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In some biological fungi, animals and plants, although some fungi and animals are also unicellular.
The term protista was first used by morphology and ecology of various protists.
Protists live in almost any environment that contains liquid water. Many protists, such as the algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Other protists include pathogenic species such as the kinetoplastid Trypanosoma brucei, which causes sleeping sickness and species of the apicomplexan Plasmodium which cause malaria.
The first groups used to classify microscopic organism were the foraminifera and amoebae. The formal taxonomic category Protoctista was first proposed in the early 1860s by John Hogg, who argued that the protists should include what he saw as primitive unicellular forms of both plants and animals. He defined the Protoctista as a "fourth kingdom of nature", in addition to the then-traditional kingdoms of plants, animals and minerals.[11] The kingdom of minerals was later removed from taxonomy by Ernst Haeckel, leaving plants, animals, and the protists as a “kingdom of primitive forms”.[12]
In 1938, monophyletic groups).[16]
Some protists, sometimes called ambiregnal protists, have been considered to be both protozoa and algae or fungi (e.g., slime molds and mixotrophic algae), and names for these have been published under either or both of the ICN and the ICZN.[17][18]
Although systematists today do not treat protists as a formal taxon, the term protist is currently used in two ways. The most popular contemporary definition is a phylogenetic one, that identifies a paraphyletic group: a protist is any eukaryote that is not an animal, (land) plant, or (true) fungus; this definition excludes many unicellular groups, like the Myxosporida (animals), the Microsporidia (fungi), many Chytridiomycetes (fungi), and yeasts (fungi). The other definition describes protists primarily by functional or biological criteria: protists are essentially those eukaryotes that are never multicellular,[19] that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues[20]; this definition excludes the brown algae, and many red and green algae. The term protozoa is used to refer to heterotrophic species of protists that do not form filaments. These terms are not used in current taxonomy, and are retained only as convenient ways to refer to these organisms.
The taxonomy of protists is still changing. Newer classifications attempt to present [21] Many are thought to be monophyletic, though there is still uncertainty. For instance, the excavates are probably not monophyletic and the chromalveolates are probably only monophyletic if the haptophytes and cryptomonads are excluded.[22]
Some protists reproduce sexually (gametes), while others reproduce asexually (binary fission).
Some species, for example [23] However, it is unclear how frequently sexual reproduction causes genetic exchange between different strains of Plasmodium in nature and most populations of parasitic protists may be clonal lines that rarely exchange genes with other members of their species.[24]
Eukaryotes emerged in evolution more than 1.5 billion years ago.[25] The earliest eukaryotes were likely protists. Although sexual reproduction is widespread among extant eukaryotes, it seemed unlikely until recently, that sex could be a primordial and fundamental characteristic of eukaryotes. A principal reason for this view was that sex appeared to be lacking in certain pathogenic protists whose ancestors branched off early from the eukaryotic family tree. However, several of these protists are now known to be capable of, or to recently have had the capability for, meiosis and hence sexual reproduction. For example, the common intestinal parasite Giardia lamblia was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex. However, G. lamblia was recently found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes.[26] These results suggested that G. lamblia is capable of meiosis and thus sexual reproduction. Furthermore, direct evidence for meiotic recombination, indicative of sex, was also found in G. lamblia.[27]
The pathogenic parasitic protists of the genus Leishmania have been shown to be capable of a sexual cycle in the invertebrate vector, likened to the meiosis undertaken in the trypanosomes.[28]
Trichomonas vaginalis, a parasitic protist, is not known to undergo meiosis, but when Malik et al.[29] tested for 29 genes that function in meiosis, they found 27 to be present, including 8 of 9 genes specific to meiosis in model eukaryotes. These findings suggest that T. vaginalis may be capable of meiosis. Since 21 of the 29 meiotic genes were also present in G. lamblia, it appears that most of these meiotic genes were likely present in a common ancestor of T. vaginalis and G. lamblia. These two species are descendants of protist lineages that are highly divergent among eukaryotes, leading Malik et al.[29] to suggest that these meiotic genes were likely present in a common ancestor of all eukaryotes.
Based on a phylogenetic analysis, Dacks and Roger proposed that facultative sex was present in the common ancestor of all eukaryotes.[30]
This view was further supported by a study of amoebae by Lahr et al.[31] Amoeba have generally been regarded as asexual protists. However these authors describe evidence that most [32]
Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock.[33] Oxidative stress, which is associated with the production of reactive oxygen species leading to DNA damage, also appears to be an important factor in the induction of sex in protists.[33]
There are some protists that are significant pathogens of animals and others that are pathogens of plants; for example there are five species of the parasitic genus Plasmodium, which cause malaria in humans; and the oomycete Phytophthora infestans, which causes late blight in potatoes.[34] A more thorough understanding of protist biology may allow these diseases to be treated more efficiently.
Recent papers have proposed the use of viruses to treat infections caused by protozoa.[35][36]
Researchers from the Agricultural Research Service are taking advantage of protists as pathogens in an effort to control red imported fire ant (Solenopsis invicta) populations in Argentina. With the help of spore-producing protists such as Kneallhazia solenopsae (this is more widely recognized as belonging to the fungus kingdom now) the red fire ant populations can be reduced by 53–100%.[37] Researchers have also found a way to infect phorid flies with the protist without harming the flies. This is important because the flies act as a vector to infect the red fire ant population with the pathogenic protist.[38]
Many protists have neither hard parts nor resistant spores, and their fossils are extremely rare or unknown. Examples of such groups include the apicomplexans,[39] most ciliates,[40] some green algae (the Klebsormidiales),[41] choanoflagellates,[42] oomycetes,[43] brown algae,[44] yellow-green algae,[45] excavates (e.g., euglenids).[46] Some of these have been found preserved in amber (fossilized tree resin) or under unusual conditions (e.g., Paleoleishmania, a kinetoplastid).
Others are relatively common in the fossil record,[47] as the diatoms,[48] golden algae,[49] haptophytes (coccoliths),[50] silicoflagellates, tintinnids (ciliates), dinoflagellates,[51] green algae,[52] red algae,[53] heliozoans, radiolarians,[54] foraminiferans,[55] ebriids and testate amoebae (euglyphids, arcellaceans).[56] Some are even used as paleoecological indicators to reconstruct ancient environments.
More probable eukaryote fossils begin to appear at about 1.8 billion years ago, the acritarchs, spherical fossils of likely algal protists.[57] Another possible representant of early fossil eukaryotes are the Gabonionta.
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Mycetozoa, Conosa, Archamoebae, Taxonomy (biology), Eukaryote
Cnidaria, Sponge, Bacteria, Fungus, Biological classification
Chlorophyta, Charophyta, Trebouxiophyceae, Ulvophyceae, Pinophyta
Rhizaria, Protist, Malaria, Apicomplexa, Animal
Bacteria, Animal, Evolution, Astrobiology, Science
Cyanobacteria, Charophyta, Fishing, Chlorophyta, Rhizaria
Medicine, Ecology, Molecular biology, Botany, Metabolism