The Oomycetes (),
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.
Etymology
Oomycota comes from
() and (), 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.
Classification
Previously the group was arranged into six orders.
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The Saprolegniales are the most widespread. Many break down decaying matter; others are parasites.
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The Leptomitales have wall thickenings that give their continuous cell body the appearance of septation. They bear chitin and often reproduce asexually.
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The Rhipidiales use rhizoids to attach their thallus to the bed of stagnant or polluted water bodies.
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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.
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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.
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The Lagenidiales are the most primitive; some are filamentous, others unicellular; they are generally parasitic.
However more recently this has been expanded considerably.
Phylogenetic relationships
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
cladistic analysis based on modern discoveries about the biology of these organisms supports a relatively close relationship with some photosynthetic organisms, such as
and
.
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 of oomycetes are composed of cellulose rather than chitin
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.
Biology
Reproduction
Most of the oomycetes produce two distinct types of spores. The main dispersive spores are asexual, self-motile
called
, 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
, that are translucent, double-walled, spherical structures used to survive adverse environmental conditions.
Ecology and pathogenicity
Many oomycetes species are economically important, aggressive algae and
plant pathology.
[Agrios, George N. (2005). Plant Pathology. 5th ed. Academic Press. link.] 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.
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The Phytophthora group is a paraphyletic genus that causes diseases such as dieback, late blight in (the cause of the Great Famine of the 1840s that ravaged Ireland and other parts of Europe),
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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.
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The third group are the , 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 ).
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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 biotrophic, which means that they are unable to survive without the presence of a living host.
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