bogs as saprotrops on difficult-to-digest substrata like pollen grains, chitin, keratin and cellulose (Moore, Robson & Trinci, 2011). Anaerobic chytrids are the most economically important members of the group. They are also present in vast majority because they occur in the rumen and hindgut of many larger mammalian herbivores including all farm animals. Although they are morphologically similar to the other chytrids, differences are sufficient for them to be placed in their own phylum called Neocallimastigomycota. Anaerobic chytrids have a crucial role in the colonization and the enzymatic degradation of lignocellulose in plants eaten by herbivores. Therefore they are crucial to the evolution of herbivores and to the prosperity of animal husbandry. These chytrids are potent producers of enzymes needed to degrade cellulose. Their own carbon metabolism relies on fermentation of glucose to acetate, lactate, ethanol and hydrogen. They possess an organelle called a hydrogenosome that generates ATP and appears to be a degenerate mitochondrion lacking a genome (Moore, Robson & Trinci, 2011). The figure shows details of metabolism of anaerobic chytrids.

[edit] History of Chytrids

Taxonomy

Correct taxonomic classification of Chytrids has been the hot topic of many debates, puzzling scientists on whether they should be classified as true Fungi. Previous classification systems suggested that the absence of flagella was a requirement for classification within the Kingdom Fungi, and since some Chytrids have flagellated spores they were placed within the Protists, a previous taxonomic group which no longer exists (Bowman et al, 1992). This classification however, was always under much deliberation as Chytrids do contain chitin in their cell walls, and use glycogen as a storage molecule, all characteristics of true fungi (Moore et al, 2011). These characteristics in combination with recent DNA sequencing confirms that they are in fact true fungi but they are currently considered a very primitive member (Bowman et al, 1992 & Moore et al, 2011).

Evolution

Chytrid-like fungi fossils found in northern Russia from the Vendian Period are the oldest known fungi fossils (Taylor, et al., 2005). Other, older fossils, previously thought to be fungi, are now considered to be the remnants of filamentous cyanobacteria. Other older fossils that look like fungi are not distinctly classifiable, leaving chytrids as the oldest fungi found in the fossil record to date (Taylor, et al., 2005).

Chytrid fossils can be found, among members of some other major fungal groups, in the Devonian Rhynie Chert in Scotland (Hass et al., 1994). Some of these parasitic forms are closely related to modern Blastocladiales, now considered a "sister-phylum" of the Chytridiomycota (James et. al., 2006). Other chytrids found in the Rhynie Chert are similar to Spizellomycetales, demonstrating that chytrids had already diversified by the Devonian Period (Remy et al., 1994). Chytrids are important because they give us a view into the past. The evolutionary relationships studied between chytrids and other fungi indicate that chytrids are probably a lot like the ancestors of fungi (James et al., 2006; Pöggler et al., 2011). Based on the characteristics of chytrids, we can presume that the original fungi were aquatic and had flagellated gametes, a trait later fungi lost in their evolutionary journey. These traits were inherited from their protistan ancestors, though phylogenetic reconstructions have not made it clear whether the last common ancestor of Fungi was of marine or freshwater origins (James et al., 2006). There is also plenty of research confirming that Fungi and Metazoa, the two major multicellular eukaryotic lineages, are close relatives (Pöggler et al., 2011).

Due to the high host-specificity of chytrids it is suggested that they have co-evolved as a parasite with their hosts (Kagami, Bruin, Ibelings,VanDonk, 2007). It is suspected that throughout this evolutionary history specific hosts (such as zooplankton) have developed various defence strategies against these deadly parasites (Kagami, Bruin et al, 2007). Kagami, Bruin, Ibelings & VanDonk (2007) suggested that some of these aquired defenses might include such strategies as, genetic diversity, chemical responses, and several indirect defence mechanisms. However, despite this being an extremely interesting topic, detailed research including data collection is still being conducted in the attempt to have a complete understanding of these evolutionary relationships.

[edit] Definitions

Anaerobic symbiont: an organism that shares a symbiotic relationship with another organism, that is they associated in such a way that both organisms benefits from their interaction with each other.

Anamorph: production, or relation to the gradual evolution of a species, from one organism to another.

Alternation of generations: interchanging between haploid and diploid states.

Chytridiomycosis: is an infectious disease of amphibians, caused by the chytrid Batrachochytrium dendrobatidis.

Endoparasitic: living within a host cell.

Flagella: whip-like appendages used for movement.

Germination: the process whereby seeds or spores sprout and begin to grow.

Holocarpic: the entire thallus functions as sporangium.

Lineage: Direct descent from a particular ancestor; ancestry.

Mitospore: haploid or diploid spore produced through the process of mitosis

Monophyletic: a phylogenetic group which contains both the common ancestor of the group and all of its descendants.

Oogamous sexual reproduction: a method of sexual reproduction in which gametes (egg and sperm) join together and form offspring.

Operculum: a lid-like structure which encloses spores within the organism’s fruiting structure.

Saprobes: any organism, especially a fungus or bacterium, that lives and feeds on dead organic matter.

Sporangium: A single or multi-celled structure in which spores are produced.

Sporic meiosis: meiosis that occurs while spores are forming.

Zoospores: a spore of certain algae, fungi, and protozoans, capable of swimming by means of a flagellum.

Zygotic meiosis: meiosis that occurs while zygotes are undergoing germination.

[edit] References

  • Blackwell, M; et al. (2004) Biodiversity of Fungi: Inventory and Monitoring Methods. Hong Kong.
  • Bowman, B.H., Taylor, J.W., Brownlee, A.G., Lee, J., Lu, S.D., White, T.J. (1992). Molecular evolution of the fungi: relationship of the basidiomycetes, ascomycetes, and chytridiomycetes. Mol Biol Evol, 9(2):285-296
  • Boynton, C. 2013. Chytridiomycota life cycle and production.
  • Gleason, F., Kagami, M., Lefevre, E., Sime-Ngando, T. (2008). The Ecology of Chytrids in Aquatic Ecosystems: Roles in Food Web Dynamics. Fungal Biology Reviews. 22: 17-25
  • Hass, H., Taylor, T. N., & Remy, W. (1994). Fungi from the Lower Devonian Rhynie Chert: Mycoparasitism. American Journal Of Botany, 81(1), 29-37.
  • Held, A. A. (1972). Improved Culture Methods for Rozella and for Olpidiopsis. Mycologia, (4), 871. doi:10.2307/3757942
  • Hibbett et al. (2007). A higher-level phylogenetic classification of the Fungi. Mycological Research. 111: 509-547.
  • James, T. Y., Letcher, P. M., Longcore, J. E., Mozley-Standridge, S. E., Porter, D., Powell, M. J., & Vilgalys, R. (2006). A Molecular Phylogeny of the Flagellated Fungi (Chytridiomycota) and Description of a New Phylum (Blastocladiomycota). Mycologia, 98(6), 860-871
  • James, M et al (2000). Life Cycle Molecular Basis of Chytridiomycota. Britian.
  • Johansson, E., Steffens, J. J., Lindqvist, Y., and Schneider, G. (2000) Crystal structure of saccharopine reductase from Magnaporthe grisea, an enzyme of the 〈-aminoadipate path- way of lysine biosynthesis. Struct. Fold. Des. 8, 1037–1047.
  • Jones, E. E. and Broquist, H. P. (1966) Saccharopine, an intermediate of the aminoadipic acid pathway of lysine biosynthesis. III. Aminoadipic semialdehyde-glutamate reductase. J. Biol. Chem. 241, 3430–3434.
  • J. P. Tewari and P. Bains, Fungi associated with the roots of clover in Alberta. I. Olpidium brassicae and Ligniera sp. Canadian Plant Disease Survey 63:2, (1983) 35, found at CPS-SCP of Canada website. Accessed March 20, 201
  • Kagami, M., Bruin, A.D., Ibelings, B.W., VanDonk, E. (2007). Parasitic chytrids: their effects on phytoplankton communities and food-web dynamics. Hydrobiologia, 578: 113-129
  • Kagami, M., Elert, E., Ibelings, B., Bruin, A., Donk, E. (2007). The parasitic chytrid, Zygorhizidium, facilitates the growth of the cladoceran zooplankter, Daphnia, in cultures of the inedible alga, Asterionella. Proceeding of the Royal society Biological Sciences. 274: 1561-1566
  • Moore et al. (2011). 21st century guidebook to fungi. New York, NY.
  • Pöggler, S. S., & Wöstemeyer, J. J. (2011). Evolution of fungi and fungal-like organisms [electronic resource] / volume editors, S. Pöggeler and J. Wöstemeyer. Heidelberg ; New York : Springer-Verlag Berlin Heidelberg.
  • Remy, W., Taylor, T. N., & Hass, H. (1994). Early Devonian fungi; a blastocladalean fungus with sexual reproduction. American Journal Of Botany, 81(6), 690-702.
  • Taylor, T. N., Hass, H. H., Kerp, H. H., Krings, M. M., & Hanlin, R. T. (2005). Perithecial Ascomycetes from the 400 Million Year Old Rhynie Chert: An Example of Ancestral Polymorphism. Mycologia, 97(1), 269-285.
  • West, H.A., and Cook, F.P. (2006). The α-Aminoadipate Pathway for Lysine Biosynthesis in Fungi. Cell Biochem Biophys. 46(1):43-64.

[edit] External links

Maine Chytrid Laboratory

University of California Museum of Paleontology Chytridiomycota Page

Amphibian Ark

The Mycological Society of America

Potato Wart (Synchytrium endobioticum)

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