| Monera | |
|---|---|
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| Scanning electron micrograph ofEscherichia colirods | |
Scientific classification | |
| Groups included | |
| Cladistically included but traditionally excluded taxa | |
DomainEukaryota | |
Monera (/məˈnɪərə/) (Greek:μονήρης (monḗrēs), "single", "solitary") is historically abiological kingdom that is made up ofprokaryotes. As such, it is composed of single-celled organisms that lack anucleus.
The taxon Monera was first proposed as a phylum byErnst Haeckel in 1866. Subsequently, the phylum was elevated to the rank of kingdom in 1925 byÉdouard Chatton. The last commonly accepted mega-classification with the taxon Monera was thefive-kingdom classification system established byRobert Whittaker in 1969.
Under thethree-domain system oftaxonomy, introduced byCarl Woese in 1977, which reflects the evolutionary history of life, the organisms found in kingdom Monera have been divided into twodomains,Archaea andBacteria (withEukarya as the third domain). Furthermore, the taxon Monera isparaphyletic (does not include all descendants of their most recent common ancestor), asArchaea andEukarya are currently believed to be more closely related than either is toBacteria. The term "moneran" is theinformal name of members of this group and is still sometimes used (as is the term "prokaryote") to denote a member of either domain.[1]
Most bacteria were classified under Monera; however, someCyanobacteria (often called the blue-green algae) were initially classified underPlantae due to their ability tophotosynthesize.
Traditionally the natural world was classified as animal, vegetable, or mineral as inSystema Naturae. After the development of themicroscope, attempts were made to fit microscopic organisms into either the plant or animal kingdoms. In 1675,Antonie van Leeuwenhoek discovered bacteria and called them "animalcules", assigning them to the class Vermes of the Animalia.[3][4][5] Due to the limited tools — the sole references for this group were shape, behaviour, and habitat — the description of genera and their classification was extremely limited, which was accentuated by the perceived lack of importance of the group.[6][7][8]
Seven years afterThe Origin of Species byCharles Darwin, in 1866Ernst Haeckel, a supporter of evolutionary theory, proposed a three-kingdom system that added the Protista as a new kingdom that contained most microscopic organisms.[2] One of his eight major divisions of Protista was composed of the monerans (called Moneres by Haeckel), which he defined as completely structure-less and homogeneous organisms, consisting only of a piece of plasma. Haeckel's Monera included not only bacterial groups of early discovery but also several small eukaryotic organisms; in fact the genusVibrio is the only bacterial genus explicitly assigned to the phylum, while others are mentioned indirectly, which ledCopeland to speculate that Haeckel considered all bacteria to belong to the genusVibrio, ignoring other bacterial genera.[7] One notable exception were the members of the modern phylumCyanobacteria, such asNostoc, which were placed in the phylum Archephyta ofAlgae (vide infra:Blue-green algae).
The Neolatin noun Monera and the German noun Moneren/Moneres are derived from the ancient Greek nounmoneres, which Haeckel stated meant "simple";[2] however, it actually means "single, solitary".[9] Haeckel also describes the protist genusMonas in the two pages about Monera in his 1866 book.[2] The informal name of a member of the Monera was initially moneron,[10] but later moneran was used.[1]
Due to its lack of features, the phylum was not fully subdivided, but the genera therein were divided into two groups:
Like Protista, the Monera classification was not fully followed at first and several different ranks were used and located with animals, plants, protists or fungi. Furthermore, Haeckel's classification lacked specificity and was not exhaustive — it in fact covers only a few pages—, consequently a lot of confusion arose even to the point that the Monera did not contain bacterial genera and others according to Huxley.[10] They were first recognized as a kingdom by Enderlein in 1925 (Bakterien-Cyclogenie. de Gruyter, Berlin).
The most popular scheme was created in 1859 byC. Von Nägeli who classified non-phototrophic Bacteria as the class Schizomycetes.[16]
The class Schizomycetes was then emended byWalter Migula (along with the coinage of the genusPseudomonas in 1894)[17] and others.[18] This term was in dominant use even in 1916 as reported byRobert Earle Buchanan, as it had priority over other terms such as Monera.[19] However, starting withFerdinand Cohn in 1872 the termbacteria (or in GermanBacterien) became prominently used to informally describe this group of species without a nucleus:Bacterium was in fact a genus created in 1828 byChristian Gottfried Ehrenberg[20] Additionally, Cohn divided the bacteria according to shape namely:
Successively, Cohn created the Schizophyta of Plants, which contained the non-photrophic bacteria in the family Schizomycetes and the phototrophic bacteria (blue green algae/Cyanobacteria) in the Schizophyceae[21] This union of blue green algae and Bacteria was much later followed by Haeckel, who classified the two families in a revised phylum Monera in the Protista.[22]
Stanier and van Neil (1941, The main outlines of bacterial classification. J Bacteriol 42: 437- 466) recognized the Kingdom Monera with two phyla, Myxophyta and Schizomycetae, the latter comprising classes Eubacteriae (3 orders), Myxobacteriae (1 order), and Spirochetae (1 order); Bisset (1962, Bacteria, 2nd ed., Livingston, London) distinguished 1 class and 4 orders: Eubacteriales, Actinomycetales, Streptomycetales, and Flexibacteriales; Orla-Jensen (1909, Die Hauptlinien des naturalischen Bakteriensystems nebst einer Ubersicht der Garungsphenomene. Zentr. Bakt. Parasitenk., II, 22: 305-346) and Bergey et al (1925, Bergey's Manual of Determinative Bacteriology, Baltimore : Williams & Wilkins Co.) with many subsequent editions) also presented classifications.
The term Monera became well established in the 20s and 30s when to rightfully increase the importance of the difference between species with a nucleus and without. In 1925, Édouard Chatton divided all living organisms into two sections, Prokaryotes and Eukaryotes: the Kingdom Monera being the sole member of the Prokaryotes section.[23]
The anthropic importance of the crown group of animals, plants and fungi was hard to depose; consequently, several other megaclassification schemes ignored on the empire rank but maintained the kingdom Monera consisting of bacteria, such Copeland in 1938 and Whittaker in 1969.[7][24] The latter classification system was widely followed, in whichRobert Whittaker proposed a five kingdom system for classification of living organisms.[24] Whittaker's system placed most single celled organisms into either the prokaryotic Monera or the eukaryotic Protista. The other three kingdoms in his system were the eukaryotic Fungi, Animalia, and Plantae. Whittaker, however, did not believe that all his kingdoms were monophyletic.[25]Whittaker subdivided the kingdom into two branches containing several phyla:
Alternative commonly followed subdivision systems were based on Gram stains. This culminated in the Gibbons and Murray classification of 1978:[26]
In 1977, aPNAS paper byCarl Woese andGeorge Fox demonstrated that thearchaea (initially called archaebacteria) are not significantly closer in relationship to thebacteria than they are toeukaryotes. The paper received front-page coverage inThe New York Times,[28] and great controversy initially. The conclusions have since become accepted, leading to replacement of the kingdom Monera with the two domainsBacteria andArchaea.[25][29] A minority of scientists, includingThomas Cavalier-Smith, continue to reject the widely accepted division between these two groups. Cavalier-Smith has published classifications in which the archaebacteria are part of a subkingdom of the Kingdom Bacteria.[30]
Although it was generally accepted that one could distinguish prokaryotes from eukaryotes on the basis of the presence of anucleus,mitosis versusbinary fission as a way of reproducing, size, and other traits, themonophyly of the kingdom Monera (or for that matter, whether classification should be according tophylogeny) was controversial for many decades. Although distinguishing between prokaryotes from eukaryotes as a fundamental distinction is often credited to a 1937 paper byÉdouard Chatton (little noted until 1962), he did not emphasize this distinction more than other biologists of his era.[25]Roger Stanier andC. B. van Niel believed that the bacteria (a term which at the time did not includeblue-green algae) and the blue-green algae had a single origin, a conviction that culminated in Stanier writing in a letter in 1970, "I think it is now quite evident that the blue-green algae are not distinguishable from bacteria by any fundamental feature of their cellular organization".[31] Other researchers, such asE. G. Pringsheim writing in 1949, suspected separate origins for bacteria and blue-green algae. In 1974, the influentialBergey's Manual published a new edition coining the term cyanobacteria to refer to what had been called blue-green algae, marking the acceptance of this group within the Monera.[25]
Kingdom monera. They belong to the prokaryotecharacteristics of kingdommonere
Copeland1938 was invoked but never defined (see thehelp page).Chatton1925 was invoked but never defined (see thehelp page).