Wikipedia:Oomycete
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 Water mold attacking an aquatic insect nymph
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Oomycota also known as Water molds (or water moulds: see spelling differences) are a group of filamentous, unicellular heterokonts, that physical resemble - but are evolutionarily distinct from - fungi. They are microscopic, absorptive organisms that reproduce both sexually and asexually and are composed of mycelia, or a tube-like vegetative body (all of an organism's mycelia are called its thallus).
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Etymology
"Oomycota" means "egg fungi," and refers to the large round oogonia, or structures containing the female gametes produced by Oomycetes.
Phylogenetic Distinction from Fungi
Traditionally, this group was classified as fungi,[1] and indeed fungi themselves were once believed to be closely related to plants. However, further research has concluded that fungi are more closely related to animals. Many species of Oomycetes are still described or listed as types of fungi and may sometimes be referred to as pseudofungi, or lower fungi.
The name "water mold" refers to their earlier classification as fungi, which stemmed from their preference for conditions of high humidity and running surface water, although they are now known to have evolved separately and show a number of differences. For instance, their cell walls 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]
Classification
Water molds are related to organisms such as brown algae and diatoms (both previously classified as algae), making up a group called the heterokonts. The name comes from the common arrangement and structure of motile cells, which typically have two unequal flagella. 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.
Pathogenity
The water molds 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), 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).
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.
