The oomycetes (), 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. Asexual reproduction involves the formation of chlamydospores and sporangia, producing motile zoospores. 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 in amber from the Cretaceous.

Etymology

Oomycota comes from oo- () and -mycete (), 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, appearing at the bases of sporangia, and sometimes in older parts of the filaments. Some are unicellular, while others are filamentous and branching.

  • Anisolpidiales <small>Dick 2001</small>
  • Anisolpidiaceae <small>Karling 1943</small>
  • Lagenismatales <small>Dick 2001</small>
  • Lagenismataceae <small>Dick 1995</small>
  • Salilagenidiales <small>Dick 2001</small>
  • Salilagenidiaceae <small>Dick 1995</small>
  • Rozellopsidales <small>Dick 2001</small>
  • Rozellopsidaceae <small>Dick 1995</small>
  • Pseudosphaeritaceae <small>Dick 1995</small>
  • Ectrogellales
  • Ectrogellaceae
  • Haptoglossales
  • Haptoglossaceae
  • Eurychasmales
  • Eurychasmataceae <small>Petersen 1905</small>
  • Haliphthorales
  • Haliphthoraceae <small>Vishniac 1958</small>
  • Olpidiopsidales
  • Sirolpidiaceae <small>Cejp 1959</small>
  • Pontismataceae <small>Petersen 1909</small> (contains Petersenia , Pontisma
  • Olpidiopsidaceae <small>Cejp 1959</small>
  • Atkinsiellales
  • Atkinisellaceae
  • Crypticolaceae <small>Dick 1995</small>
  • Saprolegniales
  • Achlyaceae
  • Verrucalvaceae <small>Dick 1984</small>
  • Saprolegniaceae <small>Warm. 1884</small> [Leptolegniaceae]
  • Leptomitales
  • Leptomitaceae <small>Kuetz. 1843</small> [Apodachlyellaceae <small>Dick 1986</small>]
  • Leptolegniellaceae <small>Dick 1971</small> [Ducellieriaceae <small>Dick 1995</small>]
  • Rhipidiales
  • Rhipidiaceae <small>Cejp 1959</small>
  • Albuginales
  • Albuginaceae <small>Schroet. 1893</small>
  • Peronosporales [Pythiales; Sclerosporales; Lagenidiales]
  • Salisapiliaceae
  • Pythiaceae <small>Schroet. 1893</small> [Pythiogetonaceae; Lagenaceae <small>Dick 1994</small>; Lagenidiaceae; Peronophythoraceae; Myzocytiopsidaceae <small>Dick 1995</small>]
  • Peronosporaceae <small>Warm. 1884</small> [Sclerosporaceae <small>Dick 1984</small>]

Phylogenetic relationships

thumb|right|[[Pythium sp. (Peronosporales), which causes pythiosis in animals, under microscope.]]

thumb|right|Symptom of [[late blight (P. infestans, Peronosporales) on the underside of a potato leaf.]]

thumb|right|[[Albugo candida (Albuginales) on shepherd's purse (Capsella bursa-pastoris, Brassicales).]]

Internal

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. A common taxonomic classification based on these data, places the class Oomycetes along with other classes such as the brown algae within the Stramenopiles.

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 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. 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.

  • The Phytophthora group is a paraphyletic genus that causes diseases such as dieback, late blight in potatoes (the cause of the Great Famine of the 1840s that ravaged Ireland and other parts of Europe), sudden oak death, rhododendron root rot, and ink disease in the European chestnut
  • The paraphyletic Pythium group is more prevalent than Phytophthora and individual species have larger host ranges, although 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, also causes Pythiosis in mammals.
  • The third group are the downy mildews, which are easily identifiable by the appearance of white, brownish or olive "mildew" on the leaf undersides (although this group can be confused with the unrelated fungal powdery mildews).
  • The fourth group are the white blister rusts, Albuginales, which cause white blister disease on a variety of flowering plants. White blister rusts sporulate beneath the epidermis of their hosts, causing spore-filled blisters on stems, leaves and the inflorescence. The Albuginales are currently divided into three genera, Albugo parasitic predominantly to Brassicales, Pustula, parasitic predominantly to Asterales, and Wilsoniana, predominantly parasitic to Caryophyllales. Like the downy mildews, the white blister rusts are obligate biotrophs, which means that they are unable to survive without the presence of a living host.

References

  • Description of The Phylum Oomycota – Systematic Biology
  • Introduction to the Oomycota – University of California Museum of Paleontology (UCMP)
  • Genome sequence and analysis of the Irish potato famine pathogen Phytophthora infestans

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