The mystery of mitochondrial origin: How is the energy factory of real nuclear cells built?

Regarding the origin of mitochondria, scientists have proposed two different hypotheses -the origin of internal symbiosis and non -symbiosis. As the name suggests, the internal symbiosis is the ancestor of the true nucleus and the primitive bacteria that swallowed it together. Compared with non -symbiotic origin, more results of the sources of symbiotic sources. However, there are also controversy in the origin of the symbiosis itself. The ancestor of the true nucleus requires a lot of energy to devour bacteria, and the mitochondria generated by energy at this time have not evolved. This is a question of a chicken or egg first: Is it devouring or mitochondria first?

Written article | Nanzhi

We will understand mitochondrion in biological courses in middle school. Parasites such as insects). Because it is the main place for cells to perform aerobic breathing, it is an energy manufacturing plant in cells, so we also call it the "power factory" of cells.

Mitochondria (matrix dyed) under the electronic microscope 丨 Image source: news.mit.edu

Where does the mitochondria in the true nucleus come from? The origin of the true nuclear creature is an important issue in the research of biological evolution.

There are two hypotheses about the origin of mitochondria, one is the Endosymbion Hypothesis, and the other is non -symbiotic origin. These two hypotheses can explain the situation of some mitochondria, so they have always supported supporters.

Mitochondria discovery

In 1850, the Swiss-German biologist and anatomist Rudolph Kolliker observed the mitochondria in the experiment and separated the mitochondria to describe the shape and size of the mitochondria, but he did not know it at the time. The functions and internal structures are not named.

Switzerland-German biologist and anatomist Rudolph Kolliker (1817-1905) is the founder of micro anatomy and is also considered the founder of modern embryonics. Picture source: wiki

By the 1880s, with the development of microscopic technology, microscopy was greatly increased, and German pathologists and historians Richard Altmann were using high -power microscope to study the subclaser micro micro -micro micro -micro -microscopy When structured, a large amount of particles were found in cells (such as muscle cells). He invented the dyeing method of identifying these particles in 1886, and clearly saw their distribution in cells under the microscope. He speculated that these particles were not part of the cell itself, but symbiotic bacteria with cells. Therefore "Bioblast".

In 1897, German biologist Carle Benda discovered that there were a large number of native particles, and the shape was sometimes linear and sometimes granules, so the original particle was named Mitochondrion. After that, the scientific community has always used mitochondria as the formal name of this granules.

Later, in the study of mitochondrial function, scientists found that mitochondria were places with trifle cycar circulation, electron transmission, and phosphoric acidization of tither cytars in cells, thereby determining that mitochondrial was a part of energy conversion in energy conversion.

Where does the mitochondria come from?

Internal symbiosis hypothesis

When Richard Altman observed mitochondria, he proposed that this structure in the cells is similar to bacteria. It is an organic body that can be born in cells and can live independently. However, there was no substantial evidence to prove this. In the 1920s, American biologist Ivan E. Wallin proposed that mitochondria originated in hypotheses that originated in the internal symbiosis, that is, mitochondria evolved from bacteria swallowed by cells, but at that time the scientific community did not recognize the recognition of the scientific community. His hypothesis.

Until the 1970s, American biologist Lynn Margulis proposed a relatively complete internal symbiotic doctrine -primitive eukaryotic creatures swallowed the Gram -negative oxygen bacteria in some cases, These oxygen bacteria have gradually evolved with the coexistence of the original eukaryotic organisms, adapt to each other, and achieve a mutually beneficial symbiosis, and gradually form mitochondria. In this symbiotic system, the host (oxygen bacteria) obtain nutrition from the host (primitive eukaryotic cells), while the host can use the energy generated by the host, which increases the competitiveness of this symbiotic body. This hypothesis has received strong support from the scientific community, and has many evidences to prove the scientific nature of the hypothesis.

First, mitochondria have independent genetic materials -mitochondrial DNA and RNA, which are different from the genetic material of the nucleus nucleus and the nucleus nucleus, which are more similar to the bacteria.

Second, when the cells are reproduced by themselves, mitochondria are also proliferated and distributed at the same time, with independence and continuity. Its split and proliferation is completed by cracking, similar to bacteria.

Third, the mitochondrial itself has an independent and complete protein synthesis system, and most of the characteristics of this synthetic system are similar to the bacterial protein synthesis system, which is different from the real nuclear cell protein synthesis system.

Fourth, mitochondria have endometrium and exterior membrane; endometrium is similar to the bacterial membrane, and the outer membrane is similar to the endometrium of the nucleus. Biologists speculated that during the formation of the symbiotic system, when the host swallowed parasitic bacteria, the host's endometrium wrapped the host to form a mitochondrial outer membrane. Fifth, hypothesis states that during the evolution, most of the original genetic information of oxygen bacteria has been transferred and merged into the host cells. Recent studies have found that there is a genetic information of breathing bacteria or blue bacteria in the nucleus of the true nucleus, which confirms the point of view of hypothesis.

Sixth, the genetic passwords of mitochondria are more similar to the genetic passwords of the deformation doors, and are considered to be mitochondria from α-ProToBaCteria.

Seventh, there are still similar symbiosis in the existing life organism, such as grass -mourning insects swallowing cyanobacteria to form symbiotic bodies.

There are also some unpredictable problems in the origin of the internal symbiosis. The degraded oxygen bacteria have oxide metabolism paths. Obviously, in survival competition, this ability has a greater advantage than the host who devour it. Then why this kind of oxygen bacteria is underneath, which is devoured as the host, but also transferred its own genetic material to the host cells, which does not conform to the law of evolution. In addition, the internal symbiotic origin hypothesis cannot explain how the control center of the cell cell cell originates.

Non -symbiosis hypothesis

After the hypothesis of the internal symbiotic origin appears, the opposition that supports non -symbiotic origin hypotheses also appears. Non -symbolic hypothesis guessing that true nucleus is derived from an oxygen bacteria. In the process of evolution, some cell membranes with breathing effects are gradually trapped. Breathing function and mitochondria of membrane structure.

There are also some evidence that the non -symbiosis origin. For example, some primitive oxygen bacteria now have mitochondria structures. This structure is formed by folding in the cells of the cells and has respiratory function. Mitochondrial is evolved instead of devouring symbiosis; the nucleus and mitochondrial membranes are continuous, indicating that the mitochondria may originate from the injecting of the cell itself's endometrial system, rather than the symbiotic bacteria.

Controversy in the symbiotic origin hypothesis

Although there are some problems that cannot be explained in the internal symbiotic origin, compared to non -internal symbiosis, the hypothesis provides more evidence, so it has become the most mainstream theory of mitochondrial origin.

In the hypothesis of the internal symbiotic origin, two genres were generated according to the different time of the symbiosis in the mitochondria.

The hypothesis proposed by a faction is called the advanced mitochondrial model ("MITO-Late" Models, which they believe that the α-deformation bacteria of the host swallowing oxygen (now there are many evidence proved that the mitochondrial is evolved from it), and has passed different channels through different ways The nucleus has been formed, which has the characteristics of the eukaryotic cells (existing nucleus, dynamic cell skeleton, and endometrial system), and has the original phagocytosis function. In other words, the time of mitochondria ancestors (α-deformation bacteria) to enter the host (primitive eukaryotic cells) is late.

The hypothesis proposed by the other faction is called the early mitochondrial model ("Mito-Early" Models. They believe that the host (eukaryotic cells) first forms a mitochondrial cells with mitochondria in the symbiosis of the α-deformation bacteria. Later, the eukaryotic biological characteristics such as nucleus and endometrial systems were evolved.

Wolbachia (Wolbachia) in insect cells under the electron microscope belongs to AlphaProteobacteria. Picture source: wiki

The dispute points of these two factions concentrate on the time point of the liter ancestor (α-deformation bacteria) into the host of the host?

Because the mitochondria ancestor entered the ancestor of the nuclear cells, it was an important discussion point. Some scientists believe that they enter through the phagocytosis, while others believe that after the cells have mitochondria, the cells have swallowed. As a result, the problem turned to the swallowing effect again.

The function of swallowing is the role of some cells swallowing microorganisms or small objects in a deformed movement. This seems simple process, in fact, a large amount of energy is required, but also the cells have dynamic cell skeleton and membrane transportation capacity.

If the cells need to devour a large amount of energy, if there is no mitochondrial energy factory for its energy, it cannot devour the ancestors of mitochondria. However, at this time, the ancestor of the mitochondria has not been swallowed, and how can mitochondria a energy factory of cells? In this way, it is like entering a time vortex. Is there chickens or eggs first? Is there a mitochondria or a devouring effect first?

If there is a swallowing effect, the ancestor of the true nucleus has evolved to a certain degree when the ancestor of the mitochondria is degraded. The arrival of the mitochondria is just the icing on the cake of the true nuclear cells. This is the advanced mitochondrial hypothesis. However, if it is mitochondrial first, mitochondria provides a lot of energy for the ancestor of the real nuclear cells. Because of this energy factory, the ancestor of the true nuclear cells is promoted by it, forming the later nuclear cells, and the evolution of the mitochondria to the authenticity cells is the evolution of the authentic cells. It is a charcoal in the snow, which is the early mitochondrial hypothesis.

The early and late models of symbiosis in mitochondria have no conclusion so far. The two schools have evidence and put forward many hypothesis.

线粒体内共生晚期模型（A）和早期模型（B）丨图片来源：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, 1998 Bill Martin and Miklos Muller [1] proposed "Hydrogen Hypothesis" -n an ancient bacteria that require hydrogen methane as the host as host Bleeding with a hydrogen-produced α-deformation bacteria, the two rely on each other to form a stable symbiotic relationship. This model shows that after the symbiosis of the ancestor of the mitochondria, the mitochondria with a large amount of energy have triggered the occurrence of true nuclear creatures, indicating that mitochondria can have the effect of authentic cells and devouring. This hypothesis is the most famous early mitochondrial model. However, scientists who questioned the hypothesis believed that the process of methane in this model was very complicated and required a large number of coenzymes, and these coenzymes were not found in today's true nuclear creatures.

The advanced mitochondrial hypothesis includes Anthony M. Poole and Nadja Neumann in 2011 [2], Joran Martijn, and Thijs J.G. ETTEMA. ) [3] In 2013, Phagocytosing Archaeon Model (PHAT); Lopez Garcia and others [4] proposed in 2006, as well Pittis Alexandros and others [5] proposed in the internal symbiotic model proposed in 2016. In these models, they all think that mitochondria formation is late, especially the devouring antique model, and believes that the devouring mechanism is a prerequisite for the fusion of mitochondria ancestors and the ancestral ancestors of the nuclear cells.

Overall, the early mitochondrial hypotheses are even more popular.

Find new evidence

In February this year, a paper published on Molecular Biology and Evolution [6] added evidence to the advanced mitochondrial hypotherapy through experiments.

This paper was published by the evolutionary microbiologist Lionel Guy of Upzara University, Sweden. Bacteria can grow in the nucleus. By analyzing the genome of 35 groups of fungi, they built the evolutionary history of the Legion bacteria and their relationship with the early hosts. Gay's team traced back through biomarker Okenone, and inferred that the first army ancestor that was adapted to the host was 1.89 billion years ago. In other words, at this time at this time of 1.89 billion years, the ancestors of the Legion bacteria had already infected the ancestors of the true nuclear creature. This infection was performed through the function of swallowing. At present, many studies believe that the first occurrence of cells containing mitochondria was nearly 1.5 billion years ago, which existed than the devouring mechanism. From this point of view, the appearance of mitochondria is after the ancestor of the eukaryotic cells has the effect of devouring -first swallowing, and then mitochondria, providing evidence for the advanced mitochondrial hypothesia.

Legion bacteria (green) enter the Zhennuyaocol cell through the function of swallowing 丨 Picture Source: Science Source

However, some scientists have proved that the coexistence of the mitochondria is very early, and it is believed that between 1.21 billion and 20.053 billion years, and even more than 2 billion to 2.4 billion years. If the time of symbiosis in the mitochondria is so early, then the evidence of Professor Gay cannot prove that the phagocyne effect is earlier than the formation of mitochondria.

Early and late debates in mitochondria are still unreasonable. There are many complex details in the origin of mitochondria to be studied and explained, but no matter what conclusions, the facts prove the great survival competition advantage produced by the ancestor of the nuclear cells and the ancestor of mitochondria. This powerful competitiveness and continuous evolution have formed our colorful world today, including us in this world.

main reference

[1] Martin W, müller 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.

[4] Lopez-Garcia P, moreira d. 2006. selective forces for the Origin of the Eukaryotic Nucleus. Bioessays 28 (5): 525–533.

[5] PITTIS AA, Gabaldón T. 2016. Late Acquisition of Mitochondria by Host with Chimaeric Prokaryotic Ancestry. Nature 531 (7592): 101–104.

[6] Hugoson E, Guliaev A, Ammunét T, Guy L. Host Adaptation in Leggellales is 1.9 GA, Coincident with Eukaryogenes [J]. Mol Biol Evol. 2022, 39 (3): msac037...

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