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Symbiogenesis

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Electron micrograph of a mitochondrion showing its mitochondrial matrix and membranes.

The endosymbiotic theory 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 that were taken inside the cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) and chloroplasts from cyanobacteria.


According to Margulis and Sagan,[1] "Life did not take over the globe by combat, but by networking" (i.e., by cooperation)[2].

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.[3]

It is also believed that 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. This hypothesis is thought to be possible because it is known today from scientific observation that transfer of DNA occurs between prokaryotic species, even if they are not closely related. Prokaryotes can take up DNA from their surroundings and have a limited ability to incorporate it into their own genome.

Evidence

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

  • 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.
  • They are surrounded by two or more membranes, and the innermost of these shows differences in composition from the other membranes of the cell. The composition is like that of a prokaryotic cell membrane.
  • Both mitochondria and plastids contain DNA that is different from that of the cell nucleus and that is similar to that of bacteria (in being circular in shape and in its size).
  • DNA sequence analysis and phylogenetic estimates suggests 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 prokaryotes.
  • 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 the eukaryotes that acquired their plastids directly from bacteria (known as Primoplantae), the glaucophyte algae have chloroplasts that strongly resemble cyanobacteria. In particular, they have a peptidoglycan cell wall between their two membranes.
  • Mitochondria and plastids are just about the same size as bacteria.

Secondary endosymbiosis

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[4]).

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.[5][6]

Problems

  • Neither mitochondria nor plastids can survive in oxygen or outside the cell, having lost many essential genes 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.)
  • The transfer of genes from mitochondria and plastids to the “host genome” or cell nucleus raises a further problem: why were all genes not transferred? In other words, why do any genes at all remain in mitochondria and plastids? This problem is addressed by the CoRR Hypothesis, which proposes that genes and respiratory chain proteins are Co-located for Redox Regulation.
  • 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 primitive eukaryote, but instead a prokaryotic cell. This in turn implies that the emergence of the eukaryotes and the formation of mitochondria were achieved simultaneously.
  • 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.

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

See also

Notes

  1. ^ Margulis, Lynn (2001). "Marvellous microbes". Resurgence. 206: 10–12. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  2. ^ Witzany G (2006). "Serial Endosymbiotic Theory (SET): The Biosemiotic Update". Acta Biotheoretica. 54 (1): 103–17. doi:10.1007/s10441-006-7831-x.
  3. ^ Gabaldón T, Snel B, van Zimmeren F, Hemrika W, Tabak H, Huynen MA (2006). "Origin and evolution of the peroxisomal proteome". Biol. Direct. 1: 8. doi:10.1186/1745-6150-1-8. PMC 1472686. PMID 16556314.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link) (Provides evidence that contradicts an endosymbiotic origin of peroxisomes. Instead it is suggested that they evolutionarily originate from the Endoplasmic Reticulum)
  4. ^ McFadden GI (2001). "Primary and secondary endosymbiosis and the origin of plastids". J Phycology. 37 (6): 951–9. doi:10.1046/j.1529-8817.2001.01126.x.
  5. ^ McFadden GI, van Dooren GG (2004). "Evolution: red algal genome affirms a common origin of all plastids". Curr. Biol. 14 (13): R514–6. doi:10.1016/j.cub.2004.06.041. PMID 15242632. {{cite journal}}: Unknown parameter |month= ignored (help)
  6. ^ Gould SB, Waller RF, McFadden GI (2008). "Plastid evolution". Annu Rev Plant Biol. 59: 491–517. doi:10.1146/annurev.arplant.59.032607.092915. PMID 18315522.{{cite journal}}: CS1 maint: multiple names: authors list (link)

References

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