IS ENDOSYMBIOSIS THE MODEL THAT ORIGINATED THE EUKARYOTIC CELL?
The endosymbiotic model is the most accepted theory among the hypothesis formulated regarding the origin of eukaryotic organisms. According to this model, the mitochondrion arose from the incorporation of a respiring bacterium into another cell while the chloroplast was created from the inclusion of a cyanobacterium capable of carrying out photosynthesis. In addition, the energy released by aerobic respiration and the ability of using sunlight to make energy allowed evolution to take place more rapidly. It is also extremely important to mention that the structure of mitochondria and chloroplast support this theory. As mentioned in the previous lines this essay will be about endosymbiosis and other theories on the evolution of the eukaryotic cell. To address these topics I will talk about evidence that support and go against each hypothesis and highlight why I believe that the endosymbiotic model is the theory that most likely happened 1.5 billion years ago.
Cellular organisms can be divided into three domain: eukaryota, archaea and bacteria.(1) Over the last decades multiple analyses of genes have been carried out to understand how the domains can be linked between each other and in particular how the eukaryotic cell was originated. It is now known that 75% of the eukaryotic genes can be tracked back to prokaryotes of bacterial origin and that eukaryotic organisms were probably evolved from the archaeal domain.(1) The previous observations drove the formation of a great variety of hypothesis regarding the origin of the eukaryotic cell, including the endosymbiotic theory. In the following lines an overview of the eukaryotic cell structure followed by a description of the different hypothesis regarding its origin will be provided.
EUKARYOTES AND PROKARYOTES
The cells that make up our bodies represent chimeras of older simpler cells and by using the word chimera scientists describe a single organism which is composed of cells from different zygotes.(2) In order to discuss how current cells evolved from ancient cells, it is essential to understand cells themselves. Cells represent the basic unit of structure and function for all living things and they are composed by the same basic properties. There are 2 basic types of cells: prokaryotic and eukaryotic.(3)
- Prokaryotic cells are simple cells with no membrane bound organelles; they evolve from protobionts and are the oldest form of life on earth. They do not have structural components that allow them to change shape and they are always single celled.(4)
- Eukaryotic cells are complex cells with membrane bound organelles and a nucleus which contain structural components that allow them to change shape. Moreover, eukaryotic cells can either be unicellular or multicellular.(4)
The structural components of eukaryotic cells and their relative functions can be seen in Fig. 1.
The exact origins of the eukaryotic cell are still unclear but a well-supported explanation known as the endosymbiotic theory represents the most widely accepted theory on the evolution of the eukaryotic cell. (5)This theory suggests that mitochondria and plastids in eukaryotic cells were once independent prokaryotic cells with the consequence of reaching the conclusion that billions of years ago there were three prokaryotic cells in the world (6). One was capable of carrying out aerobic respiration and converting energy, another one had the ability of carrying out photosynthesis while the third cell was not capable of taking part in any of the processes mentioned in the previous lines. (7)The latter cell engulfed the other cells intentionally or accidentally and that’s how it became capable of making useful energy and converting light from the sun into stored chemical energy. The phagocytosis of the respiring bacterium and more exactly the α-proteobacterium, gave rise to the formation of mitochondria while the implementation of the photosynthetic cyanobacterium created chloroplasts as shown in fig.2. (8)Endosymbionts and their hosts often represent distinct domains of life and their amalgamation can generate organisms with new biochemical capabilities which are able to survive in environments that would otherwise be hostile to either alone. (9)A symbiosis represents the close relationship between two or more different organisms and in this case the association provided the host with a new form of energy metabolism while the symbiotic bacteria received nutrients required in order to have a stable growth and evolution.(10)These originally free-living symbionts gradually became an intimate part of the eukaryotic cell.
ORGANELLES INVOLVED IN THE ENDOSYMBIOTIC THEORY
Mitochondria conserve their own genome within eukaryotic cells and have major roles in cellular metabolism.(11)Studies carried out between 1900 and 1960 proved how mitochondria are essential in order to allow the conversion of nutrient-derived energy in the form of ATP ( adenosine triphosphate) molecules through metabolic pathways known as the citric acid cycle ,β-oxidation, mitochondrial electron transfer and oxidative phosphorylation. (12)They are involved in the biosynthesis of molecules such as lipids, amino acids, haem and iron-sulphur clusters while controlling calcium homeostasis and cell death or apoptosis.(11) These organelles are made by two membranes both composed of a phospholipid bilayer. The two membranes are extremely different in appearance and physico-chemical properties that define their biochemical function. The peripheral membrane encloses the organelle while the inner membrane folds in a series of mitochondrial cristae. These invaginations increase the surface area of the inner membrane which carries out the majority of the enzymatic processes.(11) Mitochondrial dysfunction is the cause of energetic deficits associated with hereditary diseases known as mitochondrial disorders that are mainly caused by mutations in nuclear genes that encode mitochondrial proteins.(13)In addition, an alteration in mitochondrial function is at the base of conditions such as human encephalomyopathies, neurodegeneration, cancer and ageing.(11)
Chloroplasts are the site of photosynthesis in plants; the process involves the conversion of carbon dioxide and water into organic chemicals through the use of light as energy source.(14) These organelles contain a system of three membranes: the outer membrane, the inner membrane and the thylakoid system. The outer membrane is permeable to small molecules and ions while the inner membrane synthesises fatty acids, lipids and carotenoids. (15) The outer and inner membrane enclose a fluid known as stroma where the thylakoid system floats. In addition, the stroma contains the photosynthetic pigment called chlorophyll.(14)
EVIDENCE THAT SUPPORT THE ENDOSYMBIOTIC THEORY
The main point that gives this theory its credibility is the presence of genes originated in bacteria in the nucleus of eukaryotic cells. In addition, both mitochondria and chloroplast have enzymes and transport systems in their membranes that are similar to those found in membranes of current prokaryotic cells.(6) They also contain ribosomes of prokaryotic size (70S), including 16S ribosomal RNA (16S rRNA) molecule. Furthermore, they both replicate in the same way that some prokaryotic cells do; they reproduce independently from the cells in which they live in a process similar to bacterial fission.(6) Moreover, they both contain their own circular and covalently closed DNA that can replicate independently of other cell parts like bacteria that are prokaryotic organisms.(8) Through a series of experiments it was also discovered that mitochondrial DNA contains a higher ratio of guanine and cytosine base pairs ; this characteristic is usual in bacteria but isn’t exhibited in any eukaryotic organisms.(16) It’s also important to mention that antibiotics that inhibit ribosome function in bacteria also inhibit ribosome in these organelles which can additionally exhibit similar antibiotic resistance as seen in some prokaryotes. All the evidence listed above have allowed scientists to make a direct relationship between both organelles and prokaryotic cells ,indicating that the mitochondria and plastids, including chloroplast ,came from prokaryotic origins.(16)
PROBLEMS RAISED FROM THE ENDOSYMBIOTIC THEORY
Even though this theory is the most accepted hypothesis, there are some aspects that require to some extent specific clarifications. Firstly, it is possible to find chloroplast with the same size and shape as bacteria, but the range is so great that it is not possible to completely exclude the option that they might be similar due to chance.(7) In addition, the DNA of the organelles is not completely similar to bacterial DNA as it mimics the DNA of a plasmid which is supercoiled and doubly covalently linked. Furthermore, not all mitochondria contain circular DNA; ciliates (e.g. Paramecium) contain linear DNA while kinetoplastids are known to have closely-linked minicircles which are not found in any bacteria.(4)
A few hypotheses have been put forward in order to explain the formation of the eukaryotic cell including the hydrogen hypothesis. According to this theory the eukaryotic cell arose from an association between a hydrogen producing species of bacteria which eventually gave rise to the mitochondrion, and a species of hydrogen consuming archaea.(6) The host was an autotrophic methanogenic archaeon while the symbiont was an α-proteobacterium capable of living aerobically and anaerobically.(8) As a result of a lack of oxygen in the environment, the bacterium started producing hydrogen and carbon dioxide as waste products. These substances became essential in order to fuel the anaerobic system of the methanogen with the consequence of producing an association between the two organisms.(17)Following the engulfment of the α-proteobacterium within the archaea a transfer of genes from the endosymbiont occurred and as a result the host was provided with substances needed in order to import molecules from the environment and allow glycolysis to take place.(8) However, the formation of chloroplasts is not researched in depth within this hypothesis.
This theory perfectly explains why the host required the endosymbiont and the reason why the endosymbiotic gene transfer was an essential step throughout the process.(1) In addition the hydrogen hypothesis is able to prove that eukaryotes are genetic chimeras that contain genes from eurobacterial and archaeal ancestry.(7)The theory implies that the mitochondrion ancestor was capable of surviving in aerobic and anaerobic conditions, hence it would have preserved the ability of producing not only aerobic mitochondria but also anaerobic mitochondria. This would hypothetically cause the mitochondrial enzymes for anaerobic energy metabolism ,such as pyruvate–ferredoxin oxidoreductase and iron–iron hydrogenase ,to be of a-proteobacterial ancestry. Nevertheless, this theory is not completely supported due to the lack of data in regard to the presence of these genes in the eukaryotic common ancestor.(8)
The hydrogen theory and the syntrophy hypothesis are based on similar metabolic considerations such as the symbiosis being mediated by hydrogen transfer. However, in the latter theory the organisms involved were a facultative fermentative-sulphate reducing δ-proteobacterium and a methanogenic archaea.(18) The symbiont ,that eventually became a mitochondrion ,was initially an anaerobic methanotroph that depended on the methane produced by the host while the methanogen consumed the hydrogen and carbon dioxide created by the sulphate reducer.(18) This theory can be supported by a number of molecular features that not only link gram-negative bacteria or archaea to eukaryotes but also connect myxobacteria and specific methanogens with eukaryotes.(18) In fact myxobacteria exhibit complex developmental cycles and have many genes which have homologues in eukaryotic signalling pathways.(19)Additionally some methanogens share some homologous lipid-synthesis pathways with eukaryotes and contain enzymes, which interact with DNA, similar to that of eukaryotes. Nevertheless, the most important feature is the presence of histones and nucleosomes within archaea. These structures appear to be homologous in sequence and three dimensional structure to eukaryotic tetramers and additionally exhibit similar dynamics.(12)The main outcome of this model suggests that information-processing systems are originated from archaea while metabolic, social and developmental functions in eukaryotes are inherited from bacteria .This theory tries to explain the highest amount of information with the minimum number of assumptions but it is very difficult to prove whether it is accurate due to the fact that an extended research on the molecular level and ecological context within the fossil fuel record would have to be carried out.(18)
According to this theory the acquisition of mitochondria was based on the symbiosis created by an α-proteobacterial symbiont, such as Paracoccus, in two steps. The first step consisted in detoxifying the host cytoplasm by consuming oxygen while in the second phase the acquisition of proteins encoded in the host genome allowed the transport of ATP from the mitochondrion to the host cell.(13)This model was recently challenged due to the fact that a free-living bacterium such as Paracoccus would most likely not be able to carry out an active transport of ATP to a host because of a typical lack of ATP exporters within bacteria.(20) In fact only two endocellular parasitic bacteria are known to have ATP transport proteins which are related to plastid homologues but unrelated to the ATP exporters located within mitochondria.(12) This small aspect consequently destroys the credibility of this model as issues with a greater impact are raised. For example, a lack of ATP exporters causes the symbiotic relationship not to depend on the transport of ATP and consequently the reason for the formation of a symbiotic relationship is nullified.(13)
In this model eukaryotes descended from a single prokaryote ancestor following the compartmentalisation of functions created by invaginations of its plasma membrane.(19)
According to this hypothesis mitochondria and chloroplasts were formed through a compartmentalisation of plasmids within a pinched off infolding of the cell membrane. The invagination in the plasma membrane caused the genetic material to be trapped within the double membrane.(19)
CHIMERIC NATURE OF THE EUKARYOTIC CELL
All the hypothesis mentioned in the previous lines have an underlying similarity: they all describe the eukaryotic cell as an organism made up of features from bacteria and archaea.
Therefore, the eukaryotic cell can be defined as a genetical chimera as it is made by genes from both Bacteria and Archaea. (3)There is a high amount of evidence which suggests that mitochondria and chloroplasts originated from bacteria and additionally eukaryotic cells share features with Bacteria ,such as their ester-linked membrane lipids ,and other with Archaea ,such as molecular features of transcription and translation.(6) Fig.3 highlights what the three domains have in common and what are the main differences.
According to studies the gene transfer with consequent replacement from bacteria to archaea was essential in order to create a genomic combination of the two organisms. Consequently, the bacterial genome condensed and was destroyed while the cell was evolving. However, the eukaryotic cell was able to inherit DNA processing systems from the archaea domain and cellular metabolism systems from the prokaryote domain. The features of bacteria and archaea suggest that gene transfer might have possibly played an important role in the formation of eukaryotic organisms.
Over the years many theories have been formulated regarding the origins of the eukaryotic cell but only one was capable of withstanding the critiques: the endosymbiotic hypothesis. The evidence which make this model stronger are the great similarities between organelles such as mitochondria and chloroplast with prokaryotes and the fact that even though they contain their own DNA they’re incapable of living independently due to the presence of essential genes within the nuclear DNA of the host. The latter evidence goes against the theory itself since the ancestors of mitochondria and chloroplast were originally capable of living independently. Hence something must have occurred in order to stop the symbionts from being able to survive without the host and scientists believe that endosymbiotic gene transfer was responsible. (1)In addition, phylogenetic reconstructions with homologous genes from mitochondria and bacteria were able to demonstrate a strong affinity.(13) Through the combination of the capabilities of two distinct organisms endosymbiosis was capable of allowing evolution to occur at a faster rate. (9)In conclusion, until more evidences can be formulated against this hypothesis ,the endosymbiotic model is the one that most likely happened 1.5 billion years ago.
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