endosymbiosis and evolution of organelles


 Endosymbiosis is a discovery of great importance as it enables us to understand the evolution of plants and animals cell from the common evolutionary ancestor. This essay will focus on the early evolution of our eukaryote ancestor during Precambrian period, including the plastids origin along the algae family due to second endosymbiosis, discuss the evidence that supports the theory and give another example of endosymbiosis.

The theory, as discussed in the paper by Lynn Margulis, states that mitochondria originated from α-proteobacteria bacterium, which was engulfed by the ancestral anaerobic eukaryotic cell, through endocytosis, and retained within the cytoplasm due to increase in atmospheric oxygen. Prokaryote organism produced ATP through oxidative phosphorylation by receiving organic compounds from the eukaryote, causing the eukaryote to become dependent on the prokaryote for ATP production and the prokaryote to become dependent on the eukaryote for other cellular functions, resulting both organisms to evolve in symbiosis with each other and resulting in transfer of most the genes of the unicellular organism to the genome of host, getting enclosed in the nucleus. As a consequence of this, prokaryote organism was reduced into a mitochondrion, as they lost their cell wall and ability to survive independently, and transmitted to the future generation vertically (Debashish). The evolutionary history of plants involves at least two independent endosymbiotic events, because plastids such as chloroplast evolved when a primary endosymbiotic event caused photosynthetic cyanobacteria to be engulfed by some non-photosynthetic host cells as shown by the Arabidopsis thaliana genome analysis where 2%-9% of the nuclear genes have an endosymbiotic origin.

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Chlorophyta, Rhodophyta, and Glaucophyta are three clades, belonging to the group Archaeplastida, that are suggested to have been derived from the double endosymbiotic event according to John Stiller (Brooker). They all have double membrane chloroplast, but the glaucophytes plastid morphology, is most unique because it resembles cyanobacteria to some extent, as they still encompass the outer peptidoglycan layer between the chloroplast envelopes, but also resembles Chlorophyta and Rhodophyta plastid. Both Rhodophyta and Glaucophyta comprise of various features that's derived from the cyanobacteria but is absent from Chlorophyta. These are phycobilins pigments and phycobilisomes on the surface on thylakoid membranes. As a result of sequencing chloroplast genomes of algae, since they are more closely related to the ancestral cyanobacteria, has emphasised that glaucophytes, rhodophytes and chlorophytes have evolved from second endosymbiotic event.

As a result of undergoing secondary endosymbiosis, some algal groups have chloroplast with more than a double membrane such as photosynthetic dinoflagellates and stramenophile. This occurs due to heterotrophic eukaryote engulfing chloroplast containing eukaryote. The secondary endosymbiosis event is suggested as nucleomorphs, traces of primary host's nucleus are present in the periplastid space between the second and third chloroplast membranes of cryptomonads and chlorarachniophytes. In the cryptomonads the nucleomorphs is formed due to the reduction of red algal nucleus and in the chlorarachniophytes due to the reduction of green algal nucleus. Thus, plants had multiple endosymbiotic events and each evolved independently and separated into different lineages. However, in dinoflagellates and various other lineages chloroplast function was lost over time meaning the exact time when the primary endosymbiosis between the cyanobacteria and the heterotrophic eukaryote cannot be determined (current biology).

Genome sequencing of the organelle genes have shown that they encode only a small percent of organelle's proteins and the rest is encoded by nuclear genome suggesting that horizontal gene transfer has occurred in the evolutionary history of both organelles. For chloroplasts the genes were first transferred from organelle to nucleus and in chlorarachniophytes and cryptomonads the genes were transferred again from the endosymbiont to host nucleus. The genome transfer has been shown by inserting marker gene into the chloroplast and mitochondria of transformed yeast and following its progress as the gene transfer's to the nuclear genome (science mag).

There are various morphological, genetic and biochemical evidence that supports phylogenetic data and suggests mitochondria and plastids have evolved from prokaryotic organisms through endosymbiosis and some of them will be discussed in the following paragraphs. Lateral transfer of genes from the ancestor cyanobacteria to non-photosynthetic eukaryote, lead to organisms receiving the gene being able to carry out new function, such as photosynthesis, and thus a chance to occupy a new niche.

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Firstly, bacteria also generate energy through chemiosmosis. Both Bacteria and the eukaryotic organelles contain similar ATP synthase complex in their plasma membrane, which functions by means of electron transport chain and proton gradient.

Additionally horizontal gene transfer, process of genes moving between two species, of genomes encoding for both photosynthetic and non-photosynthetic ability between the aerobic bacteria and the anaerobic eukaryote's nucleus could be why both mitochondria and chloroplast contain their own DNA and ribosomes, which closely resembles bacterial ribosomes compared to its own ribosome , for the synthesis of RNA and proteins and enabling them to transcribe and translate their genes; and carrying out their own metabolic processes, such as chloroplast and mitochondria have genes encoded in the organelle genome that is important for photosynthesis and respiration respectively. Schimper Replication process of mitochondria and organelles resembles prokaryotes as they produce new organelles by independent binary fission (minireview).The process of gene transfer also ensured that the proteins that express the production of plastid is translated in the offspring and passed on throughout the lineage (Debashish).

Mitochondrial matrix and chloroplast stroma contains DNA clusters, attached to the inner membrane, known as nucleoids. These bear closer resemblance to the bacterial DNA than eukaryotes, because organelle's DNA do not contain histones and also they are both circular in shape and resemble bacterial DNA size also. Experiment involving the hybridization of plastid DNA of Euglena and cyanobacteria DNA, carried out by Pigott and Carr, emphasized the relatedness between their nuclear genome (Minireview). Chloroplast ribosomes resemble more closely to bacterial ribosome compared to the mitochondrial ribosome. Genome sequencing has shown that chloroplast nuclear genomes contain many genes that was incorporated and maintained from the photosynthetic cyanobacteria.

Additionally, evolution of RuBisCO oxygenase activity in photorespiration gives credence to the theory. Genomic sequence and structure of RuBisCO is similar in both photosynthetic bacteria and plants, and their ancestor had a protein similar to RuBisCO for CO2 fixation. Ancestor organism inhabited an anaerobic environment, and as these organisms produced oxygen during photosynthesis O2 concentration increased in the atmosphere, thus increasing RuBisCO oxygenase activity. Consequently, C3 plants and to some extent C4 plants still carry out the oxygenase activity of RuBisCO.

Chloroplasts and cyanobacteria have a monophyletic relationship as the presence of photosystems in them indicates that the plastid was derived from an endsymbiotic relationship. Electron and X-ray crystallography comparing the photosystems in both of them illustrates that they are homologous to the "photochemical reaction centre" in cyanobacteria. Also comparing the structure of both organelles show that they are surrounded by double membranes and the inner membrane differ in composition from the other membranes, as its composition is similar to that of a prokaryotic cell membrane.

Besides endosymbiosis between prokaryote and eukaryote there is also evidence that supports the early endosymbiosis between prokaryotes, through conjugation, transformation and transduction, for example evolution of the double-membrane prokaryotes due to endosymbiosis between Clostridia and Actinobacteria. Single membrane and double membrane prokaryotes are distinguished by Gram-staining techniques. Double membrane prokaryotes have a layer of peptidoglycan between the membranes and their photosynthetic machinery is situated in the inner membrane whereas single membrane prokaryotes contain the peptidoglycan layer outside the outer membrane and their photosynthetic machinery is situated in the outer membrane. Function carried out by the inner membrane in the Gram- negative bacteria is similar to the functions carried out by the outer membrane of the Gram- positive bacteria which implies that Gram- negative bacteria has evolved by being engulfed by a Gram-positive bacteria. Analysis of protein families (as shown in figure) suggest that the double-membrane has formed as a result of endosymbiosis because both, Clostridia and Actinobacteria are on each side of the Gram-negative bacteria representing the flow of genetic information from the genome donors and onto the double membrane prokaryote. In addition to this the only two prokaryotes that can carry out photosynthesis are Gram-negative bacteria and Clostridia, further implying that the double membrane prokaryotes has evolved from Clostridia because the photosynthetic machinery is composed of many genes which must have transferred from the ancestor (James A. Lake).

Molecular and biochemical evidence have shown that heterotrophic marine animals such as sea slug have evolved to carry out photosynthesis themselves by ingesting algal and through horizontal gene transfer of essential proteins for photosynthesis.

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Further evidence to show that chloroplast and mitochondrial are related is the presence of homologous genes that has similar roles in functioning as a proton pump, electron donor and acceptor in mitochondria and chloroplast. Cytochrome b6 and cytochrome found in the inner membrane of chloroplast and mitochondria respectively are the homologous proteins involved in electron transport chain.

Some protists contain organelle similar to mitochondria called hydrogenosomes as they both produce ATP but using different methods and findings suggest that they have derived from a common ancestor because same genes have been observed in both organisms even though they are distantly related to each other. As most of the proteins expressed by the mitochondria are not derived from the endosymbiont, it is more likely that hydrogenosomes received genetic material through horizontal gene transfer from mitochondria and then lost its entire ancestral genome over the period. Further experiments needs to be conducted before firmly establishing that hydrogenosomes ancestral origin is the mitochondria

In conclusion evolution of mitochondria and plastids through endosymbiosis is of significance as it greatly influenced the evolutionary emergence of the eukaryotic cell. Although the endosymbionts were reduced to organelles, Horizontal gene transfer and serial endosymbiosis made an enormous contribution to evolution, as new species can be formed by acquiring new genomes from another organism.