Evolution of multicellularity – Wikipedia

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Multicellular organisms are living beings with several cells, closely close and strongly interacting. They are present throughout the living world, and are particularly numerous among eukaryotes (plants, metazoaries, etc.). The appearance of the first multicellular organizations dates from at least 2.1 billion years [ 2 ] . Only five major lines would have developed a complex multicellularity which however appeared at least 25 times during the evolution [ 3 ] , by different mechanisms, probably due to the selective advantages which it gives, such as the possibility of an increase in the size of the organism or a specialization of the different cells. Multicellularity, however, leads to the appearance of genomic conflicts and selfish cell lines, such as tumors. These conflicts can be resolved by the appearance of prevention mechanisms against these cheating cells, or by a return to unicellularity.

Table of Contents

Definition [ modifier | Modifier and code ]

Multicelularity is defined as the possession of several cells in a living organism. On the other hand, coordination of at least one function is necessary to qualify a multicellular organism. So, for example, we cannot qualify a colony of bacteria as multicellular, because there is a lack of coordination between cells despite an occasional demonstration of organized growth. In addition, in multicellular organisms, there is cellular differentiation in order to provide different tasks.

Primitive multicellular organizations [ modifier | Modifier and code ]

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The multicellularity has appeared independently at least 25 times including in prokaryotes (in cyanobacteria, myxobacteria, etc.). On the other hand, the complex multicellularity only appeared in six eukaryotic groups: animals, mushrooms, brown, red and green algae as well as in plants [ 4 ] . The appearance of the first multicellularity, including fossils dating from the Protozoic era, can be dated around 2.1 billion years [ 5 ] . It is a tube -shaped organism of about 1 cm , considered the first multicellular organization, even if you lack information to consider it in a certain way as a multicellular organism. Some scientists consider him to be either a colony of bacteria, or the first eukaryotal alga. An in -depth study and the discovery of other fossils will make it possible to better understand this organization and to classify it with certainty.

Loss of multicellularity [ modifier | Modifier and code ]

The multicellularity was able to disappear in certain groups such as mushrooms [ 6 ] .

Classification of multicellularity in organizations [ modifier | Modifier and code ]

The multicellularity, having appeared several times during the evolution of life, can be classified according to three aspects of the cell phenotype: the first aspect is the date of the appearance. The second, the mechanism by which the multicellularity appeared. Finally, there is the level of complexity reached by the multicellular body.

History [ modifier | Modifier and code ]

Multicellular organizations have evolved at least 25 times from unicellular ancestors during life [ 7 ] .

The emergence of a first complex and organized multicellular life is attested in precambrian rocks in Gabon dated 2.1 billion years ( Gaboniona ). This form of life, probably favored by the paleoproterozoic oxygenation of the terrestrial biosphere following the disaster of oxygen and by the end of the Huronian glaciation, nevertheless seems to have been extinguished without having descendants, perhaps because of a theminization of his environment [ 8 ] .

The multicellular life will then take more than a billion and a half years to reappear and thrive in an undeniable manner, with the Cambrian explosion.

Available data [ modifier | Modifier and code ]

Multicellular fossils [ modifier | Modifier and code ]

Contemporary multicellular organizations [ modifier | Modifier and code ]

Unicellular organisms with multicellular vestiges [ modifier | Modifier and code ]

Study methods [ modifier | Modifier and code ]

Comparative biology [ modifier | Modifier and code ]

The study of the appearance of multicellularity was done by two methods. The first is comparative biology. It is a multidisciplinary approach aimed at understanding the diversity of organizations. This is done by studying the natural variation and the disparity of morphologies as well as certain biological mechanisms. With regard to multicellular organisms, comparative biology compares multicellular organisms and unicellular organisms and determines major differences in terms of structure and biochemical reactions. Thus, by observing and noting these major differences, scientists are able to withdraw information concerning the changes that have led to the appearance of multicellularity in a world dominated by unicellular organizations.

Experimentation [ modifier | Modifier and code ]

[ 9 ] Second, we find the study by experimentation. This aims to reproduce the primitive conditions of the earth. Its purpose is to determine what has influenced, which has catalyzed the transition and reproducing the event in order to confirm/understand the appearance of multicellularity.

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Different appearances of multicellularity [ modifier | Modifier and code ]

In ebacteria [ modifier | Modifier and code ]

The multicellularity appeared at least three times independent ways in ebacteria: in Myxobacteria , at the Actinobacteria and among Cyanobacteria [ ten ] . Most species in these three groups have at least one phase of multicellular life. In addition, some authors consider that a large number of bacterial colonies can be considered multicellular, given the importance of communication between different cells and the coordination of their actions [ 11 ] . In ebacteria, the appearance of multicellularity is considered to be an induced response due to an environmental stimulus. The date of the appearance of multicellularity can be drawn to around 900 million years in myxobacteria (a group of bacteria living in the soil, feeding on insoluble organic substances; also known as “Slime Bacteria”). In cyanobacteria, the appearance of multicellularity dates from around 2.5 – 2.1 billion years. Actinobacteria, a phylum of terrestrial and aquatic gram-post bacteria, the appearance of multicellularity is almost as old as in cyanobacteria. Even if we observe an appearance of multicellularity very early in ebacteria (1+ billion years), it is important to note that in certain groups, such as the green algae of the Volvocaceae family, its appearance goes back as little as 500 million years.

Among the Archaea [ modifier | Modifier and code ]

There are certain species of archaea, of the genus Methanosarcina, forming groups of cells linking each other after division. These species, a compulsory anaerobic, could benefit from multicellularity by protecting the most cells in the center of too much dioxygen in the outdoor environment. On the other hand, considering the fact that the membrane of most archeobacteria do not have organizations, which are typical and useful in the accomplishment of more complex and specialized tasks, the appearance of multicellularity is limited to species of the genus Methanosarcina . No other appearance of multicellular organisms from archaea can be noted or detected through genetic and evolving studies currently in progress. It should still be noted that even if there was only an appearance in the archaea, since they are the area closest to the eukaryotes, phylogenetically speaking, it is very possible that further through the Evolution, we will observe another divergence creating a new line of multicellular organizations.

For eukaryotes [ modifier | Modifier and code ]

The multicellularity appeared many times independent ways among eukaryotes. If the multicellularity of metazoa seems monophyletic, this is probably not the case for that of plants or mushrooms. Thus, there are multicellular organisms in opisthokonta (Metazoa, some Mycota and certain choanomonada), among the Amoebozoa, in archaeplastida (embryophytes, certain other chlorophyta and certain rhodophyta), in the stramenopila (some oomycota, certain phaeophyceae and rare Multicellular forms in bacillariophyta), in some excavata and alveolata (rare multicellular forms in ciliophora). There is no colonial or multicellular shape in the Rhizaria and the Excavata.

Several theories are proposed to explain the origin of metazoa. The symbiotic theory presumes that independent cells have developed a symbiotic relationship so close that they have lost their autonomy and had to regroup in cellular clusters, agglutinating thanks to collagen proteins. Colonial theory suggests that metazoaries derive from colonies of Choanoflagellated [ twelfth ] . Syncytial or plasmodial theory makes the metazoa derive from a multinucleated protozoan which becomes multicalel, by compartmentalizing its mass by partitions forming as many cells as there are nuclei [ 13 ] .

Hypotheses and theories of underlying mechanisms leading to the formation of multicellularity:

Symbiotic theory [ modifier | Modifier and code ]

This theory suggests that the first multicellular organisms appeared thanks to the symbiosis of several unicellular organisms each with a different specific role. The symbiosis is defined as a long -term interaction between two organisms. This relationship is usually positive. Over time, these organizations have become so dependent on each other that they could not survive individually, leading to the incorporation of their genome into a greater genome of a multicellular organism. So, each old organization would have been used to create a new line of cells specializing in the multicellular body. This kind of co-dependent relationship between species is observable through nature. On the other hand, this theory also poses a certain problem. The mechanism by which the genomes separated from the two ancient individuals could merge to create a new genome is still a mystery. No mechanism has been discovered that can describe this kind of phenomenon through multicellular organisms. Even if the symbiosis has already been observed over the course of evolution (mitochondria in eucaryotes, chloroplasts in vegetable cells), it is extremely rare and improbable. In addition, in the examples mentioned above, each element has retained a certain distinction from the primary genome. Thus, we know that mitochondria has its own genome coding its own proteins.

The theory of syncytial cellularization [ modifier | Modifier and code ]

This theory dictates that a single unicellular organism with several nuclei could have developed an internal membrane around each nucleus. Several protists such as ciliates and mold (mulds slime) can have several nuclei. This supports the proposed theory. On the other hand, the nuclei present in ciliates each have a specific function (example of the macronoyau and the micronoyau). The process of differentiation and formation of a multicellular organism from a syncytium has never been observed. So, to validate this theory, it will be necessary to find a living organism in which this phenomenon takes place.

Colonial theory [ modifier | Modifier and code ]

This theory was proposed by Ernst Haeckel, a German scientist, in 1874. Haeckel is also known for having popularized the theory that ontogenia summarizes phylogeny (theory now invalidated). This theory is that the symbiosis of several organisms of the same species (vs several organisms of different species in symbiotic theory) led to the formation of multicellular organisms. According to the definition of multicellularity, there is a requirement for cellular differentiation, therefore “colonies”, according to this definition, cannot be considered as multicellular organisms. On the other hand, some scientists do not agree on how to define a multicellular organism. There is therefore room for interpretation. This theory is supported by the fact that we have already observed this kind of phenomenon. During a lack of resources, Amibe Dictostelium tends to regroup between individuals in colonies that moves as an organism. Some Amibes then tended to slightly differentiate themselves from each other. It is sometimes difficult to differentiate from the colonies of prostists (such as amibes) of the real multicellular organisms, because the two concepts are not distinct. Some colonies are then described as multicellular in place of multicellular.

Synzoospore theory [ modifier | Modifier and code ]

Some authors suggest that the origin of the multicellularity, in metazoaries, occurred during a transition from cell differentiation from a temporal mode to a spatial mode. Instead of changing shape and structure over time, cells have changed in different places at the same time, thus creating differentiated cells. This theory is supported by the fact that several unicellular organisms contain the genes that are responsible for development, cell differentiation, cellular adhesion as well as cell-matrix adhesion in metazoans. Thus, with the presence of these genes, it is possible that different cell phenotypes be produced at the same time to make room for cells at different roles.

GK-Pid theory [ modifier | Modifier and code ]

In , some scientists have discovered that the minor mutation of a gene coding the molecule GK-PID About 800 million years ago would have given the ability to certain organizations to adapt and move to multicellularity. This molecule is used to regulate the orientation of the cytoskeleton by linking several motor proteins of the microtubule in various taxa in animals. The mutation was used to change the protein’s ability to bind to proteins of cortical markers. This change has radically changed its function, conferring an essential capacity in several animal cells to bind to each other.

The role of viruses in the appearance of multicellularity [ modifier | Modifier and code ]

Two genes serving a crucial role in the differentiation of multicellular tissues were identified in viruses. These two proteins are syncytin and protein EFF1. The latter was proven to help the formation of the skin at C. Elegans. It acts by binding the cells between them. Thus, the fact that these two proteins are of viral origin suggests that the genome of a virus, therefore the virus in itself, could have had an important role to play in the appearance of multicellularity. Their role in intercellular communication would therefore have been crucial [ 14 ] .

Profits [ modifier | Modifier and code ]

Seeing the number of times the multicellularity has appeared, it is obvious that it must have an advantage compared to the unicellular organizations. The first advantage stems from the size of the body. By being simply larger than unicellular organisms, organizations can either be better defended by predators, or better feed as predators. In addition, in some Volvox colonies, the girls’ colonies, instead of being exposed and in danger, are protected inside the mother colony. The multicellularity therefore gives an advantage to protect its fellows in order to better propagate the species as such. Another important factor providing an advantage in multicellular organisms is the storage of resources. For example, in algae, phosphate is often a limiting factor in growth. Large multicellular algae have an advantage, as they can store any excess phosphate in the liquid of the extracellular matrix. Other nutrients such as minerals, ions and water can be stored. In addition, larger organizations, such as algae colonies, are more likely to be picked up and dispersed by vectors such as birds. This allows a more effective dispersion and gives increased skill to colonize new waters. Finally, the differentiation of cells and their specializations allow each cell to accomplish the role which is assigned to them more effectively. This is one of the greatest advantages compared to unicellularity [ 15 ] , [ 16 ] .

Adaptations necessary for multicellularity [ modifier | Modifier and code ]

Cell differentiation [ modifier | Modifier and code ]

Intercellular communication [ modifier | Modifier and code ]

Intercellular membership [ modifier | Modifier and code ]

Interactions with microbes [ modifier | Modifier and code ]

As soon as living things have become multicellular, they had to socialize with microbes, first occupants of the planet, and establish with them a state of commensalism, even symbiosis through in particular microbiotes (man can thus be considered as a Primate-Microbes hybrid). The systems governing the management of these interactions, which were born from the adaptation of among the most fundamental mechanisms of development, were remarkably preserved during evolution, from the insect to the senior primates [ 17 ] .

Constraints [ modifier | Modifier and code ]

Multicelularity does not only give the advantages of the organizations. If this were the case, unicellular organizations could not survive in competition with them. There are certain costs associated with multicellular life. Several cells lose vital capacities. For example, at Volvox , the majority of 50,000 cells forming the colony are sterile sterile specialized cells. They have lost the ability to reproduce. These cells must count on the few germ cells of the colony which still have a reproductive capacity. Second, we must look at the cost of reorganizing the cytoskeleton in order to accommodate multicellular life. At Volvox, to keep the light intensity received optimal for photosynthethiser, a certain cellular position is favored in the cells of the colony. Maintaining this position requires an energy cost (due to the movement of the flagellum in individual cells). On the other hand, there is an incompatibility between colonial cellular division and motility in a cell. There is therefore competition to acquire the best positioning to optimize photosynthesis. All this is to demonstrate that the cost of specialization and cell differentiation can be high and therefore, multicellularity cannot develop in any organism as such. Only organizations that can accommodate costs will benefit from the multiple advantages given by multicellular life.

Evolutionary consequences of multicellularity [ modifier | Modifier and code ]

Multicellular life is one of the evolutionary changes that has most affected life on earth. First, the appearance of multicellularity allowed organizations to colonize places that would never have been accessible to unicellular beings. The abundance of specialized life adapted to their environment comes from the fact that multicellular animals have developed the ability to adapt more easily than a unicellular organism. This includes extreme places where unicellular beings would have no chance of survival such as deserts, the continent of Antarctica, etc. Second, in connection with the first point, the adaptive capacity of multicellular organizations is due to the variation in the transfer rate. This rate varies among different taxa and high -rate organizations have higher adaptive capacity than organizations with a relatively stable genome. Following this, multicellular organisms have evolved much longer longevity of life than unicellular organisms. In bacteria and other unicellular organisms, we usually measure the life of the organism in days and weeks. In more complex multicellular organizations, such as mammals, the lifespan is now measured in years, see decades. The specialization of each cell to do their work, the development of a protective epithelium which separates the interior from the organism from the outside as well as the coordination of all activity in order to keep homeostasis have allowed multicellular organisms survive longer and acquire increased longevity.

Genomic conflicts [ modifier | Modifier and code ]

Complexification [ modifier | Modifier and code ]

  • Mammals:
  • Bacteria :
  1. ↑ Georges B. Johnson, Peter H. Raven, Jonathan B. Losos, Susan R. Singer, Biology , Of superior Boeck, , p. 645 .
  2. ↑ (in) He Albanian Coll you. , ‘ Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago » , Nature , n O 466, , p. 100-104 (DOI  10.1038/nature09166 ) .
  3. ↑ (in) Richard K. Grosberg et Richard R. Strathmann , ‘ The evolution of multicellularity: a minor major transition » , Annual review of ecology evolution and systematics , vol. 38, , p. 621-654 (ISSN  1543-592X , DOI  10.1146/annurev.ecolsvs.36.102403.114735 , read online ) .
  4. ↑ (in) Zoe A. Popper, Gurvan Michel, Cecile Herv, David S. Domozych, William G.T. Willats, Maria G. Tuohy, Bernard Kloareg & Dagmar B. Stengel, ‘ Evolution and diversity of plant cell walls: from algae to flowering plants » , Annual Review of Plant Biology , vol. 62, n O 5, , p. 571 .
  5. ↑ Grypania Spiralis
  6. ↑ Seravin lev N « The principle of counter-directional morphological evolution and its significance for construction the megasystem of protists and other eukaryotes », Protistology , vol. 2, n O 1, (ISSN  1680-0826 , read online , consulted the )
  7. ↑ (in) Richard K. Grosberg et Richard R. Strathmann, ‘ The Evolution of Multicellularity : A Minor Major Transition ? » , The Annual Review of Ecology, Evolution, and Systematics , n O 38, , p. 622 (DOI  10.1146/annurev.ecolsys.36.102403.114735 ) .
  8. ↑ (in) Abderrazak El Albani et col. ‘ Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago » , Nature , n O 466, , p. 100–104 (DOI  10.1038/nature09166 ) .
  9. ↑ (in) Mariiny it. Boraas , Dianne B. Seale and Joseph E. Boxhorn , ‘ Phagotrophy by a flagellate selects for colonial prey: A possible origin of multicellularity » , Evolutionary Ecology , vol. twelfth, n O 2, , p. 153-164 (ISSN  0269-7653 , DOI  10.1023/A:1006527528063 , résumé ) .
  10. ↑ (in) John Tyler Bonner , ‘ The Origins of Multicellularity » , Integr. Biol. , vol. first, n O 1, , p. 27-36 ( résumé ) .
  11. ↑ (in) James A. Shapiro , ‘ Thinking about bacterial populations as multicellular organisms » , Annual review of microbiology , vol. 52, , p. 81-104 (ISSN  0066-4227 , DOI  10.1146/annurev.micro.52.1.81 , read online ) .
  12. ↑ This hypothesis is based on two arguments: presence of one or more flagelles on sperm; Larve Planula which appears in the life cycle of various lower metazoa.
  13. ↑ (in) Philippe H, Derelle R, Lopez P, et al, ‘ Phylogenomics revives traditional views on deep animal relationships » , Curr. Biol , vol. 19, n O 8, , p. 706–712 (DOI  10.1016 / J.cub.2009.02.052 )
  14. ↑ (in) Antonis Hand , ‘ The Origins of multicellularity and the early history of the genetic toolkit for animal development » , Annual review of genetics , vol. 42, , p. 235-251 (ISSN  0066-4197 , DOI  10.1146/annurev.genet.42.110807.091513 , read online ) .
  15. ↑ (in) Steven M. Stanley , ‘ An Ecological Theory for the Sudden Origin of Multicellular Life in the Late Precambrian » , PNAS , vol. 70, n O 5, , p. 1486-1489 (ISSN  0027-8424 , read online ) .
  16. ↑ (in) Richard E. Michod , ‘ Evolution of individuality during the transition from unicellular to multicellular life » , PNAS , vol. 104, n O S1, , p. 8613-8618 (ISSN  0027-8424 , DOI  10.1073/pnas.0701489104 , read online ) .
  17. ↑ Philippe Sansonetti, Microbes and men. War and peace with mucous surfaces , France secondary school, , p. 6 .
  18. ↑ (in) Claudio Murgia , Jonathan K. Pritchard , Su Yeon Needle , Ariberto Fassati you stolen A. Weiss , ‘ Clonal origin and evolution of a transmissible cancer » , Cell , vol. 126, n O 3, , p. 477-487 (ISSN  0092-8674 , DOI  10.1016/j.cell.2006.05.051 , read online ) .
  19. ↑ (in) A.-M. Pearse and K. Swift , ‘ Allograft theory: Transmission of devil facial-tumour disease : An uncanny similarity in the karyotype of these malignant tumours means that they could be infective » , Nature , vol. 439, n O 7076, , p. 549 (ISSN  0028-0836 , DOI  10.1038/439549a , résumé ) .
  20. ↑ (in) Gregory J. Veicer , Lee Kroos and Richard E. Lenski , ‘ Loss of social behaviors by Myxococcus xanthus during evolution in an unstructured habitat » , PNAS , vol. 95, n O 21, , p. 12376-12380 (ISSN  0027-8424 , read online ) .

17.anderson, D. P., Whitney, D. S., Hanson-Smith, V., Woznica, A., Campodonico-Burnett, W., Volkman, B. F., … & Vyhoda, K. E. (2016). Evolution of an ancient protein function involved in organized multicellularity in animals . Elife, 5, e10147.

18. Mikhailov, K. V., Konstantinova, A. V., Nikitin, M. A., Troshin, P. V., Rusin, L. Y., Lyubetsky, VA, … & Aleoshin, V. V. (2009). The origin of Metazoa: a transition from temporal to spatial cell differentiation . Bioessays, 31(7), 758-768.

19. Hands, A. (2008). The molecular origins of multicellular transitions. Current opinion in genetics & development, 18(6), 472-478.

20. Wolpert, L., & Szathmáry, E. (2002). Multicellularity: evolution and the egg . Nature, 420(6917), 745-745.

21. Hands, A. (2008). The molecular origins of multicellular transitions. Current opinion in genetics & development, 18(6), 472-478.

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