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* Many of these protists contain "primary" plastids that have not yet been acquired from other plastid-containing eukaryotes.
* 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.
* 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.
* Mitochondria and plastids are similar in size to bacteria.


== Secondary endosymbiosis ==
== Secondary endosymbiosis ==

Revision as of 13:58, 24 April 2012

Electron micrograph of a mitochondrion showing its mitochondrial matrix and membranes

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

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").

Evidence

Evidence that mitochondria and plastids arose from bacteria is as follows:[11][12][13]

  • 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

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[14]). 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.[15][16]

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.[17]

Extensions

  • 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

Notes

  1. ^ "Mitochondria Share an Ancestor With SAR11, a Globally Significant Marine Microbe". ScienceDaily. July 25, 2011. Retrieved 2011-07-26.
  2. ^ J. Cameron Thrash; et al. (2011). "Phylogenomic evidence for a common ancestor of mitochondria and the SAR11 clade". Scientific Reports. doi:10.1038/srep00013. {{cite journal}}: Explicit use of et al. in: |author= (help)
  3. ^ Mereschkowski C (1905). "Über Natur und Ursprung der Chromatophoren im Pflanzenreiche". Biol Centralbl. 25: 593–604.
  4. ^ Schimper AFW (1883). "Über die Entwicklung der Chlorophyllkörner und Farbkörper". Bot. Zeitung. 41: 105–14, 121–31, 137–46, 153–62.
  5. ^ Wallin IE (1923). "The Mitochondria Problem". The American Naturalist. 57 (650): 255–61. doi:10.1086/279919.
  6. ^ Ris H, Singh RN (1961). "Electron microscope studies on blue-green algae". J Biophys Biochem Cytol. 9 (1): 63–80. doi:10.1083/jcb.9.1.63. PMC 2224983. PMID 13741827. {{cite journal}}: Unknown parameter |month= ignored (help)
  7. ^ Stocking C and Gifford E (1959). "Incorporation of thymidine into chloroplasts of Spirogyra". Biochem. Biophys. Res. Comm. 1 (3): 159–64. doi:10.1016/0006-291X(59)90010-5.
  8. ^ Lynn Sagan (1967). "On the origin of mitosing cells". J Theor Bio. 14 (3): 255–274. doi:10.1016/0022-5193(67)90079-3. PMID 11541392.
  9. ^ Margulis, Lynn (2001). "Marvellous microbes". Resurgence. 206: 10–12. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ Gabaldón T, Snel B, van Zimmeren F, Hemrika W, Tabak H, Huynen MA (2006). "Origin and evolution of the peroxisomal proteome". Biol. Direct. 1 (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)
  11. ^ [1] Kimball, J. 2010. Kimball's Biology Pages. Accessed October 13, 2010. An online open source biology text by Harvard professor, and author of a general biology text, John W. Kimball.
  12. ^ Reece, J., Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson, 2010. Campbell Biology. 9th Edition Benjamin Cummings; 9th Ed. (October 7, 2010)
  13. ^ Raven, P., George Johnson, Kenneth Mason, Jonathan Losos, Susan Singer, 2010. Biology. McGraw-Hill 9th Ed. (January 14, 2010)
  14. ^ 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.
  15. ^ 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)
  16. ^ Gould SB, Waller RF, McFadden GI (2008). "Plastid evolution". Annu Rev Plant Biol. 59 (1): 491–517. doi:10.1146/annurev.arplant.59.032607.092915. PMID 18315522.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  17. ^ Georgiades K, Raoult D (2011) The rhizome of Reclinomonas americana, Homo sapiens, Pediculus humanus and Saccharomyces cerevisiae mitochondria. Biol Direct 6(1):55

References

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