Symbiogenesis , or endosymbiotic theory , is the theory of evolution from the origin of eukaryotic cells of prokaryotic organisms, first articulated in 1905 and 1910 by the Russian botanist Konstantin Mereschkowski, and advanced and proved by microbiological evidence by Lynn Margulis in 1967. He argues that the organelles that differentiate eukaryotic cells evolved through individual single-cell prokaryotic symbiosis (bacteria and archaea).
This theory suggests that mitochondria, plastids such as chloroplasts, and possibly other organelles of eukaryotic cells are free living prokaryotes taken within others in endosymbiosis. In more detail, mitochondria seem to be associated with proteobacteria rickettsiales, and chloroplasts to repair nitrile filamentous cyanobacteria.
Among the many lines of evidence supporting symbiogenesis is that new mitochondria and plastids are formed only through binary division, and that cells can not create new ones instead; that transport proteins called porins are found in the outer membranes of mitochondria, chloroplasts and bacterial cell membranes; that cardiolipin is found only in inner mitochondrial membranes and bacterial cell membranes; and that some mitochondria and plastids contain a single circular DNA molecule similar to a bacterial chromosome.
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History
Theory of symbiogenesis (from the Greek: ??? syn "together", "bios " life ", and ???????
Russian botanist Boris Kozo-Polyansky was the first to explain the theory in terms of Darwinian evolution. In his book 1924 Novyi printzip biologisi. Ocherk the theory of simbiogeneza (The new principle of biology essay on the theory of symbiogenesis was translated into English as Symbiogenesis: A New Principle of Evolution in 2010), he wrote, "Symbiogenetic theory is a theory of selection that depends on symbiotic phenomena." These theories were originally dismissed or ignored. More detailed comparative electron microscopy between cyanobacteria and chloroplasts (eg a study by Hans Ris published in 1961), combined with the discovery that plastids and mitochondria contain their own DNA (which at that stage is recognized as organism of the offspring of organisms) led to a resurgence of ideas in the 1960s -an.
This theory is advanced and proved by microbiological evidence by Lynn Margulis in a 1967 paper, on the origin of mitotic cells. In his work in 1981 Symbiosis in Cell Evolution he argues that eukaryotes cells originate as interacting entity communities, including endosymbiotic spirochaetes that develop into eelaary and cyanate flagella. This latter idea has not received much acceptance, as the flagella lacks DNA and does not exhibit ultrastructural similarity with bacteria or archaea (see also: The evolution of Procaryotic flagella and cytoskeleton). According to Margulis and Dorion Sagan, "Life does not take over the world by battle, but by network" (ie, by cooperation). The possibility that peroxisomes may have an endosymbiotic origin has also been considered, even though they lack DNA. Christian de Duve proposed that they may be the first endosymbionts, allowing cells to withstand the growing number of free oxygen molecules in the Earth's atmosphere. However, it now appears that peroxisomes can form de novo , contrary to the idea that they have a symbiotic origin. Maps Symbiogenesis
From endosymbionts to organel
According to Keeling and Archibald, the usual way to differentiate organelles from the endosymbion is by reducing their genome size. When the endosimbion evolves into an organelle, most of their genes are transferred to the host cell genome. Host cells and organelles need to develop a transport mechanism that allows the return of protein products required by organelles but now produced by cells. Cyanobacteria and -protobacteria are living organisms most closely related to plastids and mitochondria. Both cyanobacteria and -protobacteria retain large genomes (& gt; 6Mb) that encode thousands of proteins. Plastids and mitochondria show dramatic decreases in the size of the genome when compared to their bacterial relatives. The genome of chloroplasts in photosynthetic organisms is usually 120-200 kb encoding 20-200 proteins and mitochondrial genomes in humans about 16kb and encode 37 genes, 13 of which are proteins. Using the example of freshwater amoeboids, however, Paulinella chromatophora , containing chromatophores found to evolve from cyanobacteria, Keeling and Archibald argue that this is not the only possible criterion; another is that the host cell has taken control of the previous arrangement of endosymbiont division, thus synchronizing it with its own cell division. Nowack and his colleagues sequenced the gene on chromatophore (1.02 Mb) and found that only 867 proteins were encoded by these photosynthetic cells. Comparison with their nearest free survival cyanobacteria of the genus Synechococcus (having a 3 Mb genome size, with 3300 genes) reveals that chromatophores are drastically shrinking genes. Chromatophores contain genes responsible for photosynthesis but lack of genes that can perform other biosynthetic functions; these observations show that endosymbiotic cells are highly dependent on their host for survival and their growth mechanism. Thus, these chromatophores are found not to function for organel-specific purposes when compared with mitochondria and plastids. This difference could promote the early evolution of organelles photosynthesis.
The loss of genetic autonomy, the loss of many genes from endosymbionts, occurred very early in the evolutionary period. Considering the entire genome of the original endosymbiont, there are three possible major fates for genes during the evolutionary period. The first fate involves the loss of redundant functional genes, in which the genes already represented in the nucleus are eventually lost. The second fate involves the transfer of genes to the nucleus. The loss of autonomy and endosymbiont integration with its host can be primarily associated with nuclear gene transfer. Since the organelles' genome has greatly diminished over the course of evolution, nuclear genes have evolved and become more complex. As a result, many plastid and mitochondrial processes are driven by nuclear encoded gene products. In addition, many nuclear genes derived from endosimbion have acquired new functions unrelated to their organelles.
Gene transfer mechanisms are not fully known; However, some hypotheses exist to explain this phenomenon. The cDNA hypothesis involves the use of messenger RNA (mRNAs) to transport genes from organelles to nuclei where they are converted into cDNA and inserted into the genome. The cDNA hypothesis is based on the study of the genomes of flowering plants. RNA protein coding in mitochondria is spliced ​​and edited using splice and editing sites of organelles. Nuclear copies of some mitochondrial genes, however, do not contain any specialized organelle splice sites, indicating intermediate mRNAs being processed. The cDNA hypothesis has been revised because editable mitochondrial cDNAs are unlikely to rejoin nuclear genomes and are more likely to rejoin their original mitochondrial genomes. If an editable mitochondrial sequence rejoins the mitochondrial genome, the mitochondrial splice site will no longer exist in the mitochondrial genome. Further subsequent nuclear gene transfer will also not have a mitochondrial splice site.
The mass flow hypothesis is an alternative to the cDNA hypothesis, suggesting that escaped DNA, rather than mRNA, is a gene transfer mechanism. According to this hypothesis, disruption of organelles, including autophagy (normal cell damage), gametogenesis (gamet formation), and cell stress, releases imported DNA into the nucleus and inserted into nuclear DNA using a non-homologous end combination ). For example, in the early stages of endosimbiosis, due to lack of major gene transfer, the host cell has little control over the endosymbiont. Endosymbiont undergoes cell division independently of the host cell, producing multiple "copies" of the endosymbion within the host cell. Some endosimbion are dissolved (exploded), and high levels of DNA are inserted into the nucleus. A similar mechanism is expected to occur in tobacco plants, which indicates high levels of gene transfer and its cells contain many chloroplasts. In addition, the mass flow hypothesis is also supported by the presence of a group of non-random organelle genes, demonstrating the simultaneous movement of several genes.
By 2015, biologist Roberto Cazzolla Gatti provides evidence for variant theories, endogenosimbiosis, which not only endosymbiotic organelles, but the genetic material pieces of the symbiotic parasites ("gene carriers" such as viruses, retroviruses and bacteriophages), belong to the host nucleus DNA, expression of host genes and contribute to the speciation process.
Molecular and biochemical evidence suggests that mitochondria are associated with Rickettsiales proteobacteria (specifically, SAR11 clones, or close relatives), and that chloroplasts are associated with filamentous cyanobacteria that bind to nitrogen.
Organellar genome
Plastomes and mitogenomes
The third and final possible fate of the endosym- inion gene is that they remain in the organelle. Plastids and mitochondria, although they have lost many of their genomes, retain genes that encode rRNAs, tRNAs, proteins involved in redox reactions, and proteins necessary for transcription, translation, and replication. There are many hypotheses to explain why organelles retain a small part of their genome; But no one hypothesis will apply to all organisms and the topic is still quite controversial. The hydrophobic hypothesis states that highly hydrophobic (water hates) proteins (such as membrane bound proteins involved in redox reactions) are not easily transported through the cytosol and therefore these proteins must be encoded in each organel. The code disparity hypothesis states that the transfer limit is due to different genetic codes and editing of RNA between organelles and nuclei. The hypothesis of redox control states that genes coding for redox reaction proteins are maintained in order to effectively pair the need for repair and synthesis of these proteins. For example, if one of the photo systems is missing from the plastid, a medium-electron carrier may lose or gain too many electrons, signaling the need for improved photo systems. The time delay involved in signaling the nucleus and transporting the cytosolic protein to the organelle results in the production of destructive reactive oxygen species. The final hypothesis states that membrane protein assembly, especially those involved in redox reactions, requires coordinated synthesis and subunit assembly; however, the coordination of translational transport and proteins is more difficult to control in the cytoplasm.
Non-photosynthetic plastid genom
The majority of genes in mitochondria and plastids are associated with the expression (transcription, translation and replication) of proteins encoding genes involved in photosynthesis (in plastids) or cell respiration (in mitochondria). One can expect that the loss of photosynthesis or cell respiration will allow a complete loss of the genomes of the plastids or mitochondrial genomes. Although there are many examples of mitochondrial descent (myosomes and hydrogenosomes) that have lost all their organelle genomes, non-photosynthetic plastids tend to retain small genomes. There are two main hypotheses to explain this incident:
The important tRNA hypothesis noted that there is no documented gene-to-core gene transfer genes from genes encoding RNA products (tRNAs and rRNAs). As a result, plastids must create their own functional RNA or import nuclear counterparts. Genes encoding tRNA-Glu and tRNA-fmet, however, seem to be indispensable. Plastids are responsible for hem biosynthesis, requiring a plastid encoded tRNA-Glu (from the trnE gene) as a precursor molecule. Like other genes that encode RNA, trnE can not be transferred to the nucleus. In addition, it is unlikely that trnE can be replaced by tRNA-Glu cytosol since trnE is highly sustainable; a single base change in trnE has resulted in the loss of haem synthesis. The genes for tRNA-formylmethionine (tRNA-fmet) are also encoded in the plastid genome and required for translation initiation in both plastids and mitochondria. Plastids are needed to continue expressing genes for tRNA-fmet during mitochondrion while translating proteins.
The limited window hypothesis offers a more general explanation for gene retention in non-photosynthetic plastids. According to the mass flow hypothesis, genes are transferred to the nucleus after organelle disturbance. General disturbance occurs in the early stages of endosimbiosis, however, once host cells occupy the organelle division, eukaryotes can evolve into only one plastid per cell. Having only one very limiting plastid gene transfer as a lysis of a single plastid is likely to result in cell death. Consistent with this hypothesis, organisms with multiple plastids show an 80-fold increase in the transfer of plastid-to-core genes compared to single-plastide organisms.
Evidence
There is ample evidence that mitochondria and plastids including chloroplasts emerge from bacteria.
- New mitochondria and plastids are formed only through binary division, the cell division used by bacteria and archaea.
- If a mitochondrial or chloroplast cell is removed, the cell does not have the means to make a new one. For example, in some algae, such as Euglena , plastids can be destroyed by certain chemicals or the absence of prolonged light without affecting the cell. In such a case, the plastids will not regenerate.
- Transport proteins called porins are found in the outer membranes of mitochondria and chloroplasts and are also found in bacterial cell membranes.
- A cardiolipin lipol membrane is exclusively found in the inner mitochondrial membrane and bacterial cell membrane.
- Some mitochondria and some plastids contain a single circular DNA molecule similar to bacterial DNA in both size and structure.
- Genome comparison shows a close relationship between mitochondrial and Rickettsial bacteria.
- Genome comparison shows a close relationship between plastids and cyanobacteria.
- Many genes in the mitochondrial genome and chloroplasts have been lost or transferred to the host cell nucleus. Consequently, many eukaryotic chromosomes contain genes derived from the mitochondrial and plastid genomes.
- Mitochondria and ribosomal plastids are more similar to bacteria (70S) than in eukaryotes.
- Proteins made by mitochondria and chloroplasts use N-formylmethionine as an initiation amino acid, just as proteins are made by bacteria but not proteins made by eukaryotic nuclear genes or archaea.
Secondary endosymbiosis
Primary endosymbiosis involves the accumulation of cells by other free living organisms. Secondary endosymbiosis occurs when the primary endosymbiosis product is swallowed and retained by other free living eukaryotes. Secondary endosymbiosis has occurred several times and has given birth to diverse groups of algae and eukaryotes. Some organisms can take opportunistic benefits from the same process, where they ingest the algae and use their photosynthesis products, but once the prey item dies (or disappears) the host returns to a free life state. The responsible secondary endosymbionts become dependent on their organelles and can not survive if none (for a review see McFadden 2001). RedToL, the Red Alan Tree of Life Initiative funded by the National Science Foundation highlights the role of the red algae or Rhodophyta that is played in the evolution of our planet through secondary endosymbiosis.
One possibility of secondary endosymbiosis in the process has been observed by Okamoto & amp; Inouye (2005). The heterotrophic prototype Hatena behaves like a predator until it digests the green alga, which loses flagella and cytoskeleton, while Hatena , now hosted, switches to photosynthetic nutrition, gains the ability to move toward light and lost his cutlery.
The secondary endosymbiosis process leaves its evolutionary mark in the unique topography of the plastid membrane. The secondary plastid is surrounded by three (in euglenophytes and some dinoflagellates) or four membranes (in haptophytes, heterokonts, cryptophytes, and chlorarachniophytes). Two additional membranes are thought to be related to the overgrown plasma membrane membrane and the phagosome membrane of the host cell. The endosymbiotic acquisition of eukaryotic cells is represented in cryptophytes; in which the remainder of the nucleus of the red algae (nukleomorph) is present between the two outer and inner plastid membranes.
Although the diversity of organisms that contain plastids, morphology, biochemistry, genomic organization, and molecular phylogeny of plastid RNA and proteins shows the single origin of all existing plastids - although this theory is debatable.
Some species including Pediculus humanus (lice) have many chromosomes in the mitochondria. These and phylogenetic genes encoded in mitochondria suggest that mitochondria have many ancestors, which are acquired by endosimbiosis on several occasions, not just once, and that there are many mergers and rearrangements of genes on some of the original mitochondrial chromosomes.
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