Endosymbiotic theory
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1. The Endosymbiotic Theory proposes that organelles like mitochondria and chloroplasts originated from ancient endosymbiotic relationships between bacteria and host cells. 2. Evidence for this includes mitochondria and chloroplasts containing their own DNA, resembling prokaryotic cells. Their protein synthesis mechanisms also resemble bacteria. 3. Molecular sequencing shows mitochondria being most similar to proteobacteria like Rickettsia prowazekii. Chloroplast sequences are similar to cyanobacteria, supporting an endosymbiotic origin.
Endosymbiotic theory
- 1. 1 Name: JohnathanTsui Wei Yau Course: Microbiology and Virology Student number: 1222880 Date:10-04-2013 Word count: 2964 Extended essay: Reviewing the evidence for the Endosymbiotic theory- the modern, or organelle containing eukaryotic cell evolved in steps through the stable incorporation of chemo-organotrophic and phototrophic symbionts from the domain Bacteria Abstract (100 words max): An introduction to the Endosymbiotic theory is first given to provide an overview of the content of discussion, more specifically the evidence and theories for and against it. The next two sections serve to describe the two main alternative theories as opposed to the SET, preventing the assumption that it is the most validated theory. Next section that ensues discusses mainly evidences which support the SET,it is further divided into two sub-sections using two of the main eukaryotic organelles concerned- mitochondria and chloroplasts. Finally, a conclusion regarding all the surrounding arguments discussed is given.
- 2. 2 A brief introduction to Endosymbiotic theory To put it bluntly, Endosymbiotic theory (or SET, Serial Endosymbiosis Theory) is a theory that tries to explain the evolutionary origin of organelles in modern day eukaryotic cells. “As early as 1883, A.F.W Schimper proposed that the chloroplasts of plants were cyanobacteria that lived symbiotically inside plant cells…..Early scientists such as Ivan Wallin were convinced that mitochondria were bacteria that lived inside cells.” (Paracer and Ahmadjian 81) It was first revitalized and put forth in the 1960’s by Lynn Margulis, a biologist from Boston University, and published in her book “Symbiosis in Cell Evolution” in 1981. It proposes that organelles residing in eukaryotic cells, most notably mitochondria and chloroplasts (possibly peroxisomes, hydrogenosomes, etc other membrane bound organelles), originated from the endocytosis of aerobic eubacteria and cyanobacteria (respectively) by anaerobic prokaryotic host cells (archaebacteria). Unable to digest the engulfed bacteria, they developed a symbiotic relationship (where two species gain mutual benefits by living alongside each other) over billions of years, hence the name “endosymbiosis”. The prokaryotes that were once unable to utilize oxygen began to acquire abilities to carry out aerobic respirations and eventually photosynthetic reactions by successive endosymbiosis events. As time progressed, the engulfed bacteria transferred some genes to the host cell’s genome, resulting in the permanent interdependence on each other’s metabolic activities. Eventually, they developed into some of the known organelles such as mitochondria and chloroplasts.
- 3. 3 As a boldly proposed theory, the SET was questioned by the scientific community throughout the 19th and 20th century as there is no way to support it with experimental results. However, the advancement of scientific knowledge and technologies in recent years allows the structural, molecular and biochemical scrutiny of eukaryotic organelles to construct probable backup on the SET. Some of these evidences include the fact that both mitochondria and chloroplasts possess their own circular DNA (though not all in mitochondria), which strongly resembles prokaryotic cells; the fact that these organelles synthesize their own proteins and perform binary fission is also a powerful evidence. Some of the evidences which hint the validity of SET will be examined in the following sections of this essay, while arguments that the SET cannot be established will also be examined in order to provide an objective, bird eyes’ view on this particular subject. Before this, however, some of the other theories and hypothesis regarding the rise of eukaryotic organelles will also be mentioned for the sake of balancing alternative views on this matter.
- 4. 4 Autogenous Theory Figure 1- this is a simple illustration of how infoldings of an ancestral prokaryote can result in single membrane organelles such as ER and nucleus, which leads to further compartmentalization of plasmids within these invaginations that results in the appearance of organelles like mitochondria The autogenous theory proposes that organelles such as nucleus, endoplasmic reticulum, Golgi apparatus, vacuoles and lysosomes first “evolved from elaborations of the cell and other internal membranes” (Paracer and Ahmadjian 80), or in other words progressive compartmentalization as seen in figure 1. These single membrane organelles arise from the infoldings of a prokaryotic ancestor cell’s plasma membrane (Gwu.edu), and organelles such as mitochondria and chloroplasts successively evolved after the appearance of a prototype eukaryote by compartmentalization of plasmids within the invaginations of the cell membrane (Gwu.edu). The common features found between mitochondria & chloroplasts and eubacteria as proposed in SET might be explained by mosaic evolution, where “the components in the compartment evolved more slowly than other parts of the cell” (Gwu.edu), thus explaining why they possess some of the molecular and structural similarities to a prokaryotic cell. The
- 5. 5 fact that they have double membrane can also be explained by possible, secondary invaginations of the plasma membrane. The Hydrogen Hypothesis Figure 2- On the left hand side, type II amitochondriate eukaryote resembles the anaerobic proteobacterium that also utilizes hydrogenosome to generate ATP, with hydrogen, carbon dioxide and acetate discarded as waste products; this proposed endosymbiont is compared to a eukaryote with mitochondria as seen on the right
- 6. 6 Figure 3- this is a simple illustration of how the archaen (methanogen), which takes in hydrogen, carbon dioxide and acetate to produce ATP with methane discarded as waste products, engulfed the proteobacterium (hydrogen- producing symbiont) over time In short, the hydrogen hypothesis “is based on the symbiosis between an alpha proteobacterium and an archaean (methanogen)” (Paracer and Ahmadjian 81). In aerobic cell respiration performed by mitochondria (following glycolysis), pyruvate is decarboxylated into acetyl-CoA by the PDF (Pyruvate dehydrogenase complex), which is then oxidized in the citric acid cycle into NADH, CO2 and water, together with 36 mol of ATP for each mol of glucose consumed. The proteobacterium involved in the symbiosis performed respiration similar to an anaerobic amitochondriate eukaryote with hydrogenosome, where pyruvate is converted into acetate, CO2 and reduced ferredoxin by the pyruvate ferrodoxinoxidoreductase (PFO). The reduced ferredoxin is reoxidized by hydrogenase, producing ATP and hydrogen, hydrogen is discharged with carbon dioxide and acetate as waste products, as seen in figure 2. On the other hand, the archaean involved are methanogens- obligate anaerobe that uses hydrogen and carbon dioxide to produce ATP (Acetyl-CoA pathway) and methane (Methanogenesis),
- 7. 7 with methane discarded as waste products. This ensures that symbiosis took place as mutual benefits between these two organisms were guaranteed. The archaean became the host with the proteobacterium as the endosymbiont, as depicted in figure 3. Eventually, the host became dependent on the proteobacterium and genetic transfer occur from the proteobacterium to the host , “the host provided the membrane proteins for the import of substrates and enzymes for glycolysis, and ATP production began under anaerobic conditions” (Paracer and Ahmadjian 83). As time progressed, the archaean host began to utilize more complex and organic molecules instead of hydrogen and carbon dioxide. Evidences supporting the Endosymbiotic Theory In the following sections, information that point towards the SET will be discussed, and will be divided between the two major eukaryotic organelles that are often considered- mitochondria and chloroplasts.
- 8. 8 Mitochondria Figure 4- A 3D cross-sectional view of a mitochondrion, which contains its own circular mitochondrial DNA, ribosomes (note that they are 70S), ATP synthase and double membrane. It is of no surprise that some speculate its origin from a prokaryotic cell, which also possesses a circular DNA molecule and ribosomes of the same size One of the main claims that mitochondria might have originated from an ancestral proteobacterium is that a mitochondrion possesses its own single, circular DNA molecule that is not associated with histones, proteins that participate in the organization and wrapping of DNA in eukaryotic cells. In human mitochondria, it consists of 37 genes (Endosymbiosis and the Origin of Domain Eukaryotafossilmuseum.net)“16,569 base pairs of DNA in a closed circle which encodes 2 rRNA molecules, 22 tRNA molecules and 13 polypeptides, with these polypeptides constructing protein complexes such as NADH dehydrogenase complex, ATP synthase, cytochrome c oxidase and cytochrome b in the inner mitochondrial membrane” (Endosymbiosis and The Origin of Eukaryotes
- 9. 9 users.rcn.com). This indeed bears an extremely close resemblance to a prokaryote, which also has its own circular DNA that is free of histones. However, mitochondrial DNA in different organism do not share the same size, with “two or three times more genome in plant mitochondria than in animal mitochondria” and “much of the plant mitochondrial DNA consists of noncoding sequences known as introns” (Paracer and Ahmadjian 85). As prokaryotic DNA is not consisted of introns, despite the striking similarity between the molecular features of mitochondria and prokaryotes, one cannot insist that mitochondria definitely derived from ancestral eubacteria. By studying the mitochondria of fungus, it was found that the tRNA sequences are not similar to either prokaryotes or most eukaryotes (Paracer and Ahmadjian 85). Also, mitochondrial mRNA “have intervening sequences, or introns, along with post-transcriptional processing” (Paracer and Ahmadjian 85) which is again not a feature spotted in prokaryotes but only in eukaryotes. However, one can still argue in favor of the SET that mitochondrial genome consists of introns because of the transfer of genes between the host and the involved endosymbiont, which results in changes in the genetic makeup of the proto-mitochondrion. Another piece of evidence pertaining to this matter is that mitochondria have 70S ribosomes that are also found in prokaryotes but not in eukaryotes, and they have their own unique protein-synthesizing mechanism that again relates to prokaryotic cells. Antibiotics such as streptomycin block protein synthesis in bacteria but not in eukaryotic cells, and they too block protein synthesis in mitochondria; similarly, rifampicin inhibits both the RNA polymerase of bacteria and mitochondria but not that of eukaryotes (Endosymbiosis and The Origin of Eukaryotes users.rcn.com). On the other hand, inhibitors
- 10. 10 such as diphtheria toxin act on protein synthesis of eukaryotes but not that of bacteria or mitochondria Endosymbiosis and The Origin of Eukaryotes users.rcn.com). The fact that the translation machinery in mitochondria bears such likeness to that found in prokaryotes does help to validate the SET, but not fully justify it as it is not flawless. Pertaining to this evidence it is possible to counter it with the autogenous theory, which argues that mitochondria resemble prokaryotes because of mosaic evolution, where components within the compartments formed by invaginations of an ancestral prokaryotic cell’s plasma membrane evolved much slower than the other parts. Figure 5- Electron microscope images of Rhodopseudomonas (left) and R. prowazekii (right); Rhodopseudomonas is a gram-negative bacteria that can display four modes of metabolism: photoheterotrophic, photoautotrophic, chemoheterotrophic and chemoautotrophic; R. prowazekii is also a gram-negative, intracellular (obligate) and aerobic alpha proteobacteria that is the causative agent of typhus; they are both genetically and molecularly related to mitochondria Perhaps the most solid evidence that supports the SET, “the sequences of cytochromes, ferredoxin, and 5S rRNA in mitochondria are similar to those of photosynthetic prokaryotes such as Rhodopseudomonas” and by “sequencing rRNA from mitochondria of fungi, mice and humans, from
- 11. 11 nuclei of animals, yeast, and corn chloroplasts, and from E.coli that the greatest similarity in conserved sequences was between mitochondria and prokaryotes” (Paracer and Ahmadjian 85). Molecular sequencing also displays that proteobacteria are the closest modern day relatives to mitochondria, as “sequence data suggest that all extant mitochondria are derived from an ancestor of R. prowazekii (gram-negative proteobacteria) as the result of a single endosymbiotic event (2% of the protein-coding genes inE.coli are found in all examined mitochondria).” (Berg, Tymoczko and Stryer 546) Another interesting evidence to note is that mitochondria is involved in the intrinsic apoptosis pathway, which is “initiated by the formation of the cytosolic apoptosome composed of Apaf-1 (protease), procaspase 9, and the cytochrome c released from mitochondria” (Jeong and Seol, “The role of mitochondria in apoptosis”) in response to apoptotic signals. This reflects the interdependence displayed between the host and endosymbiont in ancient times after endosymbiosis had taken place for billions of years, as a result of gene transfer between the two.
- 12. 12 Chloroplasts Figure 6- A two dimensional cross-section view of a chloroplast, which, similar to mitochondria, also possesses its single circular loop of DNA, 70S ribosomes and double membrane; however, they are unique in the sense that they have these extensive system of membranous sacs called thylakoids that actively engage in light-dependent reactions by containing photosynthetic pigments Chloroplasts, like mitochondria, also have their own unique single, circular DNA. There are 128 genes that include “duplicate genes encoding each of the four subunits of the rRNA, 37 genes encoding tRNA, 4 genes encoding parts of the subunits of RNA polymerase, one gene encoding the large subunit of the enzyme RUBISCO (small subunit encoded by nuclear genome), 9 genes for components of photosystems I and II, 6 genes for parts of the ATP synthase and the rest of the genes for 19 of the 60 proteins used to construct ribosome” (Endosymbiosis and The Origin of Eukaryotes users.rcn.com). Like mitochondria, this bears a resemblance to prokaryotic cells. Furthermore, the fact that chloroplasts rely on nuclear genes to encode their structural proteins (for instance the small subunit of the enzyme RUBISCO used in the Calvin Cycle) translated in the cytosol, which are then transported into the
- 13. 13 chloroplasts supports the interdependence that had resulted from billions of years of endosymbiosis where gene transfer took place and stripped the endosymbiont of the ability to live on its own. “Plastids (major double membrane organelles in plants that specialize in the storage and assembly of compounds required by cells) typically contain some 60-100 genes, compared to cyanobacteria that have some 1500 genes.” (Endosymbiosis and the Origin of Domain Eukaryotafossilmuseum.net) This again demonstrates the gleaming possibility of the endosymbiont’s gene transfer to its host over the course of endosymbiosis. Another evidence which supports chloroplasts originated as cyanobacteria that were engulfed by prokaryotic host cell capable of phagocytosis is that chloroplasts have double phospholipid bilayers. “The inner membrane represents the plasma membrane of the original prokaryotic symbiont, and the outer membrane either represents the vacuolar membrane formed by the host around the symbiont or is the remnant outer membrane of the Gram-negative cyanobacterium that was the endosymbiont.” (Paracer and Ahmadjian 85) Other plastids such as chromoplasts and leucoplasts could have been derived from secondary endosymbiosis, where heterotrophic eukaryotes proceeded to engulf other photosynthetic eukaryotes. For instance, “a group of unicellular, motile algae called cryptomonads appear to be the evolutionary outcome of a nonphotosynthetic eukaryotic flagellate engulfing a red alga by endocytosis.” (Endosymbiosis and The Origin of Eukaryotes users.rcn.com); perhaps more specifically, Paramecium bursaria, a unicellular eukaryote, engulfs green algae named zoochlorella and develops a symbiotic relationship with it instead of digesting it. Zoochlorella provides
- 14. 14 P.bursariawith the ability to photosynthesize sugar, while they “extract nourishment from the host when it is well fed and when they are deprived of light” and are “situated within individual vacuoles and these alga-vacuole complexes grow and divide at a rate compatible with that of the paramecium.” (Karakashian, “Symbiosis in Paramecium. Bursaria”) These are solid living examples of endosymbiosis that provide insights as to the rise of eukaryotic cells. However, it is still possible to argue that plastids like chloroplasts arise as a result of secondary and even further invaginations of the ancestral prokaryotic cell’s plasma membrane, resulting in the phenomenon that the “chloroplasts of Euglena have three outer membranes, and those of brown algae and diatoms are surrounded by four membranes” (Paracer and Ahmadjian 85). Chloroplasts are very similar to mitochondria in the sense that they also possess 70S ribosomes found commonly in prokaryotes, and their own protein-synthesizing mechanism. For instance, Methionine is always the first amino acid to be translated along the mRNA transcript; antibiotics such as streptomycin prevents protein synthesis in chloroplasts just as it is in bacteria and inhibitors such as diphtheria toxin has no effect on protein synthesis unlike eukaryotes (Endosymbiosis and The Origin of Eukaryotes users.rcn.com). Just as it was mentioned under the “mitochondria” section, one can argue that this does not support the SET by applying the autogenous theory, which simply considers the prokaryotic molecular features of mitochondria and chloroplasts as a result of mosaic evolution.
- 15. 15 Perhaps the strongest piece of evidence for the SET regarding chloroplasts is from data collected from molecular sequencing, such as “DNA-RNA hybridization of chloroplasts, nucleocytoplasm, and cyanobacteria and sequencing of plastocyanin, ferredoxin, and cytochrome c….have indicated a close relationship between chloroplasts and blue-green prokaryotes.” (Paracer and Ahmadjian 86) More importantly, the 5S and 16S sequences of chloroplast rRNA is much similar to that of cyanobacteria than that of eukaryotes (Paracer and Ahmadjian 86). Conclusion and Discussion- Is the Serial Endosymbiotic Theory entirely credible? The information and evidences presented regarding the SET are largely overwhelming due to the breadth of this topic, and the sections above barely provided a glimpse of arguments supporting and against this evolutionary theory. The fact that both mitochondria and chloroplasts greatly resemble prokaryotes both morphologically and molecularly, as they both possess double phospholipid bilayers, circular DNA molecule, 70S ribosomes and DNA/RNA sequences similar to prokaryotes, increases the credibility of the endosymbiotic theory. These organelles are potentially the descendants of engulfed proteobacteria and cyanobacteria that goes back to billions of years ago. The sequencing of proteins and nucleic acids had substantially proven the genetic link of these organelles to their ancestral cells; also, there are living organisms such as Paramecium bursaria that can be presented as living proof of the possibility of endosymbiosis as a way of mutually benefitting both the host and the endosymbiont.
- 16. 16 However, the SET is not without its flaws: “All evolutionary theories must offer an explanation in mechanistic terms of how it should or could have happened in order to be tested.” (Albert de Roos,“A critique on the endosymbiotic theory for the origin of mitochondria”) The exact mechanisms of how the genetic transfer from the endosymbiont to the host occurred, how organelles like mitochondria and chloroplasts acquired their double phospholipid membranes, how the interdepence between the two evolved are not explained in this vaguely defined theory. The fact that endosymbiotic theory claims mitochondria derived from the endocytosis of aerobic proteobacteria by anaerobic archaebacteria presents a problem, as the host cell clearly could not tolerate any oxygen whilst the endosymbiont require oxygen to carry out aerobic respiration. There would also be a competition for energy resources as the endosymbiont requires ATP and would not easily transport them to the host cell. There is also the problem that “the extensive gene transfer that is needed in the endosymbiotic theory would wreak havoc in a complex genome since frequent insertion of random pieces of endosymbiont’s DNA would disrupt existing functions…the endosymbiotic theory is in contrast with the concept of gradualism that forms the basis of modern evolutionary theory.” (Albert de Roos,“A critique on the endosymbiotic theory for the origin of mitochondria”) Pertaining to the evidence that mitochondria have circular DNA similar to that of bacteria, linear mitochondrial DNA molecules exist with “eukaryotic telomeres”. (Albert de Roos,“A critique on the endosymbiotic theory for the origin of mitochondria”) Although genetic sequencing supports that mitochondria have a monophyletic origin from proteobacteria, “recent studies of protists…indicate that the mitochondrion arose in a common ancestor of all extant eukaryotes and raise the possibility that this organelle originated at essentially the same time as the
- 17. 17 nuclear component of the eukaryotic cell rather than in a separate, subsequent event.” (Gray, Burger and Lang, “Mitochondrial evolution”) In other words, the autogenous theory seems to explain the rise of mitochondria better as it claims that organelles form from invaginations of an ancestral prokaryotic cell’s plasma membrane. Speaking of the autogenous theory, it also appears to be more favorable as the evolution of eukaryotic organelles could be “driven by the advantages to sequester metabolic activity in specialized compartments.” (Albert de Roos,“A critique on the endosymbiotic theory for the origin of mitochondria”) The specific roles of the different organelles could have been based on the existing metabolic functionalities, which were expanded and amplified gradually over time. In an addition, “the mitochondrial genes could be derived from transposable elements, plastids or viruses and could come from either the nuclear genome or a bacterial genome.” (Albert de Roos,“A critique on the endosymbiotic theory for the origin of mitochondria”) In short, the true evolutionary origins of eukaryotic organelles is still undetermined due to the vast amount of time scientists are covering and the lack of substantial proof as to how every bit of the mechanism worked, with no way to experimentally testify. Although the SET offers a probable theory of endosymbiosis coupled by evolution that is backed by modern molecular data and observation, there are still a lot of questions unanswered and missing links undiscovered. Other viable options such as autogenous theory present a simpler and more direct suggestion as to the rise of eukaryotic organelles, and while they are not without faults these alternative theories should be considered before the final answer is sealed.
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- 19. 19 Karakashian, MW. "Symbiosis in Paramecium Bursaria."National Center for Biotechnology Information. U.S. National Library of Medicine, 31 Jan. 2002. Web. 01 Apr. 2013. <http://www.ncbi.nlm.nih.gov/pubmed/785659>. Roos, Albert De. "A Critique on the Endosymbiotic Theory for the Origin of Mitochondria - Telic Thoughts."A Critique on the Endosymbiotic Theory for the Origin of Mitochondria - Telic Thoughts. Telic Thoughts, 17 Oct. 2007. Web. 01 Apr. 2013. <http://telicthoughts.com/a-critique-on-the- endosymbiotic-theory-for-the-origin-of-mitochondria/>. Stryer, Lubert, John L. Tymoczko, and Jeremy M. Berg.Biochemistry. New York: W.H. Freeman, 1988. "The Endosymbiotic Theory."Endosymbiotic Theory. N.p., 14 Jan. 2002. Web. 01 Apr. 2013. <http://www.biology.iupui.edu/biocourses/N100/2k2endosymb.html>.