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| - | + | '''Definitions''' | |
| - | + | Flagellated Zoospores: | |
| - | + | Saprobes: any organism, especially a fungus or bacterium, that lives and feeds on dead organic matter. | |
| - | + | Phylogeny: The evolutionary development and history of a species or higher taxonomic grouping of organisms. | |
| - | + | Lineage: Direct descent from a particular ancestor; ancestry. | |
| + | |||
| + | Endoparasitic: A parasite, such as a tapeworm, that lives within another organism. | ||
| + | |||
| + | Microsporidia: parasite of arthropods and fishes that invade and destroy host cells. | ||
Revision as of 13:21, 20 March 2013
Contents |
Chytrids and their role in the food web
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. 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. 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.
Chytrid Life Cycle
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. Some species of Chytrid are saprobes which feed on dead and rotting organic matter while other chytrids 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. [1] Batrachochytrium dendrobanditis is currently under rigorous study, and hopefully one day this epidemic which is infecting amphibians world-wide will be contained. [2]
Another parasitic chytrid species is Olpidium brassicae. Olpidium brassicae and other Olpidum species affect plants such as melons of all kinds and gourds. These parasites leave no symptoms but are very important vectors in many soil-borne plant viruses
Rozella appears in an isolated position in the fungal phylogeny as the very earliest lineage to diverge from the rest of the fungi. Diverse lineage containing 22 different species. The largest clade and unites several orders of chytrids. Still somewhat unresolved is the placement of the endoparasitic chytrids Rozella with the primitive fungi the microsporidia, and the genus Olpidium with the Zygomycota. These genera are interesting because they are both highly reduced endoparasites (living inside the host cell) whose entire thallus consists of only a spherical body absorbing nutrients from the host material that surrounds it. Rozella appears in an isolated position in the fungal phylogeny as the very earliest lineage to diverge from the rest of the fungi (James et al., 2006a, 2006b). Rozella was circumscribed by French mycologist Marie Maxime Cornu in 1872.
Blastocladiales fungi diverged from the the core chytrid and have a life cycle with sporic meiosis wheras most core chytrids have zygotic meiosis. Blastocladiales carry out asexual reproduction by thick-walled resting spores that produce zoospores upon germination.
Definitions
Flagellated Zoospores:
Saprobes: any organism, especially a fungus or bacterium, that lives and feeds on dead organic matter.
Phylogeny: The evolutionary development and history of a species or higher taxonomic grouping of organisms.
Lineage: Direct descent from a particular ancestor; ancestry.
Endoparasitic: A parasite, such as a tapeworm, that lives within another organism.
Microsporidia: parasite of arthropods and fishes that invade and destroy host cells.
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.
(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.
<<<<<ENTER PICTURE OF BUDD FORMATION HERE>>>>>>
(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.
<<<ENTER PICTURE OF FRAGMENTATION>>>>
(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.
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.
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.
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.
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 [1]. 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 [2]. 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[3],[4].
Evolution
Chytrid-like fungi fossils found in northern Russia from the Vendian Period are the oldest known fungi fossils. Other 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.
Chytrid fossils can be found, among members of some other major fungal groups, in the Devonian Rhynie Chert in Scotland. Some of these parasitic forms are closely related to modern Blastocladiales. Other chytrids found in the Rhynie Chert are similar to Spizellomycetales, demonstrating that chytrids had already diversified by the Devonian Period. Chytrids are important because they are probably similar to the ancestors of fungi. The evolutionary relationships studied indicate that chytrids are probably like the ancestors of fungi.
Definitions
Anamorph: production, or relation to the gradual evolution of a species, from one organism to another
Flagella: whip-like appendages used for movement.
Monophyletic: a phylogenetic group which contains both the common ancestor of the group and all of its descendants.
Mitospore: haploid or diploid spore produced through the process of mitosis
Zygotic meiosis: meiosis that occurs while zygotes are undergoing germination.
Operculum: a lid-like structure which encloses spores within the organism’s fruiting structure.
Sporic meiosis: meiosis that occurs while spores are forming.
Alternation of generations: interchanging between haploid and diploid states.
Oogamous sexual reproduction: a method of sexual reproduction in which gametes (egg and sperm) join together and form offspring.
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.
Holocarpic: the entire thallus functions as sporangium.
References
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.
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
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.
Moore et al.. 2011. 21st century guidebook to fungi. New York, NY.
Pessier, Allan. 2013. Amphibian Ark. Cytrid Fungus. The Amphibian Chytrid Fungus and Chytridiomycosis. <http://www.amphibianark.org/the-crisis/chytrid-fungus/>
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.
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