Protist
Temporal range: Neoproterozoic – Recent
Scientific classification
Domain: Eukarya
Excluded groups

Many others;
classification varies

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.

Classification

Historical classifications

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]

Modern classifications

Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes

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]

Metabolism

Nutrition can vary according to the type of protist. Many protists are flagellate, for example, and filter feeding can take place where the flagella find prey. Other protists can engulf bacteria and other food particles, by extending their cell membrane around them to form a food vacuole and digest them internally, in a process termed phagocytosis.
Nutritional types in protist metabolism
Nutritional type Source of energy Source of carbon Examples
 Phototrophs   Sunlight   Organic compounds or carbon fixation  Algae, Dinoflagellates or Euglena 
 Organotrophs  Organic compounds   Organic compounds   Apicomplexa, Trypanosomes or Amoebae 

Reproduction

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]

Role as pathogens

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]

Fossil record

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.

See also

References

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  2. ^ Kirk, N. L.; Ritson-Williams, R.; Coffroth, M. A.; Miller, M. W.; Fogarty, N. D.; Santos, S. R. (2013). "Tracking Transmission of Apicomplexan Symbionts in Diverse Caribbean Corals". PLoS ONE 8 (11): e80618.  
  3. ^ Zhao, Y.; Gentekaki, E.; Yi, Z.; Lin, X. (2013). (Protista, Ciliophora)"Paramecium Subunit I Gene in Genus c"Genetic Differentiation of the Mitochondrial Cytochrome Oxidase . PLoS ONE 8 (10): e77044.  
  4. ^ Miloslavich, P.; Díaz, J. M.; Klein, E.; Alvarado, J. J.; Díaz, C.; Gobin, J.; Escobar-Briones, E.; Cruz-Motta, J. J.; Weil, E.; Cortés, J.; Bastidas, A. C.; Robertson, R.; Zapata, F.; Martín, A.; Castillo, J.; Kazandjian, A.; Ortiz, M. (2010). "Marine Biodiversity in the Caribbean: Regional Estimates and Distribution Patterns". PLoS ONE 5 (8): e11916.  
  5. ^ Martínez-Girón, R; Ribas-Barceló, A (2007). "Pitfall in sputum cytology: Protoctista resembling adenocarcinoma cells". Diagnostic Cytopathology 35 (1): 32–3.  
  6. ^ Osaka, T; Beika, A; Hattori, A; Kohno, Y; Kato, K. H.; Mizutani, T (2003). contains selenoproteins and the relevant translation apparatus"Oxyrrhis marina"The protozoa dinoflagellate . Biochemical and Biophysical Research Communications 300 (1): 236–40.  
  7. ^ Simonite T (November 2005). "Protists push animals aside in rule revamp". Nature 438 (7064): 8–9.  
  8. ^ Harper, David; Benton, Michael (2009). Introduction to Paleobiology and the Fossil Record. Wiley-Blackwell. p. 207.  
  9. ^ "Systematics of the Eukaryota". Retrieved 2009-05-31. 
  10. ^ The Flagellates. Unity, diversity and evolution. Ed.: Barry S. C. Leadbeater and J. C. Green Taylor and Francis, London 2000, p. 3.
  11. ^ a b Scamardella, J. M. (1999). "Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista". International Microbiology 2: 207–221. 
  12. ^ Rothschild LJ (1989). "Protozoa, Protista, Protoctista: what's in a name?". J Hist Biol 22 (2): 277–305.  
  13. ^ Copeland, H. F. (1938). "The Kingdoms of Organisms". Quarterly Review of Biology 13 (4): 383.  
  14. ^ Whittaker, R. H. (1959). "On the Broad Classification of Organisms". Quarterly Review of Biology 34 (3): 210.  
  15. ^ Whittaker RH (January 1969). "New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms". Science 163 (3863): 150–60.  
  16. ^ Stechmann, Alexandra; Thomas Cavalier-Smith (2003). "The root of the eukaryote tree pinpointed". Current Biology 13 (17): R665–R666.  
  17. ^ Corliss, J.O. (1995). "The ambiregnal protists and the codes of nomenclature: a brief review of the problem and of proposed solutions". Bulletin of Zoological Nomenclature 52: 11–17.
  18. ^ Barnes, Richard Stephen Kent (2001). The Invertebrates: A Synthesis. Wiley-Blackwell. p. 41. ISBN 978-0-632-04761-1.
  19. ^ O'Malley MA, Simpson AGB, and Roger AJ (2013). The other eukaryotes in light of evolutionary protistology. Biology and Philosophy 28(2): 299–330.
  20. ^ a b Adl SM, Simpson AG, Farmer MA, et al. (2005). "The new higher level classification of eukaryotes with emphasis on the taxonomy of protists". J. Eukaryot. Microbiol. 52 (5): 399–451.  
  21. ^ Cavalier-Smith T, Chao EE (October 2003). "Phylogeny and classification of phylum Cercozoa (Protozoa)". Protist 154 (3–4): 341–58.  
  22. ^ Laura Wegener Parfrey, Erika Barbero, Elyse Lasser, Micah Dunthorn, Debashish Bhattacharya, David J Patterson, and Laura A Katz (December 2006). "Evaluating Support for the Current Classification of Eukaryotic Diversity". PLoS Genet. 2 (12): e220.  
  23. ^ Talman AM, Domarle O, McKenzie FE, Ariey F, Robert V (July 2004). "Gametocytogenesis: the puberty of Plasmodium falciparum". Malar. J. 3: 24.  
  24. ^ Tibayrenc M, Kjellberg F, Arnaud J, et al. (June 1991). "Are eukaryotic microorganisms clonal or sexual? A population genetics vantage". Proc. Natl. Acad. Sci. U.S.A. 88 (12): 5129–33.  
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  26. ^ Ramesh MA, Malik SB, Logsdon JM (January 2005). "A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis". Curr. Biol. 15 (2): 185–91.  
  27. ^ Cooper MA, Adam RD, Worobey M, Sterling CR (November 2007). "Population genetics provides evidence for recombination in Giardia". Curr. Biol. 17 (22): 1984–8.  
  28. ^ Akopyants NS, Kimblin N, Secundino N, et al. (April 2009). "Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector". Science 324 (5924): 265–8.  
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  30. ^ Dacks J, Roger AJ (1999). "The first sexual lineage and the relevance of facultative sex". J. Mol. Evol. 48 (6): 779–83.  
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  38. ^ Durham, Sharon (January 28, 2010) ARS Parasite Collections Assist Research and Diagnoses. Ars.usda.gov. Retrieved on 2014-03-20.
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  41. ^ Klebsormidiales. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
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  43. ^ Introduction to the Oomycota. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
  44. ^ Introduction to the Phaeophyta. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
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Bibliography

General

  • Haeckel, E. Das Protistenreich. Leipzig, 1878.
  • Hausmann, K., N. Hulsmann, R. Radek. Protistology. Schweizerbart'sche Verlagsbuchshandlung, Stuttgart, 2003.
  • Margulis, L., J.O. Corliss, M. Melkonian, D.J. Chapman. Handbook of Protoctista. Jones and Bartlett Publishers, Boston, 1990.
  • Margulis, L., K.V. Schwartz. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3rd ed. New York: W.H. Freeman, 1998.
  • Margulis, L., L. Olendzenski, H.I. McKhann. Illustrated Glossary of the Protoctista, 1993.
  • Margulis, L., M.J. Chapman. Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth. Amsterdam: Academic Press/Elsevier, 2009.
  • Schaechter, M. Eukaryotic microbes. Amsterdam, Academic Press, 2012.

Physiology, ecology and paleontology

  • Foissner, W.; D.L. Hawksworth. Protist Diversity and Geographical Distribution. Dordrecht: Springer, 2009
  • Fontaneto, D. Biogeography of Microscopic Organisms. Is Everything Small Everywhere? Cambridge University Press, Cambridge, 2011.
  • Levandowsky, M. Physiological Adaptations of Protists. In: Cell physiology sourcebook : essentials of membrane biophysics. Amsterdam; Boston: Elsevier/AP, 2012.
  • Moore, R. C., and other editors. Treatise on Invertebrate Paleontology. Protista, part B (vol. 1, Charophyta, vol. 2, Chrysomonadida, Coccolithophorida, Charophyta, Diatomacea & Pyrrhophyta), part C (Sarcodina, Chiefly “Thecamoebians” and Foraminiferida) and part D (Chiefly Radiolaria and Tintinnina). Boulder, Colorado: Geological Society of America; & Lawrence, Kansas: University of Kansas Press.

External links

  • Tree of Life: Eukaryotes
  • A java applet for exploring the new higher level classification of eukaryotes
  • Plankton Chronicles – Protists – Cells in the Sea – video
  • Database of protist images
  • Holt, Jack R. and Carlos A. Iudica. 2013. Diversity of Life. http://comenius.susqu.edu/biol/202/Taxa.htm Last modified: 11/18/13.