Oomycete
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| Water molds | |
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| The reproductive structures of Phytophthora infestans | |
| Scientific classification | |
| Domain: | Eukarya |
| Kingdom: | Chromalveolata |
| Phylum: | Heterokontophyta |
| Class: | Oomycota |
| Orders | |
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Lagenidiales |
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Oömycota (pronounced /ˌoʊ ə maɪˈkoʊ tə/) or oömycetes (pronounced /ˌoʊ əˈmaɪ sit is/), also known as water molds (or water moulds: see spelling differences), form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms. They are filamentous microscopic, absorptive organisms that reproduce both sexually and asexually. Oomycetes occupy both saprophytic and pathogenic lifestyles – and include some of the most notorious pathogens of plants, causing devastating diseases such as Late Blight of Potato and Sudden Oak Death.
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[edit] Etymology
"Oomycota" means "egg fungi", referring to the large round oogonia, structures containing the female gametes, that are characteristic of the oomycetes.
The name "water mold" refers to their earlier classification as fungi and their preference for conditions of high humidity and running surface water.
[edit] Phylogenetic relationships and classification
Although, based on their general morphology and lifestyles, this group was traditionally classified as fungi,[1] a cladistic classification based on modern insights, supports a relatively close relationship with the photosynthetic organisms such as brown algae and diatoms, within the eukaryotic group - the heterokonts.
This relationship is supported by a number of observed differences in the characteristics of oomycetes and fungi. For instance, the cell walls of oomycetes are composed of cellulose rather than chitin[2] and generally do not have septations. Also, in the vegetative state they have diploid nuclei, whereas fungi have haploid nuclei. Oomycota spores have two flagella, while fungi spores which retain flagella (such as the Chytridiomycetes) have only one.[2] Oomycota and fungi have different metabolic pathways for synthesizing lysine and have a number of enzymes which differ.[2] The ultrastructure is also different, with oomycota having tubular mitochondrial cristae and fungi having flattened cristae.[2] In spite of this many species of oomycetes are still described or listed as types of fungi and may sometimes be referred to as pseudofungi, or lower fungi.
[edit] Biology
[edit] Reproduction
Among the water molds, these are produced as asexual spores called zoospores, which capitalize on surface water (including precipitation on plant surfaces) for movement. They also produce sexual spores, called oospores, that are translucent double-walled spherical structures used to survive adverse environmental conditions. A few produce aerial asexual spores that are distributed by wind.
[edit] Pathogenicity
The oomycetes are economically and scientifically important because they are aggressive plant pathogens. Some species can cause disease in fish. The majority of the plant pathogenic species can be classified into three groups, although more exist.
- The Phytophthora group is a genus that causes diseases such as dieback, late blight in potatoes (the cause of the Great Hunger or Potato Famine of the 1840s in Ireland and other parts of Europe)[3], sudden oak death, rhododendron root rot, and Ink Disease in the American Chestnut.
- The Pythium group is even more prevalent than Phytophthora and individual species have larger host ranges, usually causing less damage. Pythium damping off is a very common problem in greenhouses where the organism kills newly emerged seedlings. Mycoparasitic members of this group (e.g. P. oligandrum) parasitize other oomycetes and fungi, and have been employed as biocontrol agents. One Pythium species, Pythium insidiosum is also known to infect mammals.
- The third group are the downy mildews, which are easily identifiable by the appearance of white "mildew" on leaf surfaces (although this group can be confused with the unrelated fungal powdery mildews).
[edit] References
- ^ "Introduction to the Oomycota". http://www.ucmp.berkeley.edu/chromista/oomycota.html. Retrieved 2009-05-26.
- ^ a b c d Van der Auwera G, De Baere R, Van de Peer Y, De Rijk P, Van den Broeck I, De Wachter R (July 1995). "The phylogeny of the Hyphochytriomycota as deduced from ribosomal RNA sequences of Hyphochytrium catenoides". Mol. Biol. Evol. 12 (4): 671–8. PMID 7659021. http://mbe.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=7659021.
- ^ Haas, BJ; Kamoun, S; Zody, MC; Jiang, RH; Handsaker, RE; Cano, LM; Grabherr, M; Kodira, CD et al. (2009). "Genome sequence and analysis of the Irish potato famine pathogen Phytophthora infestans.". Nature 461 (7262): 393–8. doi:. PMID 19741609.
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