Artistic representation of Mitochondria https://www.the-scientist.com/tag/mitochondria

The Theory of Endosymbiosis and its Implications in the Evolution of the Three Domains

The endosymbiotic theory illuminates the idea in which an organism may reside within the cells of another while each grow to be mutually dependent (Gray). This theory was independently developed by numerous scientists and became the first recognized version of the concept of horizontal gene transfer. By this theory, it is hypothesized that the eukaryotic cell is the result of an ancient symbiotic relationship among two previously dissimilar organisms (Quammen).

Illustrated flow chard outlining the Endosymbiosis Theory https://www.google.com/url?sa=i&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwiRhvv6mfneAhWO4FQKHd56AfMQjhx6BAgBEAM&url=https%3A%2F%2Fendosymbiotichypothesis.wordpress.com%2Fhistory-the-formation-of-the-endosymbiotic-hypothesis%2F&psig=AOvVaw0JoPrVKZwjTvoKRKh_1bFp&ust=1543539438447456

In a written composition titled “Origin of Eukaryotic Cells”, an American biologist named Lynn Margulis listed a three part hypothesis. First, that chloroplasts had been derived from cyanobacteria, mitochondria had been derived from another form of bacteria, and finally that flagella had been derived from spirochetes (Quammen). The components of this hypothesis was further developed, and in one case, disproved by a number of researchers hailing from all over the globe.

The Derivation of Chloroplasts from Cyanobacteria and Mitochondria from Alpha-proteobacteria

According to the Endogenous theory, chloroplasts had taken shape inside of plant cells, formed from internal materials. Rather than as inherently self grown organs, chloroplasts had once been independent bodies foreign to the organisms they now inhabit (McFadden).

Linda Bonen was a technical assistant for Carl Woese, the microbiologist that is best known for his role in identifying the domain of Archaea. Bonen along with a man named Ford Doolittle played a part in helping to confirm the first postulate of the endosymbiosis theory; that chloroplasts had been derived from Cyanobacteria (Quammen). In some ways algal chloroplasts closely resemble cyanobacteria with the presence of DNA fibrils and a double membrane. The two compared the ribosomal RNA of the cytoplasm of red algae, the ribosomal RNA of the chloroplasts in red algae, and the ribosomal RNA of several bacteria species. If Margulis was correct, the ribosomal RNA of the chloroplasts of a complex cell should more closely resemble the ribosomal RNA of a bacteria as a result of shared origin. As expected, the results gathered displayed that the rRNA from chloroplasts varied greatly from the rRNA in algae cytoplasm (Quammen). The chloroplast rRNA matched much more closely to that of the bacteria.

Independent of both Woese and Margulis, an assistant professor named Michael W. Gray was able to confirm another component to Margulis’s hypothesis (Quammen). During his research, he ascertained that there was no resemblance in the ribosomal RNA of wheat mitochondria DNA and the rRNA of wheat mitochondria. It was later identified that the alpha-proteobacteria species Agrobacterium tumefaciens was the closest bacterial match to mitochondria (Gray). These results aided in the confirmation of the second postulate of the endosymbiosis theory in relation to mitochondrion descendance from a species of bacteria.

The Derivation of Spirochetes

Margulis’s contribution to the Endosymbiosis theory happened to be the one portion to be disproved later on. Margulis dubbed her idea undulipodia, in which she believed that flagella, cilia, and centrioles were all bacterial structures related to the bacterial group known as spirochetes (Quammen). Flagella are structures that writhe and wiggle to propel single celled organisms throughout their environment. Cilia are hair like protruding structures from eukaryotic cells that function in allowing organisms to move obstructions such as mucus and other debris through the windpipe. Finally, centrioles are structures in charge of organizing and distributing chromosomes during mitotic and meiotic cell division (Microbiology and molecular biology reviews : MMBR). By way of microscopy, Margulis was able to observe that all three of these structures had a shared arrangement of roughly nine tubules. However, there was no molecular evidence that connected this theory of undulipodia to spirochetes. The only evidence garnered was gathered by microscopy. This was an issue because in a day and age where molecular data is much more accessible, evidence gathered by microscopical means are not deemed conclusive enough. Although her independent contribution to the overarching hypothesis was later proven to be incorrect Margulis is credited with popularizing these ideas, allowing these theories to gain traction among the scientific community.

As components of these theories were both proved and disproved, there was no way to test the real world applications of their findings. At long last, a recent study was able to mimic the exact situation in which an endosymbiotic relationship could be formed and cultivated, and which conditions must be met in order to allow this relationship to persists across generations.

Lynn Margulis during her early days as a researcher Source: https://www.google.com/url?sa=i&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwiNnemNzvjeAhXGx1QKHYXdAQYQjhx6BAgBEAM&url=https%3A%2F%2Fwww.brainpickings.org%2F2018%2F04%2F19%2Flynn-margulis-talking-on-the-water%2F&psig=AOvVaw3Um2pxmD-ewSiJYUPtUK-Q&ust=1543546804628013

Endosymbiosis likely arose 1.5 billion years ago, influencing the divergence of eukaryotic cells as we know them today. Following the research done by Michael Gray, it had been hypothesized that alpha-proteobacterium had become enveloped within an archaeal host cell as an endosymbiont, triggering the evolution of mitochondria (2). Following this event, the bacteria further specialized into an organelle adept in efficiently producing energy through the synthesis of ATP. However, more recent studies have advocated that mitochondria had branched off long before the divergence of alpha-proteobacteria and had evolved from a more ancient lineage of archaean proteobacterial instead.

Gray’s hypothesis was revised with the recent discovery of the Asgard superphylum of archaea, which contains genomes that encode for both the cytoskeletal and transportation vesicle homolog predecessors of the modern day eukaryotic endomembrane system (Mehta). The discovery may indicate that the common ancestor of archaea and eukaryotes may have already contained common features found in the eukaryotic cells of today (Javaux). Before the integration of a modern mitochondria, this endosymbiont probably contained the metabolic pathways capable of carrying out the processes of glycolysis, the tricarboxylic acid (TCA) cycle, the pentose phosphate pathway, and the fatty acid biosynthesis pathway. In addition, this endosymbiont was also likely capable of living under low oxygen conditions by usage of of an electron transport chain and an ADP/ATP translocase. ADP/ATP translocase is a protein that enables the the transportation of ATP and ADP throughout the cellular membrane. This protein is likely a functional homolog to the mechanism incorporated by intracellular bacteria for the importation of ATP form the cytoplasm of a host cell (Mehta).

A recent study hoped to experimentally recreate these early stages in the evolution of eukaryotic mitochondria by artificially inducing endosymbiosis among two model species: Escherichia coli and Saccharomyces cerevisiae. These researchers hoped to engineer the Escherichia coli endosymbionts with the capability of providing ATP to host Saccharomyces cerevisiae yeast cell mutants incapable of synthesizing ATP (Mehta).

Within the model system, E. coli strains were engineered to inhabit the cytosol of a yeast mutant lacking in mitochondrial DNA. With the absence of mitochondria, the goal was to engineer the E. coli strains to produce ATP for the mutant. In return, the yeast was engineered to contribute thiamin to the auxotrophic E. coli in need of thiamin (Mehta). Thiamine pyrophosphate is a derivative of thiamin necessary to initiate the process of alcoholic fermentation in yeasts. (Pronk)

strategy incorporated to create a S. cerevisiae and E. coli hybrid. Source: Engineering yeast endosymbionts as a step toward the evolution of mitochondria

With the introduction of E. coli cells into the cytosol of the mutant yeast cells, it was expected that the engineered endosymbionts would demonstrate the ability to apply glycerol within the cell and synthesize it into ATP for the mutant host. When translocase expression was induced within the cells, ATP production was significantly increased in the bacteria (Mehta). In contradiction to this development, the parent E. coli strain lacking in plasmids encoding for translocase did not display an observable uptake in ATP production (Mehta).

An additional requirement for the formation of a stable endosymbiont was the incorporation of SNARE-like proteins. SNARE proteins are charged with the role of assisting in the fusion of vesicles to a targeted membrane (PDB101: Molecule of the Month: SNARE Proteins). Among intracellular pathogenic bacterium, a SNARE-like protein is employed. While further experimentation and research will be necessary to discern the exact role of these proteins in the establishment of a stable endosymbiont, the incorporation of SNARE-like proteins is hypothesized to aid in the avoidance of lysosomal degradation (Mehta).

Following the preceding results, the researchers came to the conclusion that the combination of the expression of ADP/ATP translocase in conjunction with two SNARE proteins was the minimum requirement for the establishment of stable endosymbiosis (Mehta).

Observed plate growths of engineered S. cerevisiae and E. coli hybrid and control cells Source: Engineering yeast endosymbionts as a step toward the evolution of mitochondria

Fluorescent microscopy imagery of S. cerevisiae and E. coli hybrid and control cells Source: Engineering yeast endosymbionts as a step toward the evolution of mitochondria

As could be evidenced by the results, the researchers were able to conclude that E. coli endosymbionts required various cofactors and amino acids from the host S. cerevisiae cytosol. In return the endosymbionts were able to supply the yeast cell with ATP. An additional requirement for establishing this relationship among S. cerevisiae and E. coli was the expression of SNARE-like proteins derived from other intracellular pathogens (Mehta).

New insights were gained through this synthetic mimicry of an endosymbiotic system. Following the study, it is now predicted that the inherently parasitic relationship among a colonizing intracellular bacteria and an archaeal host would have been disadvantageous for the archaeal host developing in its natural ecological niche. As the intracellular bacterium applies the usage of an ADP/ATP translocator for energy, the host cell may begin to deteriorate in the high concentrations of hydrogen of the otherwise favorable conditions of an archaeal host (Mehta). The metabolic processes of a parasitic bacterium allow it to generate ATP through the TCA cycle and oxygen dependent oxidative phosphorylation (Yeast Pathways Database Website Home). When working cohesively with ADP/ATP translocase, parasitically colonized archaeal cells may have adapted the ability to adapt to brand new oxygen and TCA substrate containing niches that were otherwise inaccessible to host archaeal cells without a parasitic inhabitant. In this new ecological niche, the archaeal host would have no longer produced its own ATP and thus became the parasite. Conversely, as the flow of ATP is reversed, the bacterium now takes on the role as the symbiotic host (Mehta). The ability of the endosymbiotic duo of archaean host and intracellular bacteria to expand across new ecological niches may have been the initial step in the transition from pre-mitochondrial endosymbiont to the protomitochondrions we observe today.

References:

1. Cold Spring Harbor perspectives in biology. (n.d.). Cold Spring Harbor Perspectives in Biology., 4, A011403. https://cpslo-primo.hosted.exlibrisgroup.com/primo-explore/openurl?rft_stitle=Cold%20Spring%20Harb.%20Perspect.%20Biol.&rft_aulast=Gray&rft_auinit1=M.%20W.&rft_issue=9&rft_epage=a011403&rft_atitle=Mitochondrial%20Evolution&rft_id=info:pmid%2F22952398&rft_genre=article&rft_val_fmt=info:ofi%2Ffmt:kev:mtx:journal&ctx_ver=Z39.88-2004&url_ver=Z39.88-2004&url_ctx_fmt=info:ofi%2Ffmt:kev:mtx:ctx&url_ctx_fmt=info:ofi%2Ffmt:kev:mtx:ctx&rft.jtitle=Cold%20Spring%20Harbor%20Perspectives%20in%20Biology&rft.volume=4&rft.spage=a011403&vid=01CALS_PSU&institution=01CALS_PSU&url_ctx_val=&isSerivcesPage=true&lang=en_US
2. Gray, Michael W. “Mitochondrial Evolution.” Cold Spring Harbor Perspectives in Biology, Cold Spring Harbor Lab, 1 Jan. 1970, cshperspectives.cshlp.org/content/4/9/a011403?ijkey=a2f422cd975b438ce4dbefa5d4377af358197309&keytype2=tf_ipsecsha.
3. Javaux, E. (2001). Morphological and ecological complexity in early eukaryotic ecosystems. Nature: National Academic Journal of Architecture, 412(6842), 66. https://cpslo-primo.hosted.exlibrisgroup.com/primo-explore/openurl?rft_id=info:doi%2F10.1038%2Fs41586-018-0059-5&rft_genre=article&rft_val_fmt=info:ofi%2Ffmt:kev:mtx:journal&ctx_ver=Z39.88-2004&url_ver=Z39.88-2004&url_ctx_fmt=info:ofi%2Ffmt:kev:mtx:ctx&url_ctx_fmt=info:ofi%2Ffmt:kev:mtx:ctx&rft.jtitle=Nature&rft.volume=557&rft.spage=101&vid=01CALS_PSU&institution=01CALS_PSU&url_ctx_val=&isSerivcesPage=true&lang=en_US
4. McFadden, Geoffrey I. “Chloroplast Origin and Integration.” Plant Physiology, American Society of Plant Biologists, 1 Jan. 2001, www.plantphysiol.org/content/125/1/50.
5. Mehta, Angad P., et al. “Engineering Yeast Endosymbionts as a Step toward the Evolution of Mitochondria.” PNAS, National Academy of Sciences, 13 Nov. 2018, www.pnas.org/content/115/46/11796.
6. Microbiology and molecular biology reviews : MMBR. (n.d.). Microbiology and Molecular Biology Reviews : MMBR., 81, E00008. https://cpslo-primo.hosted.exlibrisgroup.com/primo-explore/openurl?rft_genre=article&rft_val_fmt=info:ofi%2Ffmt:kev:mtx:journal&ctx_ver=Z39.88-2004&url_ver=Z39.88-2004&url_ctx_fmt=info:ofi%2Ffmt:kev:mtx:ctx&url_ctx_fmt=info:ofi%2Ffmt:kev:mtx:ctx&rft.jtitle=Microbiol%20Mol%20Biol%20Rev&rft.volume=81&rft.spage=e00008&vid=01CALS_PSU&institution=01CALS_PSU&url_ctx_val=&isSerivcesPage=true&lang=en_US
7. “PDB101: Molecule of the Month: SNARE Proteins.” PDB-101: Acetylcholine Receptor, pdb101.rcsb.org/motm/167.
8. Pronk, Jack T. “Auxotrophic Yeast Strains in Fundamental and Applied Research.” Applied and Environmental Microbiology, American Society for Microbiology, 1 May 2002, aem.asm.org/content/68/5/2095.
9. Quammen, David. The Tangled Tree: a Radical New History of Life. William Collins, 2018.
10. Yeast Pathways Database Website Home, pathway.yeastgenome.org/YEAST/NEW-IMAGE?type=PATHWAY&object=PWY3O-17.