Fossil range: नियोप्रोटेरोज़ोइक - हाल में
The term protista was first used by Ernst Haeckel in 1866. Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms: the one-celled animal-like protozoa, the plant-like protophyta (mostly one-celled algae), and the fungus-like slime molds and water molds. Because these groups often overlap, they have been replaced by phylogenetic-based classifications. However, they are still useful as informal names for describing the morphology and ecology of 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, such as the Kinetoplastids and Apicomplexa are responsible for a range of serious human diseases, such as malaria and sleeping sickness.
Classification [संपादित करें]
Historical classifications [संपादित करें]
The first division of the protists from other organisms came in the 1820's, when the German biologist Georg A. Goldfuss introduced the word protozoa to refer to organisms such as ciliates and corals. 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 1860's 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. The kingdom of minerals was later removed from taxonomy by Ernst Haeckel, leaving plants, animals, and the protists as a “kingdom of primitive forms”.
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. This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life. The kingdom Protista was later modified to separate prokaryotes into the separate kingdom of Monera, leaving the protists as a group of eukaryotic microorganisms. These five kingdoms remained the accepted classification until the development of molecular phylogenetics in the late 20th century, when it became apparent that neither protists or monera were single groups of related organisms (they were not monophyletic groups).
Modern classifications [संपादित करें]
Currently, the term protist 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. 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 monophyletic groups based on ultrastructure, biochemistry, and genetics. Because the protists as a whole are paraphyletic, such systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. The recent scheme by Adl et al. (2005) 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 protists, which may be treated as phyla, are listed in the taxobox at right. 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.
Types of protists [संपादित करें]
Protozoa, the animal-like protists [संपादित करें]
Protozoa are mostly single-celled, motile protists that feed by phagocytosis, though there are numerous exceptions. They are usually only 0.01–0.5 mm in size, generally too small to be seen without magnification. Protozoa are grouped by method of locomotion into:
|Flagellates||with long flagella||e.g., Euglena|
|Amoeboids||with transient pseudopodia||e.g., Amoeba|
|Ciliates||with multiple, short cilia||e.g., Paramecium|
|Sporozoa||non-mobile parasites; some can form spores||e.g., Toxoplasma|
Algae, the plant-like protists [संपादित करें]
They include many single-celled organisms that are also considered protozoa, such as Euglena, which many believe have acquired chloroplasts through secondary endosymbiosis. Others are non-motile, and some (called seaweeds) are truly multicellular, including members of the following groups:
|Chlorophytes||green algae, are related to higher plants||e.g., Ulva|
|Rhodophytes||red algae||e.g., Porphyra|
|Heterokontophytes||brown algae, diatoms, etc.||e.g., Macrocystis|
Fungus-like protists [संपादित करें]
Metabolism [संपादित करें]
|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 or carbon fixation||Apicomplexa, Trypanosomes or Amoebae|
Reproduction [संपादित करें]
िन्हें भी देखें [संपादित करें]
सन्दर्भ [संपादित करें]
- 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. http://www.im.microbios.org/08december99/03%20Scamardella.pdf.
- Rothschild, L. J. (1989). "Protozoa, protista, protoctista: What's in a name?". Journal of the History of Biology 22 (2): 277–305. doi:10.1007/BF00139515. http://www.springerlink.com/index/LW54T61737212643.pdf.
- Copeland, H. F. (1938). "The Kingdoms of Organisms". Quarterly Review of Biology 13 (4): 383. doi:10.1086/394568. http://links.jstor.org/sici?sici=0033-5770(193812)13%3A4%3C383%3ATKOO%3E2.0.CO%3B2-K.
- Whittaker, R. H. (1959). "On the Broad Classification of Organisms". Quarterly Review of Biology 34 (3): 210. doi:10.1086/402733. http://links.jstor.org/sici?sici=0033-5770(195909)34%3A3%3C210%3AOTBCOO%3E2.0.CO%3B2-J.
- 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 (journal) 163 (863): 150–60. PMID 5762760.
- 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.
- Cavalier-Smith, T.; Chao, E. E. Y. (2003). "Phylogeny and classification of phylum Cercozoa (Protozoa)". Protist 154 (3–4): 341–358. doi:10.1078/143446103322454112.
- 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. PMID 17194223. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1713255.
|विकिमीडिया कॉमन्स पर Protista
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आगे पाठन [संपादित करें]
Marguilis, L., Corliss, J.O., Melkonian, M.,and Chapman, D.J. (Editors) 1990. Handbook of Protoctista. Jones and Bartlett , Boston. ISBN 0-86720-052-9