Oomycete

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The Oomycetes (Template:IPAc-en),<ref>Template:Cite Collins Dictionary</ref> or Oomycota, form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms within the Stramenopiles. 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.<ref name="Agrios-2005">Template:Cite book</ref>Template:Rp Asexual reproduction involves the formation of chlamydospores and sporangia, producing motile zoospores.<ref name="Agrios-2005" /> 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.<ref>Template:Cite journal</ref> 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.<ref name="urlIntroduction to the Oomycota">Template:Cite web</ref>

Etymology

Oomycota comes from oo- (Template:Langx) and -mycete (Template:Langx), 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.

Morphology

The oomycetes rarely have septa (see hypha), and if they do, they are scarce,<ref name="Kortekamp2005">Template:Cite journal</ref> appearing at the bases of sporangia, and sometimes in older parts of the filaments.<ref name="books.google.ca">Template:Cite book</ref> Some are unicellular, while others are filamentous and branching.<ref name="books.google.ca" />

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Classification

Simplified phylogeny.Template:Citation needed
Asexual structures (sporangia) in Saprolegniales, Albuginales and PeronosporalesTemplate:Citation needed
Sexual structures (only oogonia, antheridia not shown) of Saprolegnia.Template:Citation needed

Previously the group was arranged into six orders.<ref name="books.google.ca"/>

  • The Saprolegniales are the most widespread. Many break down decaying matter; others are parasites.
  • The Leptomitales have wall thickenings that give their continuous cell body the appearance of septation. They bear chitin and often reproduce asexually.
  • The Rhipidiales use rhizoids to attach their thallus to the bed of stagnant or polluted water bodies.
  • The Albuginales are considered by some authors to be a family (Albuginaceae) within the Peronosporales, although it has been shown that they are phylogenetically distinct from this order.
  • The Peronosporales too are mainly saprophytic or parasitic on plants, and have an aseptate, branching form. Many of the most damaging agricultural parasites belong to this order.
  • The Lagenidiales are the most primitive; some are filamentous, others unicellular; they are generally parasitic.

However more recently this has been expanded considerably.<ref>Template:Citation</ref><ref>Template:Citation</ref>

Phylogenetic relationships

Pythium sp. (Peronosporales), which causes pythiosis in animals, under microscope.
Symptom of late blight (P. infestans, Peronosporales) on the underside of a potato leaf.
Albugo candida (Albuginales) on shepherd's purse (Capsella bursa-pastoris, Brassicales).

Internal

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External

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.<ref name="urlIntroduction to the Oomycota" /> 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.<ref>Template:Citation</ref> A common taxonomic classification based on these data, places the class oomycetes class along with other classes such as the brown algae within the phylum Stramenopile.

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<ref name="pmid7659021">Template:Cite journal</ref> 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.<ref name="pmid7659021" /> Oomycota and fungi have different metabolic pathways for synthesizing lysine and have a number of enzymes that differ.<ref name="pmid7659021" /> The ultrastructure is also different, with oomycota having tubular mitochondrial cristae and fungi having flattened cristae.<ref name="pmid7659021" />

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.

Biology

Reproduction

Life cycle of Phytophthora infestans (Peronosporales) on potato. M: meiosis, P: plasmogamy, K: karyogamy, n: haploid, 2n: diploid.

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.

Ecology and pathogenicity

Unidentified water mold on dead larval mayfly.
A culture of Achlya sp. (Saprolegniales) isolated from a stream.
Sea trout suffering from UDN with secondary Saprolegnia infections.

Many oomycetes species are economically important, aggressive algae and plant pathogens.<ref>Agrios, George N. (2005). Plant Pathology. 5th ed. Academic Press. link.</ref><ref>Template:Cite journal</ref> 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.

References

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