Endosymbiotic theory

The endosymbiotic theory, now generally accepted by biologists, concerns the origins of mitochondria and plastids (e.g. chloroplasts), which are organelles of eukaryotic cells. According to this theory, these organelles originated as separate prokaryotic organisms which were taken inside the cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) and chloroplasts from cyanobacteria.

History

The endosymbiotic theory was first proposed by Andreas Schimper in 1883. The idea that plastids were originally endosymbionts was first suggested by Konstantin Mereschkowsky in 1905, and the same idea for mitochondria was suggested by Ivan Wallin in the 1920s. These theories were initially dismissed on the assumption that they did not contain DNA. This was proven false in the 1960s, leading Hans Ris to resurrect the idea.

The endosymbiotic hypothesis was fleshed out and popularized by Lynn Margulis. 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, since flagella lack DNA and do not show ultrastructural similarities to prokaryotes. See also Evolution of flagella.

According to Margulis and Sagan (1996), "Life did not take over the globe by combat, but by networking" (i.e., by cooperation), and Darwin's notion of evolution driven by natural selection is incomplete (see Evolution and natural selection). However, others have argued that endosymbiosis constitutes slavery rather than mutualism.

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.

Evidence

Evidence that mitochondria and plastids arose via ancient endosymbiosis of bacteria is as follows:

• Both mitochondria and plastids contain DNA that is fairly different from that of the cell nucleus and that is similar to that of bacteria (in being circular and in its size).
• They are surrounded by two or more membranes, and the innermost of these shows differences in composition compared to the other membranes in the cell. The composition is like that of a prokaryotic cell membrane.
• 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.
• 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.
• DNA sequence analysis and phylogenetic estimates suggests that nuclear DNA contains genes that probably came from the plastid.
• 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 were not taken from outside, they evolved multiple times, in which case their close similarity to each other is difficult to explain. Many of these protists contain "secondary" plastids that have been acquired from other plastid-containing eukaryotes, not from cyanobacteria directly.
• These organelle's ribosomes are like those found in bacteria (70s).

A possible secondary endosymbiosis (i.e. involving eukaryotic plastids) has been observed by Okamoto & Inouye (2005). The heterotrophic protist Hatena behaves like a predator until it ingests a green algae, 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.

Possible objections

• Mitochondria and plastids, like nuclear DNA and unlike prokaryotic DNA, contain introns. Some form of transfer between nuclear and mitochondrial/plastid DNA must have taken place (likely organelle → nuclear transfer → organelle) or, alternatively, a mechanism for turning off portions of early proto-mitochondrial/plastid DNA must have been in place in early symbiotes.

• Neither mitochondria nor plastids can survive outside the cell, having lost many essential genes required for survival. This objection may be easily countered by accounting for the large timespan that the mitochondria/plastids have co-existed with their hosts: genes and systems which were no longer necessary may have been deleted, or in many cases, transferred into the host genome instead (In fact these transfers may constitute an important way for the host cell to regulate mitochondrial activity)

Phylogeny of the Proto-mitochondrion

The phylogenetic analyses of the few genes that are still encoded in the genomes of modern mitochondria, suggest an alpha-proteobacterial nature for this endosymbiont. Although the order Rickettsiales has been proposed as the alpha-proteobacterial sister-group of mitochondria, there is no definitive evidence as to from which alpha-proteobacterial group the proto-mitochondrion emerged.

Metabolism of the Proto-mitochondrion

Toni Gabaldón and Martijn Huynen (2003) reconstructed the proteome and corresponding metabolism of the proto-mitochondrion by comparing extant alpha-proteobacterial and eukaryotic genomes. They concluded that this organism was an aerobic alpha-proteobacterium catabolyzing lipids, glycerol and other compounds provided by the host. At least 630 gene families derived from this organism can still be found in the 9 eukaryotic genomes analyzed in the study.

Endosymbiosis and the origin of the cell nucleus

Phylogenetic analyses has shown that eukaryotic proteins involved in DNA transcription and translation, as well as H3-H4 histones have probably originated in archaea, while many proteins involved in metabolism are more closely related to bacterial homologues. This has led to the proposal that eukaryotes may have arisen through fusion of an archaeon and a bacterium.

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