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
Cretaceousamber.[6]
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,[7] appearing at the bases of sporangia, and sometimes in older parts of the filaments.[8] Some are unicellular, while others are filamentous and branching.[8]
Classification
Previously the group was arranged into six orders.[8]
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.[9][10]
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.[6] 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
phylumHeterokonta.
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[11] 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.[11] Oomycota and fungi have different metabolic pathways for synthesizing
lysine and have a number of enzymes that differ.[11] The ultrastructure is also different, with oomycota having tubular mitochondrial
cristae and fungi having flattened cristae.[11]
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
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
Many oomycetes species are economically important, aggressive algae and
plant pathogens.[12][13] 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
paraphyleticPythium group is more prevalent than Phytophthora and individual species have larger host ranges, although usually causing less damage. Pythiumdamping 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.
^Kortekamp, A. (2005). "Growth, occurrence and development of septa in Plasmopara viticola and other members of the Peronosporaceae using light- and epifluorescence-microscopy". Mycological Research. 109 (Pt 5): 640–648.
doi:
10.1017/S0953756205002418.
PMID16018320.
^
abcdVan der Auwera G, De Baere R, Van de Peer Y, De Rijk P, Van den Broeck I, De Wachter R (July 1995). "The phylogeny of the Hyphochytriomycota as deduced from ribosomal RNA sequences of Hyphochytrium catenoides". Mol. Biol. Evol. 12 (4): 671–8.
doi:
10.1093/oxfordjournals.molbev.a040245.
PMID7659021.
^Agrios, George N. (2005). Plant Pathology. 5th ed. Academic Press.
link.
^Vettraino, A. M.; Morel, O.; Perlerou, C.; Robin, C.; Diamandis, S.; Vannini, A. (2005). "Occurrence and distribution of Phytophthora species in European chestnut stands, and their association with Ink Disease and crown decline". European Journal of Plant Pathology. 111 (2): 169–180.
doi:
10.1007/s10658-004-1882-0.
S2CID2041934.