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This article comes from the official account of Wechat: back to Park (ID:fanpu2019), author: Nanzhi

With regard to the origin of mitochondria, scientists have put forward two different hypotheses-endosymbiosis and non-symbiosis. As the name suggests, endosymbiosis is the symbiotic evolution of the ancestors of eukaryotic cells and the primitive bacteria they devour. Compared with the origin of non-symbiosis, there are more results to support the origin of symbiosis. However, the origin of endosymbiosis itself is also controversial. The ancestors of eukaryotic cells needed a lot of energy to devour bacteria, and the energy-producing mitochondria had not yet evolved. This becomes a question of whether the chicken comes first or the egg comes first: whether it is swallowed first, or the mitochondria first?

We will learn about mitochondrion in high school biology courses, which is an organelle ubiquitous in eukaryotic cells (there are also a few eukaryotic cells without mitochondria, such as Giardia, Trichomonas and other parasites). Because it is the main place where cells carry out aerobic respiration and is the energy factory in cells, we also call it the "power factory" of cells.

Electron microscopic mitochondria (matrix stained)? photo Source: where does the mitochondria in news.mit.edu eukaryotic cells come from? it is related to the origin of eukaryotes and is an important topic in the study of biological evolution.

There are two hypotheses about the origin of mitochondria, one is the origin of endosymbiosis (endosymbiont hypothesis), the other is the origin of non-symbiosis. These two hypotheses can respectively explain the situation of some mitochondria, so they have always had their own supporters.

Discovery of mitochondria in 1850, Swiss-German biologist and anatomist Rudolph Kolliker observed and separated mitochondria in experiments, describing the shape and size of mitochondria, but he did not know its function and internal structure at that time, so he did not name it.

Swiss-German biologist and anatomist Crick (Rudolph Kolliker,1817-1905) is the founder of microanatomy and is also considered to be the founder of modern embryology. ? photo Source: wiki to the 1880s, with the development of microscopic technology, the magnification of the microscope increased greatly. German pathologist and histologist Richard Altman (Richard Altmann) found a large number of particles in energy-demanding cells (such as muscle cells) when using high-power microscopes to study the submicrostructure of cells. He invented a staining method to identify these particles in 1886 and clearly saw their distribution in the cells under a microscope. He speculated that the particles were not part of the cells themselves, but bacteria that symbiosis with the cells, so they were named "Bioblast".

In 1897, German biologist Carle Benda found that there were a large number of protoplasts, sometimes linear in shape and sometimes granules, so they were named mitochondria (Mitochondrion). Since then, the scientific community has been using mitochondria as the official name for such particles.

Later, in the study of the function of mitochondria, scientists found that mitochondria is the site of tricarboxylic acid cycle, electron transfer, oxidative phosphorylation in cells, thus determining that mitochondria is the site of energy conversion in eukaryotic cells.

Where do mitochondria come from? When observing mitochondria, Richard Altman proposed that this structure in cells is similar to bacteria, which are symbiotic organisms that can live independently, but there is no substantial evidence to prove this at that time. In the 1920s, American biologist Ivan E. Wallin put forward the hypothesis that mitochondria originated from endosymbiosis, that is, mitochondria evolved from bacteria swallowed by cells, but his hypothesis was not accepted by the scientific community at that time.

Until the 1970s, American biologist Lynn Margulis put forward a relatively perfect theory of endosymbiosis-primitive eukaryotes swallowed gram-negative aerobic bacteria in some cases. These aerobic bacteria gradually evolved and adapted to each other under the condition of coexistence with primitive eukaryotes, reached a mutually beneficial symbiotic relationship, and gradually formed mitochondria. In this symbiotic system, the host (aerobic bacteria) gets nutrients from the host (primitive eukaryotic cells), while the host can make use of the energy produced by the host, thus increasing the competitiveness of the symbiote. This hypothesis has been strongly supported by the scientific community, and after that there is a lot of evidence to prove the scientific nature of the hypothesis.

First, mitochondria have independent genetic material, mitochondrial DNA and RNA, which are different from those of eukaryotes and more similar to those of bacteria.

Second, when the cells reproduce themselves, the mitochondria also proliferate and distribute at the same time, which is independent and continuous, and its division and proliferation is completed through constriction, which is similar to bacteria.

Third, the mitochondria itself has an independent and complete protein synthesis system, and most of the characteristics of this synthesis system are similar to the bacterial protein synthesis system, but different from the eukaryotic protein synthesis system.

Fourth, mitochondria have intima and outer membrane, the inner membrane is similar to bacterial plasma membrane, and the outer membrane is similar to eukaryotic intima. Biologists speculate that during the formation of a symbiotic system, when the host engulfs parasitic aerobic bacteria, the intima of the host wraps the host and forms the outer membrane of the mitochondria.

Fifth, the hypothesis points out that in the process of evolution, most of the original genetic information of aerobic bacteria has been transferred and merged into the host cells. Recent studies have found that there is genetic information of respiratory bacteria or cyanobacteria in the nucleus of eukaryotic cells, which confirms the hypothesis.

Sixth, the genetic code of mitochondria is more similar to that of Proteus bacteria, and mitochondria are thought to come from α-amoeba (α-Protobacteria).

Seventh, there is still a similar symbiosis in existing living organisms, such as paramecium engulfing cyanobacteria to form symbionts.

There are also some unexplained problems about the origin of endosymbiosis. The ingested aerobic bacteria have an oxidative metabolic pathway, which clearly gives them an advantage over their host in the competition for survival. Then why the aerobic bacteria are at the bottom, not only being swallowed as the host, but also transferring their genetic material to the host cells, which is not in line with the law of evolution. In addition, the hypothesis of the origin of endosymbiosis does not explain how the nucleus, the control center of the cell, originated.

After the emergence of the hypothesis of symbiotic origin in the non-symbiotic origin hypothesis, the opposition supporting the non-symbiotic origin hypothesis also appeared. The non-symbiosis hypothesis speculates that eukaryotic cells come from an aerobic bacterium. In the process of evolution, some respiratory cell membranes gradually invaginate, enveloping part of the genetic material, forming mitochondria with both independent genetic material and respiratory function and membrane structure.

There is also some evidence for the hypothesis of non-symbiotic origin. For example, some primitive aerobic bacteria now have a quasi-mitochondrial structure, which is formed by the invagination of the plasma membrane and has respiratory function; the respiratory structure in prokaryotic cells can be regarded as the embryonic form of today's mitochondria, so it is speculated that mitochondria are evolved rather than phagocytic symbiosis. The nuclear membrane and mitochondrial membrane of eukaryotic cells are continuous, indicating that mitochondria may originate from the invagination of the intimal system of the cell itself, rather than from symbiotic bacteria.

Although there are some unexplained problems in the endosymbiosis origin hypothesis, this hypothesis provides more evidence than the non-symbiosis origin, so it has become the most mainstream theory of the origin of mitochondria.

In the hypothesis of the origin of endosymbiosis, there are two schools according to the time at which symbiosis occurs in mitochondria.

The hypothesis put forward by one school is called "Mito-late" models. They believe that before the host engulfs aerobic α-amoeba (from which there is a lot of evidence that mitochondria evolved from it), mitochondria have formed nuclei through different pathways and have the characteristics of eukaryotic cells (existing nuclei, dynamic cytoskeletons, intimal systems) and primitive phagocytosis. In other words, the mitochondrial ancestor (α-amoeba) entered the host (primitive eukaryotic cells) relatively late.

The hypothesis proposed by another school is called the early Mito-early models, which holds that the host (prokaryotic cells) first symbiosis with aerobic α-Proteus to form prokaryotic cells with mitochondria, and then evolve eukaryotic characteristics such as nucleus and intimal system after owning this power plant.

Wolbachia in insect cells under transmission electron microscope belongs to the class of α-metamorphic bacteria (Alphaproteobacteria). Photo Source: the dispute between the two factions of wiki focuses on the time when the mitochondrial ancestor (α-amoeba) entered the host. Why is this time so important?

Because exactly how the mitochondrial ancestor who entered the eukaryotic ancestor entered is a very important discussion point. Some scientists believe that it is through phagocytosis, while others believe that it is only after the cells have mitochondria that they have phagocytosis. So the problem turns to phagocytosis.

Phagocytosis is the action of some cells to devour microorganisms or small objects in the form of deformed movement. This seemingly simple process actually requires a lot of energy, as well as dynamic cytoskeleton and membrane transport capacity.

If a cell needs a lot of energy to engulf bacteria, it would not be able to devour the ancestors of mitochondria without the energy factory of mitochondria. However, at this time, the mitochondrial ancestors have not been swallowed, how can the mitochondria become the energy factory of the cell? In this way, it is like entering a time whirlpool, whether there is a chicken or an egg first? That is to say, is there mitochondria first or phagocytosis first?

If there is phagocytosis first, then the ancestors of eukaryotic cells have evolved to a certain extent when they engulfed their ancestors of mitochondria, and the arrival of mitochondria is just the icing on the cake of the evolution of eukaryotic cells, which is the late mitochondrial hypothesis. However, if there were mitochondria first, which provided a lot of energy for the eukaryotic ancestors, because of this energy factory, the eukaryotic ancestors were driven to evolve and formed the later eukaryotic cells. then the evolution of mitochondria to eukaryotic cells is a timely contribution, which is the early mitochondrial hypothesis.

Up to now, there is no final conclusion on the dispute between the early and late models of mitochondrial symbiosis, and the two schools have their own evidence and put forward many hypotheses.

Late model (A) and early model (B) of mitochondrial symbiosis? image source: Martin, William F., et al. The physiology of phagocytosis in the context of mitochondrial origin. Microbiology and Molecular Biology Reviews 81.3 (2017): e00008-17. For example, in 1998, Bill Martin and Miklos Miklos Muller [1] proposed the "hydrogen hypothesis (Hydrogen Hypothesis)"-a hydrogen-producing archaea as a host fuses with a hydrogen-producing α-amoeba, and the two depend on each other to form a stable symbiotic relationship. This model shows that after the symbiosis in the ancestral mitochondria, the occurrence of eukaryotes is triggered by the presence of large amounts of energy-producing mitochondria, indicating that eukaryotes and phagocytosis occur only after the presence of mitochondria. This hypothesis is the most famous early mitochondrial model. However, scientists who question the hypothesis believe that the process of methane production in the model is very complex and requires a large number of coenzymes, which are not found in today's eukaryotes.

The late mitochondrial hypothesis includes the phagocytic archaea model (phagocytosing archaeon model,PhAT) proposed by Anthony M. Poole and Nadja Neumann in 2011, Joran Martijn and Thijs J. G. Ettema in 2013. The symbiosis hypothesis proposed by Lopez Garcia et al. [4] in 2006, and the endosymbiosis model proposed by Petis Alexandros et al. [5] in 2016. In these models, they all believe that the formation of mitochondria is relatively late, especially in the model of phagocytosis of archaea, and that the phagocytosis mechanism is a prerequisite for the fusion of mitochondrial ancestors and eukaryotic ancestors.

Generally speaking, the early mitochondrial hypothesis is more popular today.

Looking for new evidence in February this year, a paper published in the Molecular Biology and Evolution [6] added another evidence to the late mitochondrial hypothesis through experiments.

The paper, published by team Lionel Guy, an evolutionary microbiologist at Uppsala University in Sweden, sequenced bacteria in the order Legionellales, which are intracellular parasites that can grow in eukaryotic cells. Based on the genomic analysis of 35 Legionella, they constructed the evolutionary history of Legionella and its relationship with its early host. Guy's team traced back to the biomarker Okenone and concluded that the first Legionella ancestor to adapt to the host existed 1.89 billion years ago. In other words, at the time point of 1.89 billion years, the ancestors of Legionella had already infected the ancestors of eukaryotes, and this infection was carried out through phagocytosis, which can prove that there is a phagocytosis mechanism at this time. At present, many studies believe that cells containing mitochondria first appeared nearly 1.5 billion years ago, which was later than the phagocytosis mechanism. In this way, the emergence of mitochondria is after the phagocytosis of eukaryotic ancestors-first phagocytosis, then mitochondria, providing evidence for the late mitochondrial hypothesis.

Legionella (green) enters eukaryotic host cells through phagocytosis. Photo Source: Science Source, however, some scientists have proved that mitochondrial symbiosis is very early, thought to be between 1.21 billion and 2.053 billion years, or even between 2 billion and 2.4 billion years. If the mitochondrial symbiosis occurs so early, using Professor Guy's evidence cannot prove that phagocytosis predates the formation of mitochondria.

The early and late debate on mitochondrial symbiosis is still inconclusive. There are still many complex details about the origin of mitochondria that need to be studied and explained, but no matter what the conclusion, the facts have proved the great competitive advantage of the combination of eukaryotic and mitochondrial ancestors. This strong competitiveness and continuous evolution have formed our colorful world today, including us in this world.

Main references

[1] Martin W, M ü ller M. 1998. The hydrogen hypothesis for the first eukaryote. Nature 392 (6671): 37-41.

[2] Poole AM, Neumann N. 2011. Reconciling an archaeal origin of eukaryotes with engulfment: a biologically plausible update of the Eocyte hypothesis. Res Microbiol. 162 (1): 71-76.

[3] Martijn J, Ettema TJ. 2013. From archaeon to eukaryote: the evolutionary dark ages of the eukaryotic cell. Biochem Soc Trans. 41 (1): 451-457.

[4] Lopez-Garcia P, Moreira D. 2006. Selective forces for the origin of the eukaryotic nucleus. Bioessays 28 (5): 525-533.

[5] Pittis AA, Gabald ó n T. 2016. Late acquisition of mitochondria by a host with chimaeric prokaryotic ancestry. Nature 531 (7592): 101104.

[6] Hugoson E, Guliaev A, Ammon é t T, Guy L. Host Adaptation in Legionellales Is 1.9 Ga, Coincident with Eukaryogenesis [J]. Mol Biol Evol. 2022, 39 (3): msac037.

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