The Oomycota (or Peronosporomycetes) is a group that includes more than 800 species that may be saprobic or parasitic on terrestrial or aquatic plants and animals. The oomycetes--including Phytophthora, Pythium, and downy mildews--have long been considered to be fungi because, like fungi, they obtain their nutrients via absorption and many of them produce the filamentous threads characteristic of most fungi. However, molecular phylogenetic investigations have found that the oomycetes are more closely related to the heterokont algae (i.e., the Phaeophyta [brown algae], Xanthophyta [yellow-green algae], Chrysophyta [golden algae], and Bacillariophyta [diatoms], and several smaller groups) than to the true Fungi (Chytridiomycota, Glomeromycota, Zygomycota, Ascomycota, and Basidiomycota). Thisconclusion is supported by a range of ultrastructural data (i.e., the fine structure revealed by electron microscopy).The oomycetes are now often included in a phylum Heterokonta within a kingdom Chromista or in a phylum Stramenopila (or Straminopila).
A number of characteristics distinguish the Oomycota from the true Fungi. These include differences in sexual reproduction (oomycetes produce oospores whereas true fungi produce zygospores, ascospores, or basidiospores), in the nuclear state of the vegetative mycelium (diploid in oomycetes but mostly haploid or dikaryotic in true fungi), in cell wall composition (beta glucans and cellulose in oomycetes, chitin--rarely with cellulose--in true fungi), type of flagellae (when present, flagellae are typically of two types in oomycetes--posteriorly directed whiplash and anteriorly directed fibrous--with just a posterior whiplash type present in true fungi), and mitochondria (cristae tubular in oomycetes but flattened in true fungi).
One especially notorious oomycete is Phytophthora infestans, the cause of late blight of potatoes. As a result of the famine in Ireland caused by this disease, around one million people died and another 1.5 million emigrated. A number of other plant diseases are caused by species of Phytophthora, including sudden oak death and ramorum blight caused by P. ramorum, cacao black pod caused by P. megakarya, and many root rot diseases, such as black shank of tobacco. Other members of the group include pathogens such as Pythium aphanidermatum, the cause of cottony blight of turf grasses; Peronospora tabacina (tobacco blue mold); Plasmopara viticola (downy mildew of grape); Plasmopara halstedii (sunflower downy mildew); and many others. Finally, a group traditionally placed in the oomycetes is the Saprolegniales, or water molds, which cause diseases of fish and other aquatic vertebrates.
More information on the Oomycota can be found in this online adaptation of Rossman and Palm (2006).
(Rossman and Palm 2006 and references therein; Rossman and Palm 2008 and references therein)
Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (/ˌoʊ.əˈmaɪsiːts/).[4] They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the result of contact between hyphae of male antheridia and female oogonia; these spores can overwinter and are known as resting spores.[5] Asexual reproduction involves the formation of chlamydospores and sporangia, producing motile zoospores.[5] 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. One oomycete, the mycoparasite Pythium oligandrum, is used for biocontrol, attacking plant pathogenic fungi.[6] The oomycetes are also often referred to as water molds (or water moulds), although the water-preferring nature which led to that name is not true of most species, which are terrestrial pathogens.
Oomycetes were originally grouped with fungi due to similarities in morphology and lifestyle. However, molecular and phylogenetic studies revealed significant differences between fungi and oomycetes which means the latter are now grouped with the stramenopiles (which include some types of algae). The Oomycota have a very sparse fossil record; a possible oomycete has been described from Cretaceous amber.[7]
Oomycota comes from oo- (Greek: ωόν, translit. ōon, lit. "egg") and -mycete (Greek: μύκητας, translit. mýkitas, lit. "fungus"), 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, which is characteristic for the basal taxa of the oomycetes.
The oomycetes rarely have septa (see hypha), and if they do, they are scarce,[8] appearing at the bases of sporangia, and sometimes in older parts of the filaments.[9] Some are unicellular, while others are filamentous and branching.[9]
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Previously the group was arranged into six orders.[9]
However more recently this has been expanded considerably.[10][11]
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This group was originally classified among the fungi (the name "oomycota" means "egg fungus") and later treated as protists, based on general morphology and lifestyle.[7] A cladistic analysis based on modern discoveries about the biology of these organisms supports a relatively close relationship with some photosynthetic organisms, such as brown algae and diatoms. A common taxonomic classification based on these data, places the class Oomycota along with other classes such as Phaeophyceae (brown algae) within the phylum Heterokonta.
This relationship is supported by a number of observed differences between the characteristics of oomycetes and fungi. For instance, the cell walls of oomycetes are composed of cellulose rather than chitin[12] and generally do not have septations. Also, in the vegetative state they have diploid nuclei, whereas fungi have haploid nuclei. Most oomycetes produce self-motile zoospores with two flagella. One flagellum has a "whiplash" morphology, and the other a branched "tinsel" morphology. The "tinsel" flagellum is unique to the Kingdom Heterokonta. Spores of the few fungal groups which retain flagella (such as the Chytridiomycetes) have only one whiplash flagellum.[12] Oomycota and fungi have different metabolic pathways for synthesizing lysine and have a number of enzymes that differ.[12] The ultrastructure is also different, with oomycota having tubular mitochondrial cristae and fungi having flattened cristae.[12]
In spite of this, many species of oomycetes are still described or listed as types of fungi and may sometimes be referred to as pseudo fungi, or lower fungi.
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Most of the oomycetes produce two distinct types of spores. The main dispersive spores are asexual, self-motile spores called zoospores, which are capable of chemotaxis (movement toward or away from a chemical signal, such as those released by potential food sources) in surface water (including precipitation on plant surfaces). A few oomycetes produce aerial asexual spores that are distributed by wind. They also produce sexual spores, called oospores, that are translucent, double-walled, spherical structures used to survive adverse environmental conditions.
Many oomycetes species are economically important, aggressive algae and plant pathogens.[13][14] Some species can cause disease in fish, and at least one is a pathogen of mammals. The majority of the plant pathogenic species can be classified into four groups, although more exist.
Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (/ˌoʊ.əˈmaɪsiːts/). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the result of contact between hyphae of male antheridia and female oogonia; these spores can overwinter and are known as resting spores. Asexual reproduction involves the formation of chlamydospores and sporangia, producing motile zoospores. 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. One oomycete, the mycoparasite Pythium oligandrum, is used for biocontrol, attacking plant pathogenic fungi. The oomycetes are also often referred to as water molds (or water moulds), although the water-preferring nature which led to that name is not true of most species, which are terrestrial pathogens.
Oomycetes were originally grouped with fungi due to similarities in morphology and lifestyle. However, molecular and phylogenetic studies revealed significant differences between fungi and oomycetes which means the latter are now grouped with the stramenopiles (which include some types of algae). The Oomycota have a very sparse fossil record; a possible oomycete has been described from Cretaceous amber.
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