Evolution typically refers to the changes in the proportions of biological types in a population over time. It is an important phenomenon for emerging life on earth and Endosymbiotic Theory plays a vital role in the emergence of eukaryotic cells. The beginning of the cell marked the passage from pre-biotic chemistry to partitioned modern cells and the final transition to living entities that fulfill all the delimitation of modern cells depends on the ability to evolve effectively by natural selection referred to as the Darwinian Transition.
Evolution of The Eukaryotic Cell
Evolution is the inevitable part of the race of life on earth. Scientific evidence depicted that ‘Continuous Change’ is the key to an organism for better adaption and survivability to the environment. From geological evidence, the age of the earth has been estimated at approx 4.5 billion years, and the environment was extremely hostile formerly. At the beginning the earth was in hyperthermic condition, the temperature was approx 1000C with several toxic gases from ammonia to methane to CO2, etc. Oxygen was in water vapor form which is completely ambivalent for the emergence of life.
The scientist from wide discipline introduced that emergence, adaption, and endurability of life on earth are based on predominantly three theories.
- Chemogeny (Chemical evolution)
- Biogeny (Formation of Life)
- Concogeny (Life and its evolution)
According to the modern theory of Origin of Life proposed by Russian biochemist Alexander I Oparin (1923 A.D.) life has originated from nonliving organic molecules by a series of chemical evolution. The first living body was formed approx 3.8 billion years ago. The critical evolution theory of eukaryotic cells put forward that around 4 billion years ago an ancestor of archaebacteria originated. Cells first emerged almost 3.8 billion years ago almost 750 million years after the formation of the earth.
Scientific evidence proposed, earth’s first organisms were highly resistant to extreme environments and engaged in inorganic metabolism in which they convert inorganic substances, such as sulfur and carbon, into energy to live. Around 2.7 billion years ago oxygen-generating cyanobacteria developed which over time caused an increase in the atmospheric oxygen levels and gradually aids the anoxic environment into oxygenic one. Increased oxygen level in the atmosphere drives the machinery to evolve more and more which finally triggers a drastic transformation in the intracellular mechanism and structural compartmentalization and gives rise to a new and modified one called a eukaryotic cell.
Distinguishable features of present-day eukaryotes from prokaryotes
The chief discriminating feature b/w a prokaryote and eukaryote is, a prokaryotic cell consists of a naked circular DNA without histones, whereas a eukaryotic one consists of a membrane-bound chromatin fiber and membrane-bound organelle.
The reproduction procedure is as well highly complex in larger eukaryotes in contrast to smaller prokaryotic ones. Mitosis and meiosis are the principal mechanisms of reproduction in eukaryotes whereas binary fission is in prokaryotes. Membrane-bound organelle and compartmentalizations of a eukaryotic cell are the striking attributes that have made the scientist very much inquisitive over the years, with the most noteworthy being the mitochondria and chloroplasts.
Major organelles involved in Endosymbiotic Theory
The widely accepted ‘Endosymbiotic theory’ proposed by Lynn Margulis in 1967 largely highlighted the evolution of two organelle mitochondria and chloroplast. Though the theory has a skeptical perspective, yet it has been accepted and appreciated by scientists as it shares a number of cellular mechanisms including the presence of genome, protein synthesis, antibiotic sensitivity, cell dividing machinery, and cellular constituents needed for translation with that primitive bacteria and phototrophic cyanobacteria.
Mitochondria, the membrane-bound organelle of eukaryotes are the site of respiration and oxidative phosphorylation (OP). Each cell contains thousands of mitochondria with a variety of morphological diversification including shape (sphere or rod-shaped), the chemical composition of the inner and outer membrane, and intermembrane channel for the diffusion of ions and small organic molecules.
The outer membrane is composed of lipid and protein and more permeable than the invaginated inner membrane formed cristae which are selectively permeable. OP takes place in the inner mitochondrial membrane and generates ATP with the help of enzyme ATP Synthase and serial chemical reactions hence called the powerhouse of the cell. It possesses its own DNA that encodes rRNA, tRNA, and a number of proteins required protein synthesis. Despite containing their own genome they still required a nuclear gene for the encoding of some protein.
Another membrane-bound organelle chloroplast is machines of life, upon which heterotrophs live. They are widely distributed in plants, algae, and some protists. It is an amazing molecular machine that uses carbon dioxide, water, and photons from sunlight to create sugar and oxygen. The oxygen, that is so precious to us, is a mere waste product of the reaction, called photosynthesis. Structurally they are double membrane with freely permeable outer and selectively permeable inner membrane.
Chlorophyll is found within the thylakoid. The stroma of the chloroplast contains a large amount of enzyme ribulose bisphosphate carboxylate (RubisCo), which catalysis the Calvin cycle or the dark reactions of photosynthesis. Chloroplasts also contain their own circular DNA, which is also independent of the cell’s chromosomal DNA. Genes contained within the chloroplast encode for proteins required for photosynthesis and autotrophy to occur and also rRNA and tRNA used for the processes of transcription and translation. Like mitochondria, chloroplasts have some proteins, which are encoded by nuclear genes and not just by the chloroplast genome.
Introduction to the Endosymbiotic Theory
The primary endosymbiotic theory proposed by Lynn Murgulis demonstrated that the mitochondrial ancestor was a free-living facultatively aerobic α-proteobacterium, engulfed by another cell and therefore giving rise to a eukaryotic cell. A species of cyanobacterium is thought to be the ancestor of chloroplasts, obtained by a heterotrophic eukaryote after eukaryotic cells had appeared around 1.5 billion years ago and were obtained as an internal symbiont.
Following primary endosymbiosis, non-phototrophic organisms attained to become chloroplasts and this event is referred to as secondary endosymbiosis. Both primary and secondary endosymbiosis is vital in the evolution of eukaryotes, genetic diversity, and in modern eukaryotic population.
Molecular Evidence Supports Endosymbiotic Theory
The focal points that depict the theory’s credibility are discussed below:
- Mitochondria and chloroplasts both contain DNA, with rRNA, tRNA and proteins involved and needed for the respiratory chain in mitochondria and proteins needed for photosynthesis in chloroplasts, being encoded by these small genomes within mitochondria and chloroplasts.
- Non-phototrophic eukaryotic cells are genetic chimeras containing DNA from two different sources, the endosymbiont, which is the mitochondria, and the host cell nucleus. Phototrophic eukaryotes for example algae and plants, have DNA from two endosymbionts, the mitochondria, and the chloroplasts as well as the nuclear DNA. A greater part of mitochondrial DNA and chloroplast DNA is alike to bacterial DNA in respect to its shape, which is circular and size.
- Genes originating from bacteria are found in the nucleus of the eukaryotic cell that has been proven by the sequencing of genomes. It has shown that nuclear genes encode properties are unique to mitochondria and chloroplasts, (and also which closely resemble genes of bacteria), showing that during the evolution of the eukaryotic cell these genes were transferred to the nucleus of the eukaryotic cell, from the bacterial endosymbionts, during the development of the organelle from the engulfed cell.
- Mitochondria and chloroplasts contain their own ribosomes. Ribosomes are present in eukaryotic cells and prokaryotic cells, with eukaryotic cells possessing the larger form, the 80S, and prokaryotic cells containing the smaller, 70S, ribosomes. The mitochondria and chloroplast contain these 70S ribosomes.
- Antibiotic specificity is another important evidence of this theory. Both mitochondria and chloroplast render sensitivity to antibiotics that are bactericidal or bacteriostatic. Some for example streptomycin does this by specifically interfering with the functions of the 70S ribosomes, which occurs in the same way in mitochondria and chloroplasts. Rifampicin is an antibiotic which in bacteria affects the RNA polymerase. It does not have this effect on eukaryotic RNA polymerase however does inhibit mitochondrial RNA polymerase.
- Molecular phylogenetic studies comparing organelles and rRNA support the theory of mitochondria and chloroplasts deriving from bacteria.
- In addition to the above evidence, both mitochondria and chloroplast are surrounded by two membranes. It is thought that the inner membrane is the original membrane derived from the prokaryotic cell and the outer membrane is the outcome from the process of endocytosis when the bacteria were taken into the eukaryotic cell. Protein synthesis in the endosymbionts begins with N- formyl methionine, which is a similar amino acid that initiates protein synthesis in bacteria while in eukaryotic cells protein synthesis is initiated by methionine. Also, the thylakoid membrane and the protein complexes which it contains are like those that can be found in cyanobacteria, and chloroplasts can divide in a way that is similar to the process of binary fission which is carried out by bacteria.
- This evidence all suggests that the host cell which obtained the mitochondrion by phagocytosis was an anaerobic eukaryotic cell, (which already contained a nucleus) and that the mitochondrial endosymbiont was an obligate anaerobe which is followed by the endosymbiosis of a cyanobacterium, allowing the eukaryotic cell to become photosynthetic. In this way, the host eukaryotic cell attained permanent organelles that are suited to energy production. It was also mutually favorable for the symbionts which get a stable accommodating environment for their growth.
Abundant genetic diversity in the modern population is another outcome of a specific type of endosymbiosis called secondary endosymbiosis in which one eukaryotic cell is engulfed by another eukaryote, that has already passé through primary endosymbiosis.
Conversion from a nonphotosynthetic to photosynthetic one with the help of engulfment by photosynthetic eukaryotes is an exclusive example of secondary endosymbiosis. It is thought that secondary and even further endosymbiosis gives rise to a variegated eukaryotic population.
Criticism of Endosymbiotic Theory
The evolution of mitochondria, chloroplast, and several other doubled membraned organelles of critical eukaryotic cells mostly supported by endosymbiotic theory. However, the theory has been criticized many times by different scientists with several theories.
In the late 1980s and early 90s, Tom Cavalier-Smith proposed that certain single-celled eukaryotes found at present, bear a resemblance to earlier eukaryotes, were ‘primitively amitochondriate’ (basic eukaryotes without any mitochondria), and named them ‘archezoa’. These cells are mostly anaerobic and survive in oxygen-deficient conditions.
They are not only devoid of mitochondria including peroxisome, golgidictiosome, and in some cases flagella too. Archezoa contains a nucleus which is a feature of eukaryotes, but the ribosome found in it is 70S type, they also small rRNA contain like prokaryotic cells. Due to carrying these dual features, they are thought to act as a bridge between prokaryotes and eukaryotic cells, that have supported the theory of a bacterial cell being phagocytized, but not digested, leading to the possession and establishment of mitochondria in a eukaryotic cell.
With passing time the atmosphere of the earth was also becoming more oxygenic due to photosynthetic activity of cyanobacteria or blue-green algae. A sharp rise of oxygen level triggers a slow evolution of existing cells and different cell organelle. Scientists believed that with the rising level of oxygen, cells and cell organelle began to evolve more and more and leads to aerobic respiration where oxygen act as a principal component that again supported that the phagocytosis of this bacterial cell was for the purpose of a more efficient way of generating energy.
In March 2000, Jan and Siv Andersson proposed the Ox-Tox hypothesis to clarify that aerobic respiration due to a greater percentage of oxygen in the atmosphere (roughly1.5% to 15% about 2 billion years ago) was the leading cause to the gain of the mitochondria, originally selected for the removal of oxygen by the host cell.
The demands for an oxygen-consuming symbiont to support an essentially anaerobic host would have been, more vital than they are today. The rise and fall of oxygen in the atmosphere have a wide impact on the evolution of the organism.
Oxygen deficient condition is not a very much favorable condition for many organisms, likewise, high concentration of oxygen has also a deleterious effect. Thus, a number of enzymes such as SOD, catalase, peroxidase have been synthesized to give protection against the toxic effect of oxygen respiration. These activities might not have been so widespread two billion years ago.
However, in the late 1990s, these hypotheses were questioned due to the appearance of some contradictive evidence. Genome sequencing of archezoa revealed that the ancestors of archezoa once had mitochondria, even though there was no physical indication of them in the cell. Some ‘archezoa’ such as Giardia was found to have mitochondria but in the form of mitosomes, which still carried out mitochondrial processes again implying that these eukaryotes once contained mitochondria within their cells. From extensive research, a close relationship between the structure of eukaryotic and methanogenic histones and the 3D conformation of the DNA associated with the histones has been found, and again furnish a conclusion methanogen could be the merger of eukaryotic cells.
Other problems related to Endosymbiotic Theory are that, if a bacterium was phagocytized by a eukaryotic cell, it would definitely have been digested and neither mitochondria nor chloroplasts are able to survive independently outside of the eukaryotic cell.
Alternate theory to mainstream endosymbiotic theory
- The Hydrogen Hypothesis: Martin and Muller in 1998, introduced a mechanism of energy metabolism viz associated with hydrogen instead of oxygen. According to theory a strict hydrogen dependent archaebacterium (the host) undergoes a symbiotic relationship with a eubacterium (the symbiont), that was able to respire, and generates molecular hydrogen for anaerobic heterotrophic metabolism. The hypothesis claims a common ancestry of mitochondria and hydrogenosome. It gives a clear validation of genetic chimeras of eukaryotes that possess an archaeal and eubacterial genetic ancestry. This hypothesis predicts a clear conclusion, that no amitochondrate cell has ever existed, rather a different form of anaerobic mitochondria was there as a hydrogenosome that produces ATP by converting pyruvate into hydrogen, CO2, and acetate.
- The Syntrophy Hypothesis: In 1999 another hypothesis was introduced by Purificación López-García and David Moreira on eukaryogenesis. It projects an interspecies hydrogen transfer between an ancestral sulfate-reducing myxobacterium, and a moderately thermophilic or mesophilic methanogenic archaeon endowed with histones and nucleosomes. The hypothesis proposed a symbiogenetic model of the evolution of eukaryotes. It describes wide aspects of eukaryogenesis that include environmental context, metabolic interaction, and endomembrane evolution. The most significant revelation of this theory is a tripartite metabolic symbiosis among methanogenic archaeon (future nucleus), a fermentative myxobacterial-like deltaproteobacterium (future eukaryotic cytoplasm), and a metabolically versatile methanotrophic alphaproteobacterium (future mitochondrion).
During the stages eukaryogenesis, a cellular and genomic combination of the two different organisms (which vary for the hydrogen and syntrophy hypothesis) happened with gene transfer from bacteria to archaea and then subsequent replacement. The bacterial genome is then thought to have condensed down and could have also dematerialized as the cell underwent evolution. The developing eukaryotic cell must have inherited some of the archaeal-DNA processing systems, whereas the cellular metabolism systems are thought to have come from bacterial organography.
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