The endosymbiotic theory concerns the mitochondria, plastids (e.g. chloroplasts), and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales, the SAR11 clade,^[1][2] or close relatives) and chloroplasts from cyanobacteria.

History[link]

Endosymbiotic theory

The endosymbiotic (Greek: ἔνδον endon "within", σύν syn "together" and βίωσις biosis "living") theories were first articulated by the Russian botanist Konstantin Mereschkowski in 1905.^[3] Mereschkowski was familiar with work by botanist Andreas Schimper, who had observed in 1883 that the division of chloroplasts in green plants closely resembled that of free-living cyanobacteria, and who had himself tentatively proposed (in a footnote) that green plants had arisen from a symbiotic union of two organisms.^[4] Ivan Wallin extended the idea of an endosymbiotic origin to mitochondria in the 1920s.^[5] These theories were initially dismissed or ignored. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts (for example studies by Hans Ris^[6]), combined with the discovery that plastids and mitochondria contain their own DNA^[7] (which by that stage was recognized to be the hereditary material of organisms) led to a resurrection of the idea in the 1960s.

The endosymbiotic theory was advanced and substantiated with microbiological evidence by Lynn Margulis in a 1967 paper, The Origin of Mitosing Eukaryotic Cells.^[8] 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 archaea. See also Evolution of flagella. According to Margulis and Dorion Sagan,^[9] "Life did not take over the globe by combat, but by networking" (i.e., by cooperation). The possibility that peroxisomes may have an endosymbiotic origin has also been considered, although they lack DNA. Christian de Duve proposed that they 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 they may be formed de novo, contradicting the idea that they have a symbiotic origin.^[10]

It is thought that over millennia these endosymbionts transferred some of their own DNA to the host cell's nucleus during the evolutionary transition from a symbiotic community to an instituted eukaryotic cell (called "serial endosymbiosis").

From endosymbionts to organelles[link]

Endosymbionts and organelles can be distinguished from one another on the basis of reduced genome sizes. According to Keeling and Archibald,^[11] complete gene sequencing in organelles can be attributed to their transport mechanism. This includes transfer of most of their genes to the host cells along with a need for a targeting system that would enable a re-transfer of their protein products. While the endosymbiont's experience a decrease in their overall genome sizes. For example, Paulinella Chromatophora consisted of chromatophores that were found to be associated to cyanobacteria and exhibited an endosymbiotic relationship with their host cells.^[11] Nowack and his colleagues^[12] performed gene sequencing on 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 of 3Mb with 3300 genes) revealed that chromatophores underwent a drastic genome shrinkage. Chromatophores contained genes that were accountable for photosynthesis but were deficient in genes that could carry out other biosynthetic functions signifying that these endosymbiotic cells were 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 to mitochondria and plastids. This distinction could have promoted the early evolution of photosynthetic organelles.

Evidence[link]

Evidence that mitochondria and plastids arose from bacteria is as follows:^[13][14][15]

• New mitochondria and plastids are formed only through a process similar to binary fission.
• In some algae, such as Euglena, the plastids can be destroyed by certain chemicals or prolonged absence of light without otherwise affecting the cell. In such a case, the plastids will not regenerate. This shows that the plastid regeneration relies on an extracellular source, such as from cell division or endosymbiosis.
• They are surrounded by two or more membranes, and the innermost of these shows differences in composition from the other membranes of the cell. They are composed of a peptidoglycan cell wall characteristic of a bacterial cell.
• Both mitochondria and plastids contain DNA that is different from that of the cell nucleus and that is similar to that of bacteria (both in their size and their having a circular form).
• DNA sequence analysis and phylogenetic estimates suggest that nuclear DNA contains genes that probably came from plastids.
• These organelles' ribosomes are like those found in bacteria (70S).
• Proteins of organelle origin, like those of bacteria, use N-formylmethionine as the initiating amino acid.
• Much of the internal structure and biochemistry of plastids, for instance the presence of thylakoids and particular chlorophylls, is very similar to that of cyanobacteria. Phylogenetic estimates constructed with bacteria, plastids, and eukaryotic genomes also suggest that plastids are most closely related to cyanobacteria.
• Mitochondria have several enzymes and transport systems similar to those of bacteria.
• Some proteins encoded in the nucleus are transported to the organelle, and both mitochondria and plastids have small genomes compared to bacteria. This is consistent with an increased dependence on the eukaryotic host after forming an endosymbiosis. Most genes on the organellar genomes have been lost or moved to the nucleus. Most genes needed for mitochondrial and plastid function are located in the nucleus. Many originate from the bacterial endosymbiont.
• Plastids are present in very different groups of protists, some of which are closely related to forms lacking plastids. This suggests that if chloroplasts originated de novo, they did so multiple times, in which case their close similarity to each other is difficult to explain.
• Many of these protists contain "primary" plastids that have not yet been acquired from other plastid-containing eukaryotes.
• Among eukaryotes that acquired their plastids directly from bacteria (known as Archaeplastida), the glaucophyte algae have chloroplasts that strongly resemble cyanobacteria. In particular, they have a peptidoglycan cell wall between the two membranes.

Secondary endosymbiosis[link]

Primary endosymbiosis involves the engulfment of a bacterium by another free living organism. Secondary endosymbiosis occurs when the product of primary endosymbiosis is itself engulfed and retained by another free living eukaryote. Secondary endosymbiosis has occurred several times and has given rise to extremely diverse groups of algae and other eukaryotes. Some organisms can take opportunistic advantage of a similar process, where they engulf an alga and use the products of its photosynthesis, but once the prey item dies (or is lost) the host returns to a free living state. Obligate secondary endosymbionts become dependent on their organelles and are unable to survive in their absence (for a review see McFadden 2001^[16]). RedToL, the Red Algal Tree of Life Initiative funded by the National Science Foundation highlights the role red algae or Rhodophyta played in the evolution of our planet through secondary endosymbiosis.

One possible secondary endosymbiosis in process has been observed by Okamoto & Inouye (2005). The heterotrophic protist Hatena behaves like a predator until it ingests a green alga, which loses its flagella and cytoskeleton, while Hatena, now a host, switches to photosynthetic nutrition, gains the ability to move towards light and loses its feeding apparatus.

The process of secondary endosymbiosis left its evolutionary signature within the unique topography of plastid membranes. Secondary plastids are surrounded by three (in euglenophytes and some dinoflagellates) or four membranes (in haptophytes, heterokonts, cryptophytes, and chlorarachniophytes). The two additional membranes are thought to correspond to the plasma membrane of the engulfed alga and the phagosomal membrane of the host cell. The endosymbiotic acquisition of a eukaryote cell is represented in the cryptophytes; where the remnant nucleus of the red algal symbiont (the nucleomorph) is present between the two inner and two outer plastid membranes.^{[citation needed]}

Despite the diversity of organisms containing plastids, the morphology, biochemistry, genomic organisation, and molecular phylogeny of plastid RNAs and proteins suggest a single origin of all extant plastids – although this theory is still debated.^[17][18]

Some species including Pediculus humanus have multiple chromosomes in the mitochondrion. This and the phylogenetics of the genes encoded within the mitochondrion suggests that the ancestors of mitochondria may have been acquired on several occasions rather than just once.^[19]

Extensions[link]

This unreferenced section requires citations to ensure verifiability.

• Neither mitochondria nor plastids can survive in oxygen or outside the cell, having lost many essential hormones required for survival. The standard counterargument points to the large timespan that the mitochondria/plastids have co-existed with their hosts. In this view, genes and systems that were no longer necessary were simply deleted, or in many cases, transferred into the host genome instead. (In fact these transfers constitute an important way for the host cell to regulate plastid or mitochondrial activity.) For example, most plastids are not able to produce respiratory proteins necessary for respiration. Like many living cells, plastids would die if energy is not provided to them by respiration.
• A large cell, especially one equipped for phagocytosis, has vast energetic requirements, which cannot be achieved without the internalisation of energy production (due to the decrease in the surface area to volume ratio as size increases). This implies that, for the cell to gain mitochondria, it could not have been a eukaryote, and must have been a prokaryote. This in turn implies that the emergence of the eukaryotes and the formation of mitochondria were achieved simultaneously. This may be explained by possibly a very close symbiotic relationship between two types of prokaryotes which eventually led to gene exchange and engulfing of the mitochondria precursors through partial fusion or engulfing by the host bacteria.
• Genetic analysis of small eukaryotes that lack mitochondria shows that they all still retain genes for mitochondrial proteins. This implies that all these eukaryotes once had mitochondria. This objection can be answered if, as suggested above, the origin of the eukaryotes coincided with the formation of mitochondria. Alternatively, we may postulate extinction of all other descendants of a mitochondrion-free ancestral eukaryote, perhaps due to competition from the symbiotic clade, or oxygen poisoning as levels continued to rise.

These last two problems are accounted for in the Hydrogen hypothesis.

See also[link]

• Hatena
• Lichen
• Anagenesis
• Symbiogenesis
• Transfer of mitochondrial and chloroplast DNA to the nucleus
• Viral Eukaryogenesis (hypothesis that the cell nucleus originated from endosymbiosis).
• Protobiont
• Numt
• Hydrogen hypothesis
• James A. Lake

Notes[link]

References[link]

• Alberts, Bruce (2002). Molecular biology of the cell. New York: Garland Science. ISBN 0-8153-3218-1.  (General textbook)
• Blanchard JL, Lynch M (July 2000). "Organellar genes: why do they end up in the nucleus?". Trends Genet. 16 (7): 315–20. DOI:10.1016/S0168-9525(00)02053-9. PMID 10858662. http://linkinghub.elsevier.com/retrieve/pii/S0168-9525(00)02053-9.  (Discusses theories on how mitochondria and chloroplast genes are transferred into the nucleus, and also what steps a gene needs to go through in order to complete this process.)
• Jarvis P (April 2001). "Intracellular signalling: the chloroplast talks!". Curr. Biol. 11 (8): R307–10. DOI:10.1016/S0960-9822(01)00171-3. PMID 11369220. http://linkinghub.elsevier.com/retrieve/pii/S0960-9822(01)00171-3.  (Recounts evidence that chloroplast-encoded proteins affect transcription of nuclear genes, as opposed to the more well-documented cases of nuclear-encoded proteins that affect mitochondria or chloroplasts.)
• Brinkman FS, Blanchard JL, Cherkasov A, et al. (August 2002). "Evidence that plant-like genes in Chlamydia species reflect an ancestral relationship between Chlamydiaceae, cyanobacteria, and the chloroplast". Genome Res. 12 (8): 1159–67. DOI:10.1101/gr.341802. PMC 186644. PMID 12176923. http://www.genome.org/cgi/content/full/12/8/1159. 
• Okamoto N, Inouye I (October 2005). "A secondary symbiosis in progress?". Science 310 (5746): 287. DOI:10.1126/science.1116125. PMID 16224014. http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=16224014. 
• Cohen WD, Gardner RS (1959). "Viral Theory and Endosymbiosis". http://www.psychoneuroendocrinology.com/symbiosis.pdf.  (Discusses theory of origin of eukaryotic cells by incorporating mitochondria and chloroplasts into anaerobic cells with emphasis on 'phage bacterial and putative viral mitochondrial/chloroplast interactions.)

External links[link]

• Tree of Life Eukaryotes