
Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory
) is the leading
evolution
Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
ary theory of the origin of
eukaryotic
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
cells from
prokaryotic organisms.
The theory holds that
mitochondria
A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
,
plastid
A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria.
Examples of plastids include chloroplasts ...
s such as
chloroplast
A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s, and possibly other organelles of eukaryotic cells are descended from formerly free-living prokaryotes (more closely related to the
Bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
than to the
Archaea
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
) taken one inside the other in
endosymbiosis
An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...
. Mitochondria appear to be
phylogenetically
In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
related to
Rickettsiales bacteria, while chloroplasts are thought to be related to
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
.
The idea that chloroplasts were originally independent organisms that merged into a symbiotic relationship with other one-celled organisms dates back to the 19th century, when it was espoused by researchers such as
Andreas Schimper. The endosymbiotic theory was articulated in 1905 and 1910 by the Russian botanist
Konstantin Mereschkowski, and advanced and substantiated with microbiological evidence by
Lynn Margulis in 1967.
Among the many lines of evidence supporting symbiogenesis are that mitochondria and plastids contain their own
chromosome
A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
s and reproduce by
splitting in two, parallel but separate from the
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
of the rest of the cell; that the chromosomes of some mitochondria and plastids are single circular DNA molecules similar to the
circular chromosomes of bacteria; that the
transport proteins called
porins are found in the outer membranes of mitochondria and chloroplasts, and also bacterial cell membranes; and that
cardiolipin is found only in the inner mitochondrial membrane and bacterial cell membranes.
History

The
Russian botanist
Konstantin Mereschkowski first outlined the theory of symbiogenesis (from
Greek
Greek may refer to:
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group
*Greek language, a branch of the Indo-European language family
**Proto-Greek language, the assumed last common ancestor of all kno ...
: σύν ''syn'' "together", βίος ''bios'' "life", and γένεσις ''genesis'' "origin, birth") in his 1905 work, ''The nature and origins of chromatophores in the plant kingdom'', and then elaborated it in his 1910 ''The Theory of Two Plasms as the Basis of Symbiogenesis, a New Study of the Origins of Organisms''.
[(journal URL]
) Mereschkowski proposed that complex life-forms had originated by two episodes of symbiogenesis, the incorporation of
symbiosis, symbiotic bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
to form successively
nuclei and
chloroplast
A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s.
Mereschkowski knew of the work of botanist
Andreas Schimper. In 1883, Schimper had observed that the division of
chloroplast
A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s in green plants closely resembled that of free-living
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
. Schimper had tentatively proposed (in a footnote) that green plants had arisen from a
symbiotic
Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...
union of two organisms. In 1918 the French scientist
Paul Jules Portier published ''Les Symbiotes'', in which he claimed that the
mitochondria
A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
originated from a symbiosis process.
Ivan Wallin advocated the idea of an endosymbiotic origin of
mitochondria
A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
in the 1920s.
The Russian botanist
Boris Kozo-Polyansky became the first to explain the theory in terms of
Darwinian evolution. In his 1924 book ''A New Principle of Biology. Essay on the Theory of Symbiogenesis'', he wrote, "The theory of symbiogenesis is a theory of selection relying on the phenomenon of symbiosis."
These theories did not gain traction until more detailed electron-microscopic comparisons between cyanobacteria and chloroplasts were made, such as by
Hans Ris in 1961 and 1962. These, combined with the discovery that plastids and mitochondria contain their own DNA, led to a resurrection of the idea of symbiogenesis in the 1960s.
Lynn Margulis advanced and substantiated the theory with microbiological evidence in a 1967 paper, ''On the origin of mitosing cells.'' In her 1981 work ''Symbiosis in Cell Evolution'' she argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic
spirochaetes that developed into eukaryotic
flagella
A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
and
cilia
The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
. This last idea has not received much acceptance, because flagella lack DNA and do not show ultrastructural similarities to
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
or to
archaea
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
(see also:
Evolution of flagella and
Prokaryotic cytoskeleton). According to Margulis and
Dorion Sagan, "Life did not take over the globe by combat, but by networking" (i.e., by cooperation).
Christian de Duve proposed that the
peroxisome
A peroxisome () is a membrane-bound organelle, a type of microbody, found in the cytoplasm of virtually all eukaryotic cells. Peroxisomes are oxidative organelles. Frequently, molecular oxygen serves as a co-substrate, from which hydrogen perox ...
s may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular oxygen in the Earth's atmosphere. However, it now appears that peroxisomes may be formed
''de novo'', contradicting the idea that they have a symbiotic origin.
[
(Provides evidence that contradicts an endosymbiotic origin of peroxisomes, and suggests instead that they originate evolutionarily from the ]endoplasmic reticulum
The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...
) The fundamental theory of symbiogenesis as the origin of mitochondria and chloroplasts is now widely accepted.
Symbiogenesis revolutionized the history of evolution by proposing a mechanism for evolutionary development not encompassed in the original Darwininan vision. Symbiogenesis demonstrated that major evolutionary advancements, particularly the origin of eukaryotic cells, may have resulted from symbiotic mergers rather than from gradual mutations and individual competition, i.e., classical natural selection. Accordingly, symbiogenic theory suggests that endosymbiosis may be a powerful force in generating evolutionary novelty, beyond that which can be explained by natural selection alone.
From endosymbionts to organelles

Biologists usually distinguish
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...
s from
endosymbiont
An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...
s – whole organisms living inside other organisms – by their reduced
genome size
Genome size is the total amount of DNA contained within one copy of a single complete genome. It is typically measured in terms of mass in picograms (trillionths or 10−12 of a gram, abbreviated pg) or less frequently in daltons, or as the tot ...
s.
As an endosymbiont evolves into an organelle, most of its genes are transferred to the host cell
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
. The host cell and organelle therefore need to develop a transport mechanism that enables the return of the
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
products needed by the organelle but now manufactured by the cell.
Free-living ancestors
Alphaproteobacteria were formerly thought to be the free-living organisms most closely related to mitochondria.
Later research indicates that mitochondria are most closely related to
Pelagibacterales bacteria, in particular, those in the SAR11 clade.
Nitrogen-fixing
Nitrogen fixation is a chemical process by which molecular dinitrogen () is converted into ammonia (). It occurs both biologically and abiological nitrogen fixation, abiologically in chemical industry, chemical industries. Biological nitrogen ...
filamentous
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
are the free-living organisms most closely related to plastids.
Both cyanobacteria and alphaproteobacteria maintain a large (>6
Mb) genome encoding thousands of proteins.
Plastid
A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria.
Examples of plastids include chloroplasts ...
s and
mitochondria
A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
exhibit a dramatic reduction in genome size when compared with their bacterial relatives.
Chloroplast genomes in photosynthetic organisms are normally 120–200kb
encoding 20–200 proteins
and mitochondrial genomes in humans are approximately 16kb and encode 37 genes, 13 of which are proteins.
Using the example of the freshwater
amoeboid
An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and r ...
, however, ''
Paulinella chromatophora'', which contains
chromatophore
Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopod
A cephalopod is any member o ...
s found to be evolved from cyanobacteria, Keeling and Archibald argue that this is not the only possible criterion; another is that the host cell has assumed control of the regulation of the former endosymbiont's division, thereby synchronizing it with the cell's
own division.
Nowack and her colleagues gene sequenced the chromatophore (1.02Mb) and found that only 867 proteins were encoded by these photosynthetic cells. Comparisons with their closest free living cyanobacteria of the genus ''
Synechococcus'' (having a genome size 3Mb, with 3300 genes) revealed that chromatophores had undergone a drastic genome shrinkage. Chromatophores contained genes that were accountable for
photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
but were deficient in genes that could carry out other biosynthetic functions; this observation suggests that these endosymbiotic cells are highly dependent on their hosts for their survival and growth mechanisms. Thus, these chromatophores were found to be non-functional for organelle-specific purposes when compared with mitochondria and plastids. This distinction could have promoted the early
evolution
Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
of photosynthetic organelles.
The loss of genetic autonomy, that is, the loss of many genes from endosymbionts, occurred very early in evolutionary time.
Taking into account the entire original endosymbiont genome, there are three main possible fates for genes over evolutionary time. The first is the loss of functionally redundant genes,
in which genes that are already represented in the nucleus are eventually lost. The second is the
transfer of genes to the nucleus, while the third is that genes remain in the organelle that was once an organism.
The loss of autonomy and integration of the endosymbiont with its host can be primarily attributed to nuclear gene transfer.
As organelle genomes have been greatly reduced over evolutionary time,
nuclear genes have expanded and become more complex.
As a result, many plastid and mitochondrial processes are driven by nuclear encoded gene products.
In addition, many nuclear genes originating from endosymbionts have acquired novel functions unrelated to their organelles.
Gene transfer mechanisms
The mechanisms of gene transfer are not fully known; however, multiple hypotheses exist to explain this phenomenon. The possible mechanisms include the
Complementary DNA (cDNA) hypothesis and the bulk flow hypothesis.
The cDNA hypothesis involves the use of
messenger RNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
mRNA is created during the ...
(mRNAs) to transport genes from organelles to the nucleus where they are converted to cDNA and incorporated into the genome.
The cDNA hypothesis is based on studies of the genomes of flowering plants. Protein coding RNAs in mitochondria are spliced and edited using organelle-specific splice and editing sites. Nuclear copies of some mitochondrial genes, however, do not contain organelle-specific splice sites, suggesting a processed mRNA intermediate. The cDNA hypothesis has since been revised as edited mitochondrial cDNAs are unlikely to recombine with the nuclear genome and are more likely to recombine with their native mitochondrial genome. If the edited mitochondrial sequence recombines with the mitochondrial genome, mitochondrial splice sites would no longer exist in the mitochondrial genome. Any subsequent nuclear gene transfer would therefore also lack mitochondrial splice sites.
The bulk flow hypothesis is the alternative to the cDNA hypothesis, stating that escaped DNA, rather than mRNA, is the mechanism of gene transfer.
According to this hypothesis, disturbances to organelles, including
autophagy
Autophagy (or autophagocytosis; from the Greek language, Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-depe ...
(normal cell destruction),
gametogenesis (the formation of gametes), and cell stress release DNA which is imported into the nucleus and incorporated into the nuclear DNA using
non-homologous end joining
Non-homologous end joining (NHEJ) is a pathway that repairs double-strand breaks in DNA. It is called "non-homologous" because the break ends are directly ligated without the need for a homologous template, in contrast to homology directed repair ...
(repair of double stranded breaks).
For example, in the initial stages of endosymbiosis, due to a lack of major gene transfer, the host cell had little to no control over the endosymbiont. The endosymbiont underwent cell division independently of the host cell, resulting in many "copies" of the endosymbiont within the host cell. Some of the endosymbionts
lysed (burst), and high levels of DNA were incorporated into the nucleus. A similar mechanism is thought to occur in tobacco plants, which show a high rate of gene transfer and whose cells contain multiple chloroplasts.
In addition, the bulk flow hypothesis is also supported by the presence of non-random clusters of organelle genes, suggesting the simultaneous movement of multiple genes.
Ford Doolittle proposed that (whatever the mechanism) gene transfer behaves like a ratchet, resulting in unidirectional transfer of genes from the organelle to the nuclear genome.
When genetic material from an organelle is incorporated into the nuclear genome, either the organelle or nuclear copy of the gene may be lost from the population. If the organelle copy is lost and this is fixed, or lost through genetic drift, a gene is successfully transferred to the nucleus. If the nuclear copy is lost, horizontal gene transfer can occur again, and the cell can 'try again' to have successful transfer of genes to the nucleus.
In this ratchet-like way, genes from an organelle would be expected to accumulate in the nuclear genome over evolutionary time.
Endosymbiosis of protomitochondria
Endosymbiotic theory for the origin of mitochondria suggests that the proto-eukaryote engulfed a protomitochondrion, and this endosymbiont became an organelle, a major step in
eukaryogenesis, the creation of the eukaryotes.
Mitochondria
Mitochondria are organelles that synthesize the energy-carrying molecule
ATP for the cell by
metabolizing carbon-based
macromolecule
A macromolecule is a "molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass." Polymers are physi ...
s. The presence of
DNA
Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
in mitochondria and proteins, derived from
mtDNA, suggest that this organelle may have been a
prokaryote
A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
prior to its integration into the proto-
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
.
Mitochondria are regarded as organelles rather than endosymbionts because mitochondria and the host cells share some parts of their
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
, undergo division simultaneously, and provide each other with means to produce energy.
The
endomembrane system and
nuclear membrane
The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer polar membrane, membranes that in eukaryotic cells surround the Cell nucleus, nucleus, which encloses the genome, genetic material.
The nuclear envelope con ...
were hypothesized to have derived from the
protomitochondria.
Nuclear membrane
The presence of a nucleus is one major difference between eukaryotes and
prokaryote
A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
s. Some conserved
nuclear proteins between eukaryotes and prokaryotes suggest that these two types had a common ancestor. Another theory behind nucleation is that early nuclear membrane proteins caused the
cell membrane
The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
to fold and form a sphere with pores like the
nuclear envelope
The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material.
The nuclear envelope consists of two lipid bilayer membran ...
.
As a way of forming a nuclear membrane, endosymbiosis could be expected to use less energy than if the cell was to develop a metabolic process to fold the cell membrane for the purpose.
Digesting engulfed cells without energy-producing mitochondria would have been challenging for the host cell.
On this view, membrane-bound bubbles or
vesicles leaving the protomitochondria may have formed the nuclear envelope.
The process of symbiogenesis by which the early
eukaryotic cell integrated the proto-
mitochondrion
A mitochondrion () is an organelle found in the cell (biology), cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosine tri ...
likely included protection of the
archaea
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
l host
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
from the release of
reactive oxygen species
In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...
. These would have been formed during
oxidative phosphorylation
Oxidative phosphorylation(UK , US : or electron transport-linked phosphorylation or terminal oxidation, is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order ...
and ATP production by the proto-mitochondrion. The
nuclear membrane
The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer polar membrane, membranes that in eukaryotic cells surround the Cell nucleus, nucleus, which encloses the genome, genetic material.
The nuclear envelope con ...
may have evolved as an adaptive innovation for protecting against nuclear genome
DNA damage caused by reactive oxygen species. Substantial transfer of genes from the ancestral proto-mitochondrial genome to the nuclear genome likely occurred during early eukaryotic evolution.
The greater protection of the nuclear genome against reactive oxygen species afforded by the nuclear membrane may explain the adaptive benefit of this gene transfer.
Endomembrane system

Modern eukaryotic cells use the endomembrane system to transport products and wastes in, within, and out of cells. The membrane of nuclear envelope and endomembrane vesicles are composed of similar membrane proteins. These vesicles also share similar membrane proteins with the organelle they originated from or are traveling towards.
This suggests that what formed the nuclear membrane also formed the endomembrane system. Prokaryotes do not have a complex internal membrane network like eukaryotes, but they could produce extracellular vesicles from their outer membrane.
After the early prokaryote was consumed by a proto-eukaryote, the prokaryote would have continued to produce vesicles that accumulated within the cell.
Interaction of internal components of vesicles may have led to the
endoplasmic reticulum
The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...
and the
Golgi apparatus
The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...
, both being parts of the endomembrane system.
Cytoplasm
The syntrophy hypothesis, proposed by López-García and Moreira around the year 2000, suggested that eukaryotes arose by combining the metabolic capabilities of an archaean, a fermenting deltaproteobacterium, and a methanotrophic alphaproteobacterium which became the mitochondrion. In 2020, the same team updated their syntrophy proposal to cover an
promethearchaeon that produced hydrogen with deltaproteobacterium that oxidised sulphur. A third organism, an alphaproteobacterium able to respire both aerobically and anaerobically, and to oxidise sulphur, developed into the mitochondrion; it may possibly also have been able to photosynthesise.
Date
The question of when the transition from prokaryotic to eukaryotic form occurred and when the first
crown group
In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...
eukaryotes appeared on earth is unresolved. The oldest known body fossils that can be positively assigned to the Eukaryota are acanthomorphic
acritarchs from the 1.631
Gya Deonar Formation of India. These fossils can still be identified as derived post-nuclear eukaryotes with a sophisticated, morphology-generating
cytoskeleton
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...
sustained by mitochondria. This fossil evidence indicates that endosymbiotic acquisition of
alphaproteobacteria must have occurred before 1.6 Gya. Molecular clocks have also been used to estimate the last eukaryotic common ancestor, however these methods have large inherent uncertainty and give a wide range of dates. Reasonable results include the estimate of c. 1.8 Gya. A 2.3 Gya estimate also seems reasonable, and has the added attraction of coinciding with one of the most pronounced biogeochemical perturbations in Earth history, the early Palaeoproterozoic
Great Oxygenation Event. The marked increase in atmospheric oxygen concentrations at that time has been suggested as a contributing cause of eukaryogenesis, inducing the evolution of oxygen-detoxifying mitochondria. Alternatively, the Great Oxidation Event might be a consequence of eukaryogenesis, and its impact on the export and burial of organic carbon.
Organellar genomes
Plastomes and mitogenomes

Some endosymbiont genes remain in the organelles. Plastids and mitochondria retain genes encoding rRNAs, tRNAs, proteins involved in redox reactions, and proteins required for transcription, translation, and replication. There are many hypotheses to explain why organelles retain a small portion of their genome; however no one hypothesis will apply to all organisms, and the topic is still quite controversial. The hydrophobicity hypothesis states that highly
hydrophobic
In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, thu ...
(water hating) proteins (such as the membrane bound proteins involved in
redox
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...
reactions) are not easily transported through the cytosol and therefore these proteins must be encoded in their respective organelles. The code disparity hypothesis states that the limit on transfer is due to differing genetic codes and RNA editing between the organelle and the nucleus. The redox control hypothesis states that genes encoding redox reaction proteins are retained in order to effectively couple the need for repair and the synthesis of these proteins. For example, if one of the
photosystems is lost from the plastid, the intermediate electron carriers may lose or gain too many electrons, signalling the need for repair of a photosystem. The time delay involved in signalling the nucleus and transporting a cytosolic protein to the organelle results in the production of damaging
reactive oxygen species
In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...
. The final hypothesis states that the assembly of membrane proteins, particularly those involved in redox reactions, requires coordinated synthesis and assembly of subunits; however, translation and protein transport coordination is more difficult to control in the cytoplasm.
Non-photosynthetic plastid genomes
The majority of the genes in the mitochondria and plastids are related to the expression (transcription, translation and replication) of genes encoding proteins involved in either photosynthesis (in plastids) or cellular respiration (in mitochondria). One might predict that the loss of photosynthesis or cellular respiration would allow for the complete loss of the plastid genome or the mitochondrial genome respectively.
While there are numerous examples of mitochondrial descendants (
mitosomes and
hydrogenosomes) that have lost their entire organellar genome,
non-photosynthetic plastids tend to retain a small genome. There are two main hypotheses to explain this occurrence:
The essential tRNA hypothesis notes that there have been no documented functional plastid-to-nucleus gene transfers of genes encoding RNA products (tRNAs and rRNAs). As a result, plastids must make their own functional RNAs or import nuclear counterparts. The genes encoding tRNA-Glu and tRNA-fmet, however, appear to be indispensable. The plastid is responsible for
haem biosynthesis, which requires plastid encoded tRNA-Glu (from the gene trnE) as a precursor molecule. Like other genes encoding RNAs, trnE cannot be transferred to the nucleus. In addition, it is unlikely trnE could be replaced by a
cytosol
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...
ic tRNA-Glu as trnE is highly conserved; single base changes in trnE have resulted in the loss of haem synthesis. The gene for tRNA-
formylmethionine
''N''-Formylmethionine (fMet, HCO-Met, For-Met) is a derivative of the amino acid methionine in which a formyl group has been added to the amino group. It is specifically used for initiation of protein synthesis from bacterial and organellar g ...
(tRNA-fmet) is also encoded in the plastid genome and is required for translation initiation in both plastids and mitochondria. A plastid is required to continue expressing the gene for tRNA-fmet so long as the mitochondrion is translating proteins.
The limited window hypothesis offers a more general explanation for the retention of genes in non-photosynthetic plastids.
According to this hypothesis, genes are transferred to the nucleus following the disturbance of organelles.
Disturbance was common in the early stages of endosymbiosis, however, once the host cell gained control of organelle division, eukaryotes could evolve to have only one plastid per cell. Having only one plastid severely limits gene transfer
as the lysis of the single plastid would likely result in cell death.
Consistent with this hypothesis, organisms with multiple plastids show an 80-fold increase in plastid-to-nucleus gene transfer compared with organisms with single plastids.
Evidence
There are many lines of evidence that mitochondria and plastids including chloroplasts arose from bacteria.
[Kimball, J. 2010]
''Kimball's Biology Pages''.
Accessed October 13, 2010. An online open source biology text by Harvard professor, and author of a general biology text, John W. Kimball.[Reece, J., Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson, 2010. ''Campbell Biology.'' 9th Edition Benjamin Cummings; 9th Ed. (October 7, 2010)]
* New mitochondria and plastids are formed only through
binary fission
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two values (0 and 1) for each digit
* Binary function, a function that takes two arguments
* Binary operation, a mathematical o ...
, the form of cell division used by bacteria and archaea.
* If a cell's mitochondria or chloroplasts are removed, the cell does not have the means to create new ones. In some
algae
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
, such as ''
Euglena
''Euglena'' is a genus of Unicellular organism, single-celled, flagellate eukaryotes. It is the best-known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 200 species. Species of '' ...
'', the plastids can be destroyed by certain chemicals or prolonged absence of light without otherwise affecting the cell: the plastids do not regenerate.
*
Transport proteins
A transport protein (variously referred to as a transmembrane pump, transporter, escort protein, acid transport protein, cation transport protein, or anion transport protein) is a protein that serves the function of moving other materials within ...
called
porins are found in the outer membranes of mitochondria and chloroplasts and are also found in bacterial cell membranes.
* A
membrane lipid cardiolipin is exclusively found in the inner mitochondrial membrane and bacterial cell membranes.
* Some mitochondria and some plastids contain single circular DNA molecules that are similar to the DNA of
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
both in size and structure.
*
Genome comparisons suggest a close relationship between mitochondria and
Alphaproteobacteria.
* Genome comparisons suggest a close relationship between plastids and
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
.
* Many genes in the genomes of mitochondria and chloroplasts have been lost or transferred to the nucleus of the host cell. Consequently, the chromosomes of many eukaryotes contain genes that originated from the genomes of mitochondria and plastids.
* Mitochondria and plastids contain their own
ribosome
Ribosomes () are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (messenger RNA translation). Ribosomes link amino acids together in the order s ...
s; these are more similar to those of bacteria (70S) than those of eukaryotes.
* Proteins created by mitochondria and chloroplasts use
N-formylmethionine as the initiating amino acid, as do proteins created by bacteria but not proteins created by eukaryotic nuclear genes or archaea.
Secondary endosymbiosis
Primary endosymbiosis involves the engulfment of a cell by another free living organism. Secondary endosymbiosis occurs when the product of primary endosymbiosis is itself engulfed and retained by another free living eukaryote. Secondary endosymbiosis has occurred several times and has given rise to extremely diverse groups of algae and other eukaryotes. Some organisms can take opportunistic advantage of a similar process, where they engulf an alga and use the products of its photosynthesis, but once the prey item dies (or is lost) the host returns to a free living state. Obligate secondary endosymbionts become dependent on their organelles and are unable to survive in their absence. A secondary endosymbiosis event involving an ancestral
red alga
Red algae, or Rhodophyta (, ; ), make up one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest Phylum, phyla of algae, containing over 7,000 recognized species within over 900 Genus, genera amidst ongoing taxon ...
and a
heterotrophic
A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
eukaryote resulted in the evolution and diversification of several other photosynthetic lineages including
Cryptophyta
The cryptomonads (or cryptophytes) are a Class (biology), superclass of algae, most of which have chloroplast, plastids. They are traditionally considered a Division (taxonomy), division of algae among phycologists, under the name of Cryptophyta ...
,
Haptophyta
The haptophytes, classified either as the Haptophyta, Haptophytina or Prymnesiophyta (named for '' Prymnesium''), are a clade of algae.
The names Haptophyceae or Prymnesiophyceae are sometimes used instead. This ending implies classification at ...
,
Stramenopiles (or Heterokontophyta), and
Alveolata.
A possible secondary endosymbiosis has been observed in process in the heterotrophic protist ''
Hatena''. This organism behaves like a predator until it ingests a
green alga
The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ( ...
, which loses its flagella and cytoskeleton but continues to live as a symbiont. ''Hatena'' meanwhile, now a host, switches to photosynthetic nutrition, gains the ability to move towards light, and loses its feeding apparatus.
Despite the diversity of organisms containing plastids, the morphology, biochemistry, genomic organisation, and molecular phylogeny of plastid RNAs and proteins suggest a single origin of all extant plastids – although this theory was still being debated in 2008.
Nitroplasts
A unicellular marine alga, ''
Braarudosphaera bigelowii'' (a
coccolithophore
Coccolithophores, or coccolithophorids, are single-celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdom ...
, which is a eukaryote), has been found with a cyanobacterium as an endosymbiont. The cyanobacterium forms a nitrogen-fixing structure, dubbed the
nitroplast. It divides evenly when the host cell undergoes mitosis, and many of its proteins derive from the host alga, implying that the endosymbiont has proceeded far along the path towards becoming an organelle. The cyanobacterium is named
''Candidatus'' Atelocyanobacterium thalassa, and is abbreviated UCYN-A. The alga is the first eukaryote known to have the ability to fix nitrogen.
See also
* ''
Angomonas deanei'', a protozoan that harbours an obligate bacterial symbiont
* ''
Hatena arenicola'', a species that appears to be in the process of acquiring an endosymbiont
*
Hydrogen hypothesis, the theory that mitochondria were acquired by hydrogen-dependent archaea, their endosymbionts being facultatively anaerobic bacteria
*
Kleptoplasty
Kleptoplasty or kleptoplastidy is a process in symbiosis, symbiotic relationships whereby plastids, notably chloroplasts from algae, are sequestered by the host. The word is derived from ''Kleptes'' (κλέπτης) which is Greek language, Greek ...
, the sequestering of plastids from ingested algae
* ''
Mixotricha paradoxa'', which itself is a symbiont, contains numerous endosymbiotic bacteria
* ''
Parakaryon myojinensis
''Parakaryon myojinensis'', also known as the Myojin parakaryote, is a highly unusual species of single-celled organism known only from a single specimen, described in 2012. It has features of both prokaryotes and eukaryotes but is apparently ...
'', a possible result of endosymbiosis independent of eukaryotes
* ''
Parasite Eve'', fiction about endosymbiosis
* ''
Strigomonas culicis'', another protozoan that harbours an obligate bacterial symbiont
*
Viral eukaryogenesis, hypothesis that the cell nucleus originated from endosymbiosis
References
Further reading
* (General textbook)
*
* (Discusses theory of origin of eukaryotic cells by incorporating mitochondria and chloroplasts into anaerobic cells with emphasis on 'phage bacterial and putative viral mitochondrial/chloroplast interactions.)
* (Recounts evidence that chloroplast-encoded proteins affect transcription of nuclear genes, as opposed to the more well-documented cases of nuclear-encoded proteins that affect mitochondria or chloroplasts.)
* (Discusses theories on how mitochondria and chloroplast genes are transferred into the nucleus, and also what steps a gene needs to go through in order to complete this process.)
*
*
External links
Tree of Life Eukaryotes
{{DEFAULTSORT:Symbiogenesis
Biological hypotheses
Endosymbiotic events
Evolutionary biology
Symbiosis
Microbiology
Eukaryote genetics