Jump to content

Protist

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by 213.120.222.194 (talk) at 15:33, 13 September 2013. The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

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

Many others;
classification varies

Protoctists /ˈproʊt[invalid input: 'ɨ']st/ are a diverse group of eukaryotic microorganisms. Historically, protoctists were treated as a biological kingdom formally called the Protista, and included mostly unicellular organisms that did not fit into the other kingdoms. Molecular information has been used to redefine this group in modern taxonomy as diverse and often distantly related phyla. The group of protoctists is now considered to mean diverse phyla that are not closely related through evolution and have different life cycles, trophic levels, modes of locomotion, and cellular structures.[1][2] Besides their relatively simple levels of organization, the protoctists do not have much in common.[3] They are unicellular, or they are multicellular without specialized tissues, and this simple cellular organization distinguishes the protoctists from other eukaryotes, such as fungi, animals and plants.

The term protista was first used by Ernst Haeckel in 1866. Protoctists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms: the unicellular "animal-like" protozoa, the "plant-like" protophyta (mostly unicellular algae), and the "fungus-like" slime molds and water molds. These traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on phylogenetics (evolutionary relatedness among organisms). However, the older terms are still used as informal names to describe the morphology and ecology of various protoctists.

Protoctists live in almost any environment that contains liquid water. Many protoctists, such as the algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Other protoctists, such as the Kinetoplastids and Apicomplexa, are responsible for a range of serious human diseases, such as malaria and sleeping sickness.

Classification

Historical classifications

The first division of the protoctists from other organisms came in the 1830s, when the German biologist Georg August Goldfuss introduced the word protozoa to refer to organisms such as ciliates and corals.[4] This group was expanded in 1845 to include all "unicellular animals", such as Foraminifera and amoebae. The formal taxonomic category Protoctista was first proposed in the early 1860s by John Hogg, who argued that the protoctists 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.[4] The kingdom of minerals was later removed from taxonomy by Ernst Haeckel, leaving plants, animals, and the protoctists as a “kingdom of primitive forms”.[5]

Herbert Copeland resurrected Hogg's label almost a century later, arguing that "Protoctista" literally meant "first established beings", Copeland complained that Haeckel's term protista included anucleated microbes such as bacteria. Copeland's use of the term protoctista did not. In contrast, Copeland's term included nucleated eukaryotes such as diatoms, green algae and fungi.[6] This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protoctista as the four kingdoms of life.[7] The kingdom Protoctista was later modified to separate prokaryotes into the separate kingdom of Monera, leaving the protoctists as a group of eukaryotic microorganisms.[8] These five kingdoms remained the accepted classification until the development of molecular phylogenetics in the late 20th century, when it became apparent that neither protoctists nor monera were single groups of related organisms (they were not monophyletic groups).[9]

Modern classifications

Currently, the term protoctist is used to refer to unicellular eukaryotes that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues.[10] The term protozoa is used to refer to heterotrophic species of protoctists 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 protoctists is still changing. Newer classifications attempt to present monophyletic groups based on ultrastructure, biochemistry, and genetics. Because the protoctists as a whole are paraphyletic, such systems often split up or abandon the kingdom, instead treating the protoctist groups as separate lines of eukaryotes. The recent scheme by Adl et al. (2005)[10] is an example that does not bother with formal ranks (phylum, class, etc.) and instead lists organisms in hierarchical lists. This is intended to make the classification more stable in the long term and easier to update. Some of the main groups of protoctists, which may be treated as phyla, are listed in the taxobox at right.[11] 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.[12]

Metabolism

Nutrition in some different types of protoctists is variable. In flagellates, for example, filter feeding may sometimes occur where the flagella find the prey. Other protoctists can engulf bacteria and digest them internally, by extending their cell membrane around the food material to form a food vacuole. This is then taken into the cell via endocytosis (usually phagocytosis; sometimes pinocytosis).

Nutritional types in protoctist 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 protoctists reproduce sexually (gametes), while others reproduce asexually (binary fission).

Some species, for example Plasmodium falciparum, have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually.[13] However, it is unclear how frequently sexual reproduction causes genetic exchange between different strains of Plasmodium in nature and most populations of parasitic protoctists may be clonal lines that rarely exchange genes with other members of their species.[14]

Sexual reproduction

Eukaryotes emerged in evolution more than 1.5 billion years ago.[15] The earliest eukaryotes were likely protoctists. 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 protoctists whose ancestors branched off early from the eukaryotic family tree. However, several of these protoctists 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 intestinalis was once considered to be a descendant of a protoctist lineage that predated the emergence of meiosis and sex. However, G. intestinalis was recently found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes.[16] These results suggested that G. intestinalis is capable of meiosis and thus sexual reproduction. Furthermore, direct evidence for meiotic recombination, indicative of sex, was also found in G. intestinalis.[17]

Parasitic protozoa of the genus Leishmania cause human diseases. These protoctists have recently been shown to have a sexual cycle as well.[18]

Trichomonas vaginalis, a parasitic protoctist, is not known to undergo meiosis, but when Malik et al.[19] 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 the capability for meiosis was present in recent ancestors of T. vaginalis. Since 21 of the 29 meiotic genes were also present in G. intestinalis, it appears that most of these meiotic genes were likely present in a common ancestor of T. vaginalis and G. intestinalis. These two species are descendants of protoctist lineages that are highly divergent among eukaryotes, leading Malik et al.[19] 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.[20]

This view was further supported by a study of amoebae by Lahr et al.[21] Amoeba have generally been regarded as asexual protoctists. However these authors describe evidence that most amoeboid lineages are anciently sexual, and that the majority of asexual groups likely arose recently and independently.

Protoctists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock. [22] 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 protoctists.[22]

Role as pathogens

Some protoctists are significant pathogens of both animals and 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.[23] A more thorough understanding of protoctist biology may allow these diseases to be treated more efficiently.

Recent papers have proposed the use of viruses to treat infections caused by protozoa.[24][25]

Researchers from the Agricultural Research Service are taking advantage of protoctists as pathogens in an effort to control red imported fire ant (Solenopsis invicta) populations in Argentina. With the help of spore-producing protoctists 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%.[26] Researchers have also found a way to infect phorid flies with the protoctist without harming the flies. This is important because the flies act as a vector to infect the red fire ant population with the pathogenic protoctist.[27]

See also

References

  1. ^ Simonite T (2005). "Protists push animals aside in rule revamp". Nature. 438 (7064): 8–9. doi:10.1038/438008b. PMID 16267517. {{cite journal}}: Unknown parameter |month= ignored (help)
  2. ^ Harper, David; Benton, Michael (2009). Introduction to Paleobiology and the Fossil Record. Wiley-Blackwell. p. 207. ISBN 1-4051-4157-3.{{cite book}}: CS1 maint: multiple names: authors list (link)
  3. ^ "Systematics of the Eukaryota". Retrieved 2009-05-31.
  4. ^ a b Scamardella, J. M. (1999). "Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista" (PDF). International Microbiology. 2: 207–221.
  5. ^ Rothschild LJ (1989). "Protozoa, Protista, Protoctista: what's in a name?" (PDF). J Hist Biol. 22 (2): 277–305. doi:10.1007/BF00139515. PMID 11542176.
  6. ^ Copeland, H. F. (1938). "The Kingdoms of Organisms". Quarterly Review of Biology. 13 (4): 383. doi:10.1086/394568. JSTOR 2808554.
  7. ^ Whittaker, R. H. (1959). "On the Broad Classification of Organisms". Quarterly Review of Biology. 34 (3): 210. doi:10.1086/402733. JSTOR 2816520.
  8. ^ Whittaker RH (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. doi:10.1126/science.163.3863.150. PMID 5762760. {{cite journal}}: Unknown parameter |month= ignored (help)
  9. ^ Stechmann, Alexandra (2003). "The root of the eukaryote tree pinpointed" (PDF). Current Biology. 13 (17): R665 – R666. doi:10.1016/S0960-9822(03)00602-X. PMID 12956967. Retrieved 15 May 2011. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ 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. doi:10.1111/j.1550-7408.2005.00053.x. PMID 16248873. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  11. ^ Cavalier-Smith T, Chao EE (2003). "Phylogeny and classification of phylum Cercozoa (Protozoa)". Protist. 154 (3–4): 341–58. doi:10.1078/143446103322454112. PMID 14658494. {{cite journal}}: Unknown parameter |month= ignored (help)
  12. ^ Laura Wegener Parfrey, Erika Barbero, Elyse Lasser, Micah Dunthorn, Debashish Bhattacharya, David J Patterson, and Laura A Katz (2006 December). "Evaluating Support for the Current Classification of Eukaryotic Diversity". PLoS Genet. 2 (12): e220. doi:10.1371/journal.pgen.0020220. PMC 1713255. PMID 17194223. {{cite journal}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)
  13. ^ Talman AM, Domarle O, McKenzie FE, Ariey F, Robert V (2004). "Gametocytogenesis: the puberty of Plasmodium falciparum". Malar. J. 3: 24. doi:10.1186/1475-2875-3-24. PMC 497046. PMID 15253774. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)
  14. ^ Tibayrenc M, Kjellberg F, Arnaud J; et al. (1991). "Are eukaryotic microorganisms clonal or sexual? A population genetics vantage". Proc. Natl. Acad. Sci. U.S.A. 88 (12): 5129–33. doi:10.1073/pnas.88.12.5129. PMC 51825. PMID 1675793. {{cite journal}}: Explicit use of et al. in: |author= (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  15. ^ Javaux EJ, Knoll AH, Walter MR (2001). "Morphological and ecological complexity in early eukaryotic ecosystems". Nature. 412 (6842): 66–9. doi:10.1038/35083562. PMID 11452306. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  16. ^ Ramesh MA, Malik SB, Logsdon JM (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. doi:10.1016/j.cub.2005.01.003. PMID 15668177. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  17. ^ Cooper MA, Adam RD, Worobey M, Sterling CR (2007). "Population genetics provides evidence for recombination in Giardia". Curr. Biol. 17 (22): 1984–8. doi:10.1016/j.cub.2007.10.020. PMID 17980591. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  18. ^ Akopyants NS, Kimblin N, Secundino N; et al. (2009). "Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector". Science. 324 (5924): 265–8. doi:10.1126/science.1169464. PMC 2729066. PMID 19359589. {{cite journal}}: Explicit use of et al. in: |author= (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  19. ^ a b Malik SB, Pightling AW, Stefaniak LM, Schurko AM, Logsdon JM (2008). "An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis". PLoS ONE. 3 (8): e2879. doi:10.1371/journal.pone.0002879. PMC 2488364. PMID 18663385.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)
  20. ^ Dacks J, Roger AJ (1999). "The first sexual lineage and the relevance of facultative sex". J. Mol. Evol. 48 (6): 779–83. PMID 10229582. {{cite journal}}: Unknown parameter |month= ignored (help)
  21. ^ Lahr DJ, Parfrey LW, Mitchell EA, Katz LA, Lara E (2011). "The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms". Proc. Biol. Sci. 278 (1715): 2081–90. doi:10.1098/rspb.2011.0289. PMC 3107637. PMID 21429931. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  22. ^ a b Bernstein H, Bernstein C, Michod RE (2012). DNA repair as the primary adaptive function of sex in bacteria and eukaryotes. Chapter 1: pp.1-49 in: DNA Repair: New Research, Sakura Kimura and Sora Shimizu editors. Nova Sci. Publ., Hauppauge, N.Y. ISBN: 978-1-62100-808-8 https://www.novapublishers.com/catalog/product_info.php?products_id=31918
  23. ^ N. Campbell, J. Reese. Biology. 2008. pp. 583, 588
  24. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.2217/FMB.13.48, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.2217/FMB.13.48 instead.
  25. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1016/j.tim.2013.02.006, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1016/j.tim.2013.02.006 instead.
  26. ^ "ARS Parasite Collections Assist Research and Diagnoses". USDA Agricultural Research Service. January 28, 2010.
  27. ^ http://www.ars.usda.gov/is/pr/2010/100128.htm
Wikimedia Commons has media related to Protista.
Wikispecies has information related to Protista.
Retrieved from "https://en.wikipedia.org/enwiki/w/index.php?title=Protist&oldid=572770005"