ENDOSYMBIOTIC THEORY: HISTORY, WHAT IT PROPOSES, EXAMPLES - BIOLOGY - 2025

The endosymbiotic theory oendosybiosis (endo, inside and symbiosis, living together) raises the origin of the different organelles present in eukaryotes from symbiotic relationships between ancestral prokaryotes.

In this relationship, a prokaryote was swallowed by a larger one. Eventually, the smallest organism was not digested, but survived and was incorporated into the cytoplasm of its host.

The endosymbiotic theory seeks to explain the origin of eukaryotic organelles, such as chloroplasts. Source: pixabay.com

For evolutionary continuity to occur, both organisms had to have similar replication times. The consequence of a synchronized division is that the offspring of the host contained the symbiotic organism.

Thus, it is suggested that chloroplasts are the result of an endosymbiosis between a heterotrophic organism and a cyanobacterium that, over time, became a plastid. Similarly, it is speculated that mitochondria have their evolutionary origin in the group of alpha-proteobacteria.

Although these ideas began to form in the minds of various 19th century scientists, they were appropriately taken up, modified, and sustained in the mid-1960s by Lynn Margulis.

Some of the evidences of this theory are the similarity between organelles and bacteria in terms of size, organization of the genome, structure of the ribosomes and molecular homology.

History

1900-1920: contributions from Mereschkowsky, Portier, and Wallien

For most biologists, the endosymbiotic theory is instantly associated with Lynn Margulis. However, several scientists prior to Margulis proposed incipient hypotheses to explain the origin of eukaryotic organelles.

The first ideas related to endosymbiotic theories are attributed to the Russian botanist Constantin Mereschkowsky, who made a description of the origin of plastids (organelles of plants that include chloroplasts, chromoplasts, amyloplasts, among others) in 1905.

This author's approach basically consists of a symbiotic event between a "reduced" cyanobacteria and a host. Although the thought of the scientist Mereschkowsky was innovative for the time, it did not provide an explanation for the origin of the other eukaryotic organelles.

In 1918, the French biologist Paul Portier noticed a certain resemblance between bacteria and mitochondria. Although the ideas were on the right track, the author proposed culturing mitochondria outside cells, a methodology that was rejected by his colleagues.

In the mid-1920s, the origin of mitochondria was extended by the native biologist from the United States Ivan Wallin, who was convinced that these organelles were descendants of prokaryotic organisms.

Unfortunately, Wallin did not find a plausible mechanism for the proposed transformation, so the endosymbiotic theory was forgotten for a few years.

1960: contributions from Lynn Margulis

It wasn't until the 1960s that a young Boston University researcher named Lynn Margulis proposed the endosymbiotic theory quite robustly, based on cytological, biochemical, and paleontological evidence.

Today, the endosymbiotic theory is accepted with normality, but in Margulis's time his ideas were treated with marked skepticism - which caused the rejection of his work by more than 15 scientific journals.

What does the endosymbiotic theory propose?

The origin of the first prokaryotic cell dates back to more than 3.5 billion years, and it remained this way for approximately 1.5 billion additional years. After this time period, it is presumed that the first eukaryotic cells appeared, identified by their complexity, presence of nucleus and organelles.

Within biology, one of the most important topics is the origin and evolution of the eukaryotic cell, and one of the theories that seeks to explain it is the endosymbiotic theory.

This proposes the origin of organelles from symbiosis events between ancestral prokaryotic organisms, which with the passage of time were integrated and the engulfed organism was reduced and became a cytoplasmic component of the largest.

One of the consequences of the endosymbiotic theory is the horizontal transfer of genes between the prokaryotic organisms involved in the symbiotic association, from the new "organelle" to the host's nuclear genome.

Evidence

Below we will present a series of general evidences that support the endosymbiotic theory:

Size

The size of eukaryotic organelles (call it chloroplasts or mitochondria) is quite similar to that of modern bacterial organisms.

Ribosomes

One of the most acclaimed differences between eukaryotic and prokaryotic lineages resides in the size of the large and small subunits that make up the ribosomes - structural involved in protein synthesis.

Chloroplasts and mitochondria have ribosomes within them and these exhibit the ribosome traits described in eubacteria.

Genetic material

Both chloroplasts and mitochondria are characterized by having their own circular genome - just like prokaryotic organisms.

Mitochondrial genome

The mitochondrial genome is made up of the genes that code for a small number of ribosomal RNA and transfer RNA that are involved in protein synthesis of the organelle in question.

The vast majority of structural proteins and mitochondrial enzymes are encoded by genes that reside in nuclear genetic material.

The organization of the mitochondrial genome varies widely among eukaryotic lineages. In humans, for example, the mitochondrial genome is a circular molecule that harbors some 16,569 base pairs that code for two ribosomal RNAs, 22 transfer RNAs, and only 13 proteins.

Chloroplast genome

In contrast to the mitochondrial genome, the chloroplast genome is slightly larger and contains the information necessary for the synthesis of approximately 120 proteins.

Consequences of the organelle genome

The peculiar mode of division of chloroplasts and mitochondria shows a non-Mendelian pattern. That is, reproduction occurs through its reproduction (as in bacteria) and not by de novo cellular synthesis.

This phenomenon occurs due to the existence of unique genetic material that we cannot find in the nucleus of the cell. Mitochondria are inherited by the maternal line and in the vast majority of plants with sexual reproduction, chloroplasts are contributed to the zygote in formation by the mother plant.

Molecular homologies

Thanks to gene sequencing, it has become clear that the sequences of ribosomal RNA and other genes in mitochondria and chloroplasts are more closely related to the sequences of bacteria than to the sequences housed in the nucleus of eukaryotes.

The DNA sequences of mitochondria are remarkably similar to the sequences found in a specific group of bacteria called alpha-proteobacteria. This evidence suggests that the possible organism that participated in the endosymbiotic event was an ancestral alpha-proteobacterium.

In contrast, chloroplast sequences appear to be closely related to cyanobacteria, a group of eubacteria with the necessary enzymatic machinery to carry out photosynthetic reactions.

The origin of the mitochondria

Today's mitochondria may have arisen from an event that occurred between 1 and 1.5 billion years ago, where a large anaerobic cell engulfed a smaller aerobic bacterium, with the enzymatic machinery necessary for oxidative phosphorylation.

The aerobic organism provided its host with the ability to generate more ATP for each degraded organic molecule.

As the endosymbiotic theory gained acceptance in the scientific community, the taxonomic identity of the ancestral organisms involved in the symbiosis was hotly debated.

Today, the idea is used that the large host was an archaea and the engulfed organism (as we mentioned previously) was an alpha-proteobacterium - although some variants of the theory propose an anaerobic bacterium, since there are several anaerobic forms of mitochondria such as hydrogensomes.

The origin of plastids

Origin of primary plastids

Although in the late 1960s the symbiotic theory was supported by robust evidence from multiple fields of biology, it was not until the 1990s that rapid advances in bioinformatics processing and sequencing techniques provided evidence at the molecular level.

Comparative studies based on molecular phylogenies were able to trace the origin of the genes encoded by plastids in plants to cyanobacteria. Furthermore, they demonstrated the transfer of genes from the endosymbiont genome to the host nuclear genome.

The establishment of the first plastids is estimated to have occurred 1.5 billion years ago, although the temporal figure remains controversial among scientists.

Origin of secondary plastids

Although the establishment of an ancestral cyanobacteria within a prokaryotic host explains the origin of primary plastids, the evolutionary history becomes complex when one seeks to explain the origin of the secondary plastics that we find in some photosynthetic organisms.

These secondary plastids are characterized by the presence of additional membranes, that is, one or two extra membranes to the two membranes that usually surround the organelle. This number of membranes complicates the interpretation, since if an ancestral prokaryote swallowed a cyanobacterium it would not obtain all three or four membranes.

Thus, an ingenious answer to this evolutionary problem was to propose multiple endosymbiosis events. In this context, a plant cell with an already existing plastid was engulfed by a second host, eventually shrinking to a plastid.

In some cases, a third endosymbiotic event occurs. The number of symbioses and the nature of the host are discussed in the literature, although there is empirical evidence that supports the idea of ​​multiple endosymbiosis events.

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