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Multicellular organism

Multicellular organisms are organisms that consist of more than one cell, in contrast to unicellular organisms.{{cite book | last = Becker| first = Wayne M. | title = The world of the cell | publisher = Pearson Benjamin Cummings | series = | year = 2008 | doi = | isbn = 978-0-321-55418-5 | page = 480|display-authors=etal}} All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium. Multicellular organisms arise in various ways, for example by cell division or by aggregation of many single cells. Colonial organisms are the result of many identical individuals joining together to form a colony. However, it can often be hard to separate colonial protists from true multicellular organisms, because the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular".{{cite book |author1=Brian Keith Hall |author2=Benedikt Hallgrímsson |author3=Monroe W. Strickberger | year = 2008 | title = Strickberger's evolution: the integration of genes, organisms and populations | edition = 4th | publisher = Hall/Hallgrímsson | location = | isbn = 978-0-7637-0066-9 | page = 149 }}

Evolutionary history


Multicellularity has evolved independently at least 46 times, including in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and land plants.http://public.wsu.edu/~lange-m/Documnets/Teaching2011/Popper2011.pdf It evolved repeatedly for Chloroplastida (green algae and land plants), once or twice for animals, once for brown algae, three times in the fungi ( chytrids, ascomycetes and basidiomycetes) and perhaps several times for slime molds and red algae. The first evidence of multicellularity is from cyanobacteria-like organisms that lived 3–3.5 billion years ago. To reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cells (i.e. sperm and egg cells), an issue that is studied in evolutionary developmental biology. Animals have evolved a considerable diversity of cell types in a multicellular body (100–150 different cell types), compared with 10–20 in plants and fungi.Margulis, L. & Chapman, M.J. (2009). Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth ( ed.. ed.). Amsterdam: Academic Press/Elsevier. p. 116.

Loss of multicellularity

Loss of multicellularity occurred in some groups.Seravin L. N. (2001) The principle of counter-directional morphological evolution and its significance for constructing the megasystem of protists and other eukaryotes. Protistology 2: 6–14, linkhttp://cyberleninka.ru/article/n/the-principle-of-counter-directional-morphological-evolution-and-its-significance-for-construction-the-megasystem-of-protists-and-other. Fungi are predominantly multicellular, though early diverging lineages are largely unicellular (e.g. Microsporidia) and there have been numerous reversions to unicellularity across fungi (e.g. Saccharomycotina, Cryptococcus, and other yeasts).Parfrey, L.W. & Lahr, D.J.G. (2013), p. 344. It may also have occurred in some red algae (e.g. Porphyridium), but it is possible that they are primitively unicellular.Seckbach, Joseph, Chapman, David J. eds.. (2010). Red algae in the genomic age. New York, NY, U.S.A.: Springer, p. 252, linkhttps://books.google.com/books?id=fegCa9G-c90C&lpg=PP1&hl=pt-BR&pg=PA252#v=onepage&q&f=false. Loss of multicellularity is also considered probable in some green algae (e.g. Chlorella vulgaris and some Ulvophyceae).Richter, Daniel Joseph: The gene content of diverse choanoflagellates illuminates animal origins, 2013. In other groups, generally parasites, a reduction of multicellularity occurred, in number or types of cells (e.g. the myxozoans, multicellular organisms, earlier thought to be unicellular, are probably extremely reduced cnidarians).http://tolweb.org/Myxozoa/2460


Multicellular organisms, especially long-living animals, face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development. Changes in tissue morphology can be observed during this process. Cancer in animals ( metazoans) has often been described as a loss of multicellularity. There is a discussion about the possibility of existence of cancer in other multicellular organismsRichter, D. J. (2013), p. 11. or even in protozoa.Lauckner, G. (1980). Diseases of protozoa. In: Diseases of Marine Animals. Kinne, O. (ed.). Vol. 1, p. 84, John Wiley & Sons, Chichester, UK. For example, plant galls have been characterized as tumors but some authors argue that plants do not develop cancer.

Separation of somatic and germ cells

In some multicellular groups, which are called Weismannists, a separation between a sterile somatic cell line and a germ cell line evolved. However, Weismannist development is relatively rare (e.g. vertebrates, arthropods, Volvox), as great part of species have the capacity for somatic embryogenesis (e.g. land plants, most algae, many invertebrates).Ridley M (2004) Evolution, 3rd edition. Blackwell Publishing, p. 295-297.Niklas, K. J. (2014) The evolutionary-developmental origins of multicellularity.

Hypotheses for origin

consists of four cells.]] One hypothesis for the origin of multicellularity is that a group of function-specific cells aggregated into a slug-like mass called a grex, which moved as a multicellular unit. This is essentially what slime molds do. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a coenocyte. A membrane would then form around each nucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected cells in one organism (this mechanism is observable in Drosophila). A third hypothesis is that as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues. This is what plant and animal embryos do as well as colonial choanoflagellates. Multicellular development in a choanoflagellate; Stephen R. Fairclough, Mark J. Dayel and Nicole King In a Single-Cell Predator, Clues to the Animal Kingdom’s Birth Because the first multicellular organisms were simple, soft organisms lacking bone, shell or other hard body parts, they are not well preserved in the fossil record.A H Knoll, 2003. Life on a Young Planet. Princeton University Press. (hardcover), (paperback). An excellent book on the early history of life, very accessible to the non-specialist; includes extensive discussions of early signatures, fossilization, and organization of life. One exception may be the demosponge, which may have left a chemical signature in ancient rocks. The earliest fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon ( Gabonionta). This kind of severely co-dependent symbiosis can be seen frequently, such as in the relationship between clown fish and Riterri sea anemones. In these cases, it is extremely doubtful whether either species would survive very long if the other became extinct. However, the problem with this theory is that it is still not known how each organism's DNA could be incorporated into one single genome to constitute them as a single species. Although such symbiosis is theorized to have occurred (e.g. mitochondria and chloroplasts in animal and plant cells— endosymbiosis), it has happened only extremely rarely and, even then, the genomes of the endosymbionts have retained an element of distinction, separately replicating their DNA during mitosis of the host species. For instance, the two or three symbiotic organisms forming the composite lichen, although dependent on each other for survival, have to separately reproduce and then re-form to create one individual organism once more.

The cellularization (syncytial) theory

This theory states that a single unicellular organism, with multiple nuclei, could have developed internal membrane partitions around each of its nuclei.

The colonial theory

The Colonial Theory of Haeckel, 1874, proposes that the symbiosis of many organisms of the same species (unlike the symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism. At least some, it is presumed land-evolved, multicellularity occurs by cells separating and then rejoining (e.g. cellular slime molds) whereas for the majority of multicellular types (those that evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division.{{Cite journal | doi = 10.1038/420745a | title = Multicellularity: Evolution and the egg | year = 2002 | last1 = Wolpert | first1 = L. | last2 = Szathmáry | first2 = E. | journal = Nature | volume = 420 | pages = 745 | pmid = 12490925 | issue = 6917 }} The mechanism of this latter colony formation can be as simple as incomplete cytokinesis, though multicellularity is also typically considered to involve cellular differentiation.{{Cite journal | doi = 10.1002/bies.20197 | title = A twelve-step program for evolving multicellularity and a division of labor | year = 2005 | last1 = Kirk | first1 = D. L. | journal = BioEssays | volume = 27 | pages = 299–310 | pmid = 15714559 | issue = 3 }} The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently in 16 different protoctistan phyla. For instance, during food shortages the amoeba Dictyostelium groups together in a colony that moves as one to a new location. Some of these amoeba then slightly differentiate from each other. Other examples of colonial organisation in protista are Volvocaceae, such as Eudorina and Volvox, the latter of which consists of up to 500–50,000 cells (depending on the species), only a fraction of which reproduce.AlgaeBase. Volvox Linnaeus, 1758: 820. For example, in one species 25–35 cells reproduce, 8 asexually and around 15–25 sexually. However, it can often be hard to separate colonial protists from true multicellular organisms, as the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular".

The Synzoospore theory

Some authors suggest that the origin of multicellularity, at least in Metazoa, occurred due to a transition from temporal to spatial cell differentiation, rather than through a gradual evolution of cell differentiation, as affirmed in Haeckel’s Gastraea theory.Mikhailov K. V., Konstantinova A. V., Nikitin M. A., Troshin P. V., Rusin L., Lyubetsky V., Panchin Y., Mylnikov A. P., Moroz L. L., Kumar S. & Aleoshin V. V. (2009). The origin of Metazoa: a transition from temporal to spatial cell differentiation. Bioessays, 31(7), 758–768, linkhttp://www.kumarlab.net/pdf_new/MikhailovAleoshin09.pdf.


About 800 million years ago, a minor genetic change in a single molecule called guanylate kinase protein-interaction domain (GK-PID) may have allowed organisms to go from a single cell organism to one of many cells.

The role of viruses

Genes borrowed from viruses have recently been identified as playing a crucial role in the differentiation of multicellular tissues and organs and even in sexual reproduction, in the fusion of egg cell and sperm. Such fused cells are also involved in metazoan membranes such as those that prevent chemicals crossing the placenta and the brain body separation. Two viral components have been identified. The first is syncytin, which came from a virus. The second identified in 2007 is called EFF1, which helps form the skin of Caenorhabditis elegans, part of a whole family of FF proteins. Felix Rey, of the Pasteur Institute in Paris has constructed the 3D structure of the EFF1 proteinJamin, M, H Raveh-Barak, B Podbilewicz, FA Rey (2014) "Structural basis of eukaryotic cell-cell fusion" (Cell, Volume 157, Issue 2, 10 April 2014), Pages 407–419 and shown it does the work of linking one cell to another, in viral infections. The fact that all known cell fusion molecules are viral in origin suggests that they have been vitally important to the inter-cellular communication systems that enabled multicellularity. Without the ability of cellular fusion, colonies could have formed, but anything even as complex as a sponge would not have been possible.Slezak, Michael (2016), "No Viruses? No skin or bones either" (New Scientist, No. 2958, 1 March 2014) p.16


Multicellularity allows an organism to exceed the size limits normally imposed by diffusion: single cells with increased size have a decreased surface-to-volume ratio and have difficulty absorbing sufficient nutrients and transporting them throughout the cell. Multicellular organisms thus have the competitive advantages of an increase in size without its limitations. They can have longer lifespans as they can continue living when individual cells die. Multicellularity also permits increasing complexity by allowing differentiation of cell types within one organism.

See also


External links

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This article based upon the http://en.wikipedia.org/wiki/multicellular_organism, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=multicellular_organism&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany