The Evolution of the Eukaryotic Cell - Essay Example

Evolution of the Eukaryotic Cell Introduction In the Proterozoic Era, 2.5 billion to 544 million years ago, the atmospheric level of oxygen increased to 15% due to oxygen producing cyanobacteria. The levels of oxygen in the atmosphere produced a fatal environment in which anaerobic organisms needed to evolve methods of coping with the presence of oxygen. These organisms evolved to be the organisms know today as eukaryotes. Direct filiation is the classical view of the evolution of eukaryotic organisms. This theory states that all organisms derived directly from a unique ancestral population by the accumulation of single step mutations, and that the same mutational mechanisms known to operate in the evolution of higher animals and plants also operated in the differentiation of higher eukaryotic cells from lower prokaryotic cells. This theory maintains that advantageous mutations, which enabled organisms to survive with oxygen in the atmosphere, were the basis for evolutionary change in prokaryotes. (Conduah, 2005). The second theory, referred to as the "botanical myth" by Lynn Margulis states that primitive photosynthetic bacteria evolved gradually into algae and plants, and some of these lost photosynthetic competence and evolved into fungi and animals. Due to the abundance of oxygen in the atmosphere, the fungi and animals evolved mitochondria, which utilize oxygen instead of light, as the energy producing organelle. (Conduah, 2005). The third theory of serial endosymbiosis relies on symbiogenesis, or long term symbiotic relationships between different species that lead to new forms of life states the following: blue green algae produced oxygen as a by-product of photosynthesis, allowing oxygen to buildup in the atmosphere; other bacteria, prokaryotic cells, developed and grew, some of them with aerobic capabilities; anaerobic, heterotrophic cells known as proto- eukaryotes, ingested these aerobes and developed a mutually beneficial relationship. Ingested organisms that prevented or avoided being digested evolved into the energy producing eukaryotic organelle mitochondria, from proteobacteria, and chloroplasts, from cyanobacteria. (Conduah, 2005). SET also explains the origin of eukaryotic flagella and cilia. Proponents propose that flagella derived from the symbiotic relationship of a host cell with a parasitic spirochete. A parasitic spirochete attached to surface of the host cell to gain food through the cell membrane, and the host cell gained motility from its whip-like motions. The beneficial relationship between the organisms evolved in the same manner as that of mitochondria and chloroplasts. Serial Endosymbiotic Theory (SET) The endosymbiotic theory related to the primitive origins of the organelles: mitochondria and chloroplasts. According to the endosymbiotic theory, these originated as prokaryotic organisms, which were engulfed by a larger prokaryote through phagocytosis. This larger prokaryote was probably a rickettsia bacterium, which is an anaerobic proteobacteria that was a precursor to the mitochondria organelle. Similarly, chloroplasts come from an autotrophic prokaryote called endosymbiotic cyanobacteria. The theory has in origins in 1905. Konstantin Mereschkowsky with chloroplasts and Ivan Wallin in the 20s advanced a similar idea for mitochondria. Later on, Henry Ris found that they contain DNA. The modern attribution goes to Lynn Margulis for her work in 1981, Symbiosis in Cell Evolution. She contended that eukaryotic cells started as communities of networking bodies such as endosymbiotic spirochetes that developed cilia and flagella. The problem with this is that cilia and flagella do not contain DNA. Another organelle, the peroxisome, is thought to have emerged this way. They, too, do not contain DNA, however. Christian de Duve’s peroxisome idea did not last long. (Cooper, 2005) Modern evidence that suggests the endosymbiotic theory is viable: Mitochondria and chloroplasts contain DNA, which is 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 chloroplasts are formed only through a process similar to binary fission. In some algae, such as Euglena, the chloroplasts can be destroyed by certain chemicals or prolonged absence of light without otherwise affecting the cell. In such a case, the chloroplasts will not regenerate. Much of the internal structure and biochemistry of chloroplasts, for instance the presence of thylakoids and particular chlorophylls, is very similar to that of cyanobacteria. Phylogenies built with bacteria, chloroplasts, and eukaryotic genomes also suggest that chloroplasts are most closely related to cyanobacteria. DNA sequence analysis and phylogeny suggests that nuclear DNA contains genes that probably came from the chloroplast. Some proteins encoded in the nucleus are transported to the organelle, and both mitochondria and chloroplasts have unusually small genomes compared to other organisms. This is consistent with an increased dependence on the eukaryotic host after forming an endosymbiosis. Chloroplasts appear in very different groups of protists, which are in general more closely related to forms lacking them than to each other. This suggests that if chloroplasts originated as part of the cell, they did so multiple times, in which case their close similarity to each other is difficult to explain. The size of both organelles is comparable to bacteria. These organelle's ribosomes are like those found in bacteria. (http://en.wikipedia.org/wiki/Endosymbiotic_hypothesis, 2005) Phylogenetic Trees Source: Wikipedia Other evolutionary Theories as They Relate to SED Archezoa hypothesis. The Archaezoa Hypothesis presented by Cavalier-Smith (1983) proposes that there was a subkingdom, Archaezoa, which diverged prior to the eukaryotes acquisition of the mitochondria. The evidence that is presented for the acceptance of this theory is the phylogenetic relationship of the amitochondriate taxa in comparison to the mitochondriate taxa and presence or absence of mitochondrial-like products within both types. Acquisition of the mitochondrion is monophyletic which would have the amitochondriate organisms coming from the mitochondriate organisms. Furthermore, since the amitochondriates are thought to be derived from the mitochondriates, they contain genes that were transferred by the eukaryotes. This would go against the evidence supported by in which mitochondriates served as precursors for mitochondriates. (Fulginiti, 2005). All of these observations do seem to indicate that the recent addition to SET is indeed a valid one. As Margulis herself admits, however, there probably is not enough evidence at the moment to say conclusively that flagella originated through endosymbiosis, as we know very nearly can say about mitochondria and chloroplasts. Although the Archeazoa theory was the first theory to provide an explanation on the acquisition of the mitochondria within Eukaryotes, it is not a valid argument because at the time that this theory was developed, there was not any substantial knowledge on phylogentic relationships to accurately support this theory (Roger, 1999). In addition, there were no appropriate tests developed to see if the genetic hypothesis was true. With time however, it appears that this part of the theory will more than likely begin to gain more and more acceptance. The key to unlocking the connection between flagella and spirochetes would be finding more conclusive molecular evidence. Since the eukaryotic flagella have microtubules arranged in a 9+2 manner (nine bundles of paired microtubules around the outer wall with two tubules in the center), Margulis (1982) believes that a strong match between DNA or RNA sequences of spirochetes and flagellar tubulin would greatly help her theory. There does not seem to be any current research that has confirmed this hope, but we would remain optimistic about such a potential as Margulis has been. (Fulginiti, 2005). There are three primary organelles of the eukaryotic cell that have arisen due to endosymbiosis. The current version of modified SET as proposed by Lynn Margulis proposes that the development of mitochondria, chloroplasts, and flagella (undulipodia) all occurred through endosymbiosis. However, the Archaezoa Hypothesis as presented by Cavalier-Smith proposes that there is a phylogenetic relationship between the amitochondriate taxa and mitochondriate taxa because of the presence or absence of mitochondrial-like products within both types. The point of this review will be to provide evidence that each of these three organelles did indeed originate as endosymbionts in order to show that SET is the most valid explanation for the rise of eukaryotic cells. (Fulginiti, 2005). These first two organelles, mitochondria and chloroplasts, are agreed to have evolved as endosymbionts by all supporters of any form of SET. The flagella proposal however, which is still fairly new and controversial, has yet to be adopted by all SET supporters. Lee indicates that support for the endosymbiotic and bacterial origins of mitochondria can be provided through comparative studies involving ribosome size, protein synthesis inhibition patterns, and enzyme structural properties. They also point out that mitochondria show great similarities in metabolic properties and protein structure to some types of present-day bacteria. (Cooper, 2005) Cyanobacteria Theory. Many sources indicate that members of the alpha-proteobacteria appear to be the most similar eubacteria relatives to the mitochondria. The proteobacteria are more commonly known as purple bacteria and are aerobic when oxygen is present, meaning that they reduce O2 to H2O. This oxygen-producing ability would make the proteobacteria good candidates to serve as the bacteria that were engulfed by anaerobic bacteria as the earth’s atmosphere became predominantly filled with oxygen, as mentioned in the introduction. The sequencing of both mitochondrial and proteobacteria genomes done by Gray and Gillham has given further support to this theory as the genomes of the two were shown to be quite similar in many regards. Lee discovered that the alpha-proteobacteria serve as the precursors to eukaryotic mitochondria one step further by proposing specific genera that may serve as the most likely ancestral models for the mitochondria. Paracoccus, Rhizobium, and Rhodospirillum seem to be the best candidates according to Lee. Of these three, the genus Paracoccus appears to be the best model as members of this genus exhibit the greatest number of similarities to the mitochondria. Whatley performed extensive comparative studies between Paracoccus denitrificans and the eukaryotic mitochondria, concluding that they shared similarities in their electron transport chains as well as their respiratory processes, phosphorylation techniques, and membrane structure. Lee explains that they include the genus Rhizobium as a potential mitochondrial ancestor because their electron transport chain and manner of respiration is very similar to that of Paracoccus and mitochondria. Similar evidence exists for the endosymbiotic origins of chloroplasts as well. McFadden notes that the basis of the argument that chloroplasts arose through endosymbiosis which can be traced to an observation made by A.F.W. Schimper, dating back as far as 1883. Schimper recorded in his studies that the plastids (chloroplasts) of plant cells very much resembled free-living cyanobacteria. This observation, which was made well over a century ago, still carries quite a bit of weight today as almost all current versions of SET. (Fannon, 2005) The cyanobacteria, which are often split into the blue-green bacteria and grass green bacteria (chloroxybacteria), undergo oxygenic photosynthesis . Different types of cyanobacteria contain chlorophylls (either A or A with B) as well as other types of pigments that allow them to convert carbon dioxide and energy from the sun into organic material. Oxygen is released through this process. When we compare chloroplasts and cyanobacteria using their functions alone, we can already see why the belief that chloroplasts are actually symbiotic cyanobacteria is quite convincing. Chloroplasts perform these same types of photosynthetic actions within eukaryotic plant and algae cells. (Fannon, 2005) The argument for cyanobacteria as the precursors becomes even more convincing with DNA studies. The theory really began to gain credibility in the late 1950s when Stocking and Gifford (1959) discovered that chloroplasts actually contain their own DNA that is separate and unique from the nuclear DNA of the eukaryotic cell. Evidence through the sequencing of DNA, RNA, and protein, demonstrates that the chloroplasts have evolved from cyanobacteria since the genetic makeup of each is so similar. Most work in this area, such as that done by Reith and Munholland. They sequenced the chloroplast DNA of Euglena and Porphyra and found it to be quite similar to sequenced DNA of the cyanobacteria. The cyanobacteria have however, undergone some extreme size reduction throughout the process of becoming endosymbionts. Kaneko et al. (1996) have demonstrated that the fully traced genome of a species of cyanobacteria from the genus Synechocystis contains 3,229 genes, while a chloroplast genome from Porphyra traced by Reith and Munholland was shown to contain just 200 genes. Conclusion All three theories propose that eukaryotes evolved from a prokaryotic ancestor because of a buildup of toxic substances from waste products such as oxygen and hydrogen ions in the earth’s environment during the Proterozoic era. Each theory differs in the mechanism of organelle evolution, but SET provides the most evidence. (Conduah, 2005). Works Cited Cooper, Geoffrey. The Cell: A Molecular Approach. The Origin and Evolution of Cells http://www.ncbi.nlm.nih.gov /books/bv.fcgi?db=Books&rid=cooper.section.90 Conduah, Daisy, Jeff Garofalo, Steve Heverley, Bryant Upton. An Investigation into the Evidence for the Serial Endosymbiosis Theory of Eukaryotic Development. http://www.susqu.edu/students/c/conduah/SET.htm IUPUI Department of Biology. The Endosymbiotic Theory. http://www.biology.iupui.edu/ biocourses/N100/2k3endosymb.html Fannon, Meghan. The Evolution of Eukaryotic Cells According to the Serial Endosymbiosis Theory. Journal of Systematic Biology at Susquehanna University. http://comenius.susqu. edu /bi/ 202/Journal/Vol8/number2/dragonfliespub.htm Fulginiti, Mike, Chris Gehman, Donald Halke, Melissa Wright. A Review of the Serial Endosymbiosis Theory of Eukaryotic Origin.  http://www.susqu.edu/students/w/ wrightmelissa/ %20Review%20of%20the%20Serial%20Endosymbiosis %20Theory%20of%20Eukaryotic%20Origin.html Wikipedia. Endosymbiotic theory. http://en.wikipedia.org/wiki/Endosymbiotic_hypothesis. Read More