Symbiogenesis, also known as the endosymbiotic theory or serial endosymbiotic theory,) is the leading scientific theory for the evolutionary origin of eukaryotes from prokaryotic ancestors. The theory holds that multi-membraned eukaryotic organelles such as mitochondria and chloroplasts are endosymbionts descended from formerly free-living prokaryotes (more closely related to the Bacteria than to the Archaea) that were phagocytized into another prokaryote's endomembrane system. Mitochondria appear to be phylogenetically related to Alphaproteobacteria such as Rickettsiales, while chloroplasts are thought to be related to beta-cyanobacteria.
The idea that chloroplasts were originally independent unicellular organisms that merged into a symbiotic relationship with other unicellular organisms dates back to the 19th century, when it was espoused by researchers such as Andreas Schimper. The endosymbiotic theory was articulated in 1905 and 1910 by the Russian botanist Konstantin Mereschkowski, and advanced and substantiated with microbiological evidence by Lynn Margulis in 1967.
Among the many lines of evidence supporting symbiogenesis are that mitochondria and plastids contain their own chromosomes and reproduce by binary fission, parallel but separate from the sexual reproduction of the rest of the cell; that the chromosomes of some mitochondria and plastids are single circular DNA molecules similar to the circular chromosomes of bacteria; that the transport proteins called porins are found in the outer membranes of mitochondria and chloroplasts, and also bacterial cell membranes; and that cardiolipin is found only in the inner mitochondrial membrane and bacterial cell membranes. They also contain a separate layer of biomembrane in addition to the ones enveloping other organelles.
History
The Russian botanist Konstantin Mereschkowski first outlined the theory of symbiogenesis (from Greek: σύν syn "together", βίος bios "life", and γένεσις genesis "origin, birth") in his 1905 work, The nature and origins of chromatophores in the plant kingdom, and then elaborated it in his 1910 The Theory of Two Plasms as the Basis of Symbiogenesis, a New Study of the Origins of Organisms.See:
(journal URL: ) Mereschkowski proposed that complex life-forms had originated by two episodes of symbiogenesis, the incorporation of symbiotic bacteria to form successively nuclei and chloroplasts. Mereschkowski knew of the work of botanist Andreas Schimper. In 1883, Schimper had observed that the division of chloroplasts in green plants closely resembled that of free-living cyanobacteria. Schimper had tentatively proposed (in a footnote) that green plants had arisen from a symbiotic union of two organisms.See: From p. 105: "Inzwischen theilte mir Herr Professor Schmitz mit, dass … die höheren Pflanzen sich ebenso verhalten würden." (Meanwhile, Prof. Schmitz reported to me that among algae, the creation of chlorophyll granules from the cell plasm doesn't occur, but that they arise exclusively from one another by division. The spores receive from the mother plant chlorophyll granules, which create, by division, all of the chlorophyll granules of the plants that arises from them [i.e., the spores]. This finding in algae made it seem likely to Prof. Schmitz that the higher plants would behave likewise.) From p. 106: "Meine Untersuchungen haben ergeben, … aus dem Scheitelmeristem sich entwickelnden Gewebe erzeugen." (My investigations have revealed that the vegetation points [i.e., points of vegetative growth] always contain differentiated chlorophyll bodies or their colorless rudiments; that they arise not by creation from the cell plasm, but from one another by division, and that they create all chlorophyll bodies and starch-forming [bodies] of the tissues developing from the apical meristem.) From p. 112, footnote 2: "Sollte es sich definitiv bestätigen, … an eine Symbiose erinnern." (If it should definitely be confirmed that the plastids in egg cells are not formed anew, then their relation to the organism containing them would somewhat suggest a symbiosis.)
In 1918 the French scientist Paul Jules Portier published Les Symbiotes, in which he claimed that the mitochondria originated from a symbiosis process. From p. 293: "Cette modification dans les rapports des appareils nucléaire et mitochondrial peut être le résultat de deux mécanismes. … Cette la parthénogénèse." (This modification in the relations of the nuclear and mitochondrial systems could be the result of two mechanisms: (a) There is a combination of two factors: contribution of new symbionts by the spermatozoid and reduction division. That is fertilization. (b) A single factor exists: reduction division: in this case, the egg contains sufficiently active symbionts. That is parthenogenesis.) Ivan Wallin advocated the idea of an endosymbiotic origin of mitochondria in the 1920s. The Russian botanist Boris Kozo-Polyansky became the first to explain the theory in terms of Darwinian evolution. In his 1924 book A New Principle of Biology. Essay on the Theory of Symbiogenesis, English translation: Reviewed in: he wrote, "The theory of symbiogenesis is a theory of selection relying on the phenomenon of symbiosis."
These theories did not gain traction until more detailed electron-microscopic comparisons between cyanobacteria and chloroplasts were made, such as by Hans Ris in 1961 and 1962. These, combined with the discovery that plastids and mitochondria contain their own DNA, led to a resurrection of the idea of symbiogenesis in the 1960s. Lynn Margulis advanced and substantiated the theory with microbiological evidence in a 1967 paper, On the origin of mitosing cells. In her 1981 work Symbiosis in Cell Evolution she argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic spirochaetes that developed into eukaryotic flagella and cilia. This last idea has not received much acceptance, because flagella lack DNA and do not show ultrastructural similarities to bacteria or to archaea (see also: Evolution of flagella and Prokaryotic cytoskeleton). According to Margulis and Dorion Sagan, "Life did not take over the globe by combat, but by networking" (i.e., by cooperation). Christian de Duve proposed that the peroxisomes may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular oxygen in the Earth's atmosphere. However, it now appears that peroxisomes may be formed de novo, contradicting the idea that they have a symbiotic origin.(Provides evidence that contradicts an endosymbiotic origin of peroxisomes, and suggests instead that they originate evolutionarily from the endoplasmic reticulum) The fundamental theory of symbiogenesis as the origin of mitochondria and chloroplasts is now widely accepted.
Symbiogenesis revolutionized the history of evolution by proposing a mechanism for evolutionary development not encompassed in the original Darwininan vision. Symbiogenesis demonstrated that major evolutionary advancements, particularly the origin of eukaryotic cells, may have resulted from symbiotic mergers rather than from gradual mutations and individual competition, i.e., classical natural selection. Accordingly, symbiogenic theory suggests that endosymbiosis may be a powerful force in generating evolutionary novelty, beyond that which can be explained by natural selection alone.
From endosymbionts to organelles
Biologists usually distinguish organelles from endosymbionts – whole organisms living inside other organisms – by their reduced genome sizes. As an endosymbiont evolves into an organelle, most of its genes are transferred to the host cell genome. The host cell and organelle therefore need to develop a transport mechanism that enables the return of the protein products needed by the organelle but now manufactured by the cell.
Free-living ancestors
Alphaproteobacteria are thought to be the free-living organisms most closely related to mitochondria. More recent research has indicated further that mitochondria are most closely related to the subclade of Alphaproteobacteria known as Pelagibacterales bacteria, in particular, those in the SAR11 clade.
Nitrogen-fixing filamentous cyanobacteria are the free-living organisms most closely related to plastids.
Both cyanobacteria and alphaproteobacteria maintain a large (>6Mb) genome encoding thousands of proteins. Plastids and mitochondria exhibit a dramatic reduction in genome size when compared with their bacterial relatives. Chloroplast genomes in photosynthetic organisms are normally 120–200kb encoding 20–200 proteins and mitochondrial genomes in humans are approximately 16kb and encode 37 genes, 13 of which are proteins. Using the example of the freshwater amoeboid, however, Paulinella chromatophora, which contains chromatophores found to be evolved from cyanobacteria, Keeling and Archibald argue that this is not the only possible criterion; another is that the host cell has assumed control of the regulation of the former endosymbiont's division, thereby synchronizing it with the cell's own division. Nowack and her colleagues gene sequenced the chromatophore (1.02Mb) and found that only 867 proteins were encoded by these photosynthetic cells. Comparisons with their closest free living cyanobacteria of the genus Synechococcus (having a genome size 3Mb, with 3300 genes) revealed that chromatophores had undergone a drastic genome shrinkage. Chromatophores contained genes that were accountable for photosynthesis but were deficient in genes that could carry out other biosynthetic functions; this observation suggests that these endosymbiotic cells are highly dependent on their hosts for their survival and growth mechanisms. Thus, these chromatophores were found to be non-functional for organelle-specific purposes when compared with mitochondria and plastids. This distinction could have promoted the early evolution of photosynthetic organelles.
The loss of genetic autonomy, that is, the loss of many genes from endosymbionts, occurred very early in evolutionary time. Taking into account the entire original endosymbiont genome, there are three main possible fates for genes over evolutionary time. The first is the loss of functionally redundant genes, in which genes that are already represented in the nucleus are eventually lost. The second is the transfer of genes to the nucleus, while the third is that genes remain in the organelle that was once an organism. The loss of autonomy and integration of the endosymbiont with its host can be primarily attributed to nuclear gene transfer. As organelle genomes have been greatly reduced over evolutionary time, nuclear genes have expanded and become more complex. As a result, many plastid and mitochondrial processes are driven by nuclear encoded gene products. In addition, many nuclear genes originating from endosymbionts have acquired novel functions unrelated to their organelles.
Gene transfer mechanisms
The mechanisms of gene transfer are not fully known; however, multiple hypotheses exist to explain this phenomenon. The possible mechanisms include the Complementary DNA (cDNA) hypothesis and the bulk flow hypothesis.