Difference between revisions of "Endosymbiotic hypothesis"

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The endosymbiotic theory, known to academics as the serial endosymbiosis theory (SET), is a scientific theory concerning the origins of mitochondria, plastids (e.g. chloroplasts), and nuclei in eukaryotic cells. According to this theory, these organelles arose from free-living bacteria that were taken inside another cell as endosymbionts.

History

The endosymbiotic theory was first articulated by famed Russian botanist Konstantin Mereschkowski in 1905^[1]. Mereschkowski based his theory in part on work done by botanist Andreas Schimper, who had observed in 1883 that the division of chloroplasts in green plants closely resembled that of cyanobacteria. Schimper had himself tentatively proposed (in a footnote) that green plants could have arisen from the symbiosis of two simpler organisms.^[2] In the 1920s, Irvin Wallin, a professor at the University of Colorado Medical School, extended the idea of an endosymbiotic origin to the mitochondria.^[3] These theories were initially dismissed (or ignored) by the scientific community. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts (for example, studies by Hans Ris^[4]) combined with the discovery that plastids and mitochondria contain their own DNA^[5] (which had recently been identified as 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 in 1967 by American biologist Lynn Margulis. A young faculty member at Boston University, Margulis authored a theoretical paper entitled The Origin of Mitosing Eukaryotic Cells^[6]. The paper, however, was "rejected by about fifteen scientific journals," Margulis recalled^[7](see Best of the Public). It was finally accepted by The Journal of Theoretical Biology and is considered today a landmark in modern endosymbiotic theory. Drawing heavily from the ideas of Mereschkowski and Wallin, Margulis's endosymbiotic theory formulation is the first to rely on direct microbiological observations as opposed to paleontological or zoological observations, which were previously the norm for new works in evolutionary biology.

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^[8]. It is believed 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"). This hypothesis is thought to be possible because it is known today from scientific observation that transfer of DNA occurs between bacteria species, even if they are not closely related. Bacteria can take up DNA from their surroundings and have a limited ability to incorporate it into their own genome.

Evidence

A great body of evidence exists in the fields of molecular biology, biochemistry, and genetics that supports the theory that mitochondria and plastids arose from bacteria^[9][10][11].

• 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. 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 (in being circular in shape and in its size).
• 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 Primoplantae), 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.

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.) For example, most plastids are not able to produce respiratory proteins necessary for respiration. Like any living cell, 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 bacterium. 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 bacteria 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 by the hydrogen hypothesis.

Serial Endosymbiosis Theory and Evolution

The endosymbiotic theory is part of a large body of evidence supporting the theory of evolution. Prominent evolutionary biologist Richard Dawkins had the following to say of Lynn Margulis and her work:

I greatly admire Lynn Margulis's sheer courage and stamina in sticking by the endosymbiosis theory, and carrying it through from being an unorthodoxy to an orthodoxy. I'm referring to the theory that the eukaryotic cell is a symbiotic union of primitive prokaryotic cells. This is one of the great achievements of twentieth-century evolutionary biology, and I greatly admire her for it.^[12]

There are essentially two differing views concerning endosymbiotic theory, one slightly more logical than the other:

• Creationists (rightfully) assume that God, in His infinite wisdom, designed a complex cellular system that allows modern organisms to function perfectly on a molecular level. Endosymbiotic theory speaks to the majesty of His creation. Indeed, when confronted with endosymbiotic theory, Christians often think of the words of Daniel: "I will praise thee, for I am fearfully and wonderfully made."
• The general consensus of the scientific community is that endosymbiotic theory is a demonstrated fact, backed up by a mountain of molecular/biochemical evidence. This fact, in conjunction with a wealth of biological research, confirms Darwin's theory of evolution as a scientific truth.

See Also

• Theory of Evolution
• Symbiosis
• Cell
• Biology

References

1. ↑ Mereschkowski C (1905). "Über Natur und Ursprung der Chromatophoren im Pflanzenreiche". Biol Centralbl 25: 593–604.
2. ↑ Schimper AFW (1883). "Über die Entwicklung der Chlorophyllkörner und Farbkörper". Bot. Zeitung 41: 105–14, 121–31, 137–46, 153–62.
3. ↑ Wallin IE (1923). "The Mitochondria Problem". The American Naturalist 57 (650): 255–61. doi:10.1086/279919.
4. ↑ Ris H, Singh RN (January 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.
5. ↑ 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.
6. ↑ 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.
7. ↑ John Brockman, The Third Culture, New York: Touchstone, 1995, 135.
8. ↑ 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. (Provides evidence that contradicts an endosymbiotic origin of peroxisomes. Instead it is suggested that they evolutionarily originate from the Endoplasmic Reticulum)
9. ↑ 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.
10. ↑ 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)
11. ↑ Raven, P., George Johnson, Kenneth Mason, Jonathan Losos, Susan Singer, 2010. Biology. McGraw-Hill 9th Ed. (January 14, 2010)
12. ↑ John Brockman, The Third Culture, New York: Touchstone, 1995, 144.