Oomycota forms a distinct
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
lineage of
fungus
A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
-like
eukaryotic
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...
microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s, called oomycetes (). They are
filamentous and
heterotrophic, and can reproduce both
sexually and
asexually. Sexual reproduction of an
oospore
An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and oomycetes. They are believed to have evolved either through the Somatic fusion, fusion of two species or the chemically-induced stimulat ...
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
A chlamydospore is the thick-walled large resting spore of several kinds of fungi, including Ascomycota such as '' Candida'', Basidiomycota such as ''Panus'', and various Mortierellales species. It is the life-stage which survives in unfavourable c ...
and
sporangia, producing
motile
Motility is the ability of an organism to move independently, using metabolic energy.
Definitions
Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
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
''Phytophthora infestans'' is an oomycete or water mold, a fungus-like microorganism that causes the serious potato and tomato disease known as late blight or potato blight. Early blight, caused by ''Alternaria solani'', is also often called "pot ...
and
sudden oak death. One oomycete, the
mycoparasite
A mycoparasite is an organism with the ability to parasitize fungi.
Mycoparasites might be biotrophic or necrotrophic, depending on the type of interaction with their host.
Types of mycoparasitic organisms
Myco-heterotrophy
Various plants may ...
''
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
A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
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
Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from ...
). The Oomycota have a very sparse fossil record; a possible oomycete has been described from
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
amber
Amber is fossilized tree resin that has been appreciated for its color and natural beauty since Neolithic times. Much valued from antiquity to the present as a gemstone, amber is made into a variety of decorative objects."Amber" (2004). In M ...
.
Etymology
Oomycota comes from the
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
ωόν (oon, 'egg') and μύκητας (mykitas, '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.
Morphology
The oomycetes rarely have septa (see
hypha
A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium.
Structure
A hypha consists of one o ...
), 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.
* 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
Albuginaceae is a family of oomycetes.
Genera and species
Albuginaceae contains the following subtaxa:
*'' Albugo''
**'' Albugo achyranthis''
**'' Albugo aechmantherae''
**'' Albugo arenosa''
**'' Albugo austroafricana''
**''Albugo candida''
* ...
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.
*
Anisolpidiales Dick 2001
**
Anisolpidiaceae Karling 1943
*
Lagenismatales Dick 2001
**
Lagenismataceae Dick 1995
*
Salilagenidiales Dick 2001
**
Salilagenidiaceae Dick 1995
*
Rozellopsidales Dick 2001
**
Rozellopsidaceae Dick 1995
**
Pseudosphaeritaceae Dick 1995
*
Ectrogellales
**
Ectrogellaceae
*
Haptoglossales
**
Haptoglossaceae
*
Eurychasmales
**
Eurychasmataceae Petersen 1905
*
Haliphthorales
**
Haliphthoraceae Vishniac 1958
*
Olpidiopsidales
**
Sirolpidiaceae Cejp 1959
**
Pontismataceae Petersen 1909 (contains ''Petersenia'' , ''Pontisma''
**
Olpidiopsidaceae Cejp 1959
*
Atkinsiellales
**
Atkinisellaceae
**
Crypticolaceae Dick 1995
*
Saprolegniales
**
Achlyaceae
**
Verrucalvaceae Dick 1984
**
Saprolegniaceae Warm. 1884 eptolegniaceae*
Leptomitales
**
Leptomitaceae Kuetz. 1843 Dick 1986">podachlyellaceae Dick 1986**
Leptolegniellaceae Dick 1971 Dick 1995">ucellieriaceae Dick 1995*
Rhipidiales
**
Rhipidiaceae Cejp 1959
*
Albuginales
Albuginaceae is a family of oomycetes.
Genera and species
Albuginaceae contains the following subtaxa:
*'' Albugo''
**'' Albugo achyranthis''
**'' Albugo aechmantherae''
**'' Albugo arenosa''
**'' Albugo austroafricana''
**''Albugo candida''
* ...
**
Albuginaceae Schroet. 1893
*
Peronosporales ythiales; Sclerosporales; Lagenidiales**
Salisapiliaceae
**
Pythiaceae Schroet. 1893 Dick 1994; Lagenidiaceae; Peronophythoraceae; Myzocytiopsidaceae Dick 1995">ythiogetonaceae; Lagenaceae Dick 1994; Lagenidiaceae; Peronophythoraceae; Myzocytiopsidaceae Dick 1995**
Peronosporaceae Warm. 1884 Dick 1984">clerosporaceae Dick 1984
Phylogenetic relationships
Internal
External
This group was originally classified among the
fungi
A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
(the name "oomycota" means "egg fungus") and later treated as
protists, based on general morphology and lifestyle.
A
cladistic
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived ch ...
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
In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclatu ...
Heterokonta.
This relationship is supported by a number of observed differences between the characteristics of oomycetes and fungi. For instance, the
cell wall
A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...
s of oomycetes are composed of
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
rather than
chitin
Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
and generally do not have
septations. Also, in the vegetative state they have
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respecti ...
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.
Oomycota and fungi have different metabolic pathways for synthesizing
lysine
Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated &minu ...
and have a number of enzymes that differ.
The ultrastructure is also different, with oomycota having tubular mitochondrial
cristae and fungi having flattened cristae.
In spite of this, many
species
In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
of oomycetes are still described or listed as types of fungi and may sometimes be referred to as pseudofungi, or lower fungi.
Biology
Reproduction
Most of the oomycetes produce two distinct types of spores. The main dispersive spores are asexual, self-motile
spore
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...
s called
zoospore
A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves.
Diversity Flagella types
Zoospores may possess one or m ...
s, 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
oospore
An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and oomycetes. They are believed to have evolved either through the Somatic fusion, fusion of two species or the chemically-induced stimulat ...
s, 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 pathogens. 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
''Phytophthora'' (from Greek (''phytón''), "plant" and (), "destruction"; "the plant-destroyer") is a genus of plant-damaging oomycetes (water molds), whose member species are capable of causing enormous economic losses on crops worldwide, a ...
'' group is a
paraphyletic
In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...
that causes diseases such as
dieback,
late blight in
potato
The potato is a starchy food, a tuber of the plant ''Solanum tuberosum'' and is a root vegetable native to the Americas. The plant is a perennial in the nightshade family Solanaceae.
Wild potato species can be found from the southern Un ...
es (the cause of the
Great Famine of the 1840s that ravaged
Ireland
Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...
and other parts of Europe),
sudden oak death,
rhododendron root rot, and
ink disease
''Phytophthora cinnamomi'' is a soil-borne water mould that produces an infection which causes a condition in plants variously called "root rot", "dieback", or (in certain ''Castanea'' species), "ink disease". The plant pathogen is one of the wor ...
in the
European chestnut
* The
paraphyletic
In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
''
Pythium'' group is more prevalent than ''Phytophthora'' and individual species have larger host ranges, although usually causing less damage. ''Pythium''
damping off
Damping off (or damping-off) is a horticultural disease or condition, caused by several different pathogens that kill or weaken seeds or seedlings before or after they germinate. It is most prevalent in wet and cool conditions.
Symptoms
There a ...
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 mildew
Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to the Peronosporaceae family. In commercial agriculture, they are a particular problem for growers of c ...
s, 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
White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White on ...
,
Albuginales
Albuginaceae is a family of oomycetes.
Genera and species
Albuginaceae contains the following subtaxa:
*'' Albugo''
**'' Albugo achyranthis''
**'' Albugo aechmantherae''
**'' Albugo arenosa''
**'' Albugo austroafricana''
**''Albugo candida''
* ...
, which cause white blister disease on a variety of flowering plants. White blister rusts sporulate beneath the
epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the ...
of their hosts, causing spore-filled blisters on stems, leaves and the
inflorescence
An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed ...
. The Albuginales are currently divided into three genera, ''
Albugo'' parasitic predominantly to
Brassicales, ''
Pustula'', parasitic predominantly to
Asterales
Asterales () is an order of dicotyledonous flowering plants that includes the large family Asteraceae (or Compositae) known for composite flowers made of florets, and ten families related to the Asteraceae. While asterids in general are chara ...
, and ''
Wilsoniana'', predominantly parasitic to
Caryophyllales. Like the
downy mildews, the
white blister rusts
White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White on ...
are obligate
biotrophs, which means that they are unable to survive without the presence of a living host.
References
External links
Description of The Phylum Oomycota– Systematic Biology
– University of California Museum of Paleontology (UCMP)
{{Taxonbar, from1=Q223597, from2=Q61997516