The
endosymbiotic theory concerns the origins of
mitochondriaIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5–10 micrometers in diameter...
and plastids (e.g.
chloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.The word chloroplast is...
s), which are
organelleIn cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane....
s of
eukaryoticA eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried...
cells. According to this theory, these organelles originated as separate
prokaryoticThe prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles...
organisms that were taken inside the cell as
endosymbiontAn endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
s. Mitochondria developed from
proteobacteriaThe Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....
(in particular,
RickettsialesThe Rickettsiales, also called rickettsias, are an order of small proteobacteria. Most of those described survive only as endosymbionts of other cells. Some are notable pathogens, including Rickettsia, which causes a variety of diseases in humans...
or close relatives) and chloroplasts from
cyanobacteriaCyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria = blue)...
.
History
The endosymbiotic theory was first articulated by the Russian botanist
Konstantin MereschkowskiKonstantin Sergejewicz Mereschkowsky was a prominent Russian biologist and botanist active mainly around Kazan, whose research on lichens...
in 1905. Mereschkowsky was familiar with work by botanist
Andreas SchimperAndreas Franz Wilhelm Schimper was a botanist and phytogeographer who made major contributions in the fields of histology, ecology and plant geography.-Biography:...
, who had observed in 1883 that the division of
chloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.The word chloroplast is...
s in green plants closely resembled that of free-living
cyanobacteriaCyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria = blue)...
, and who had himself tentatively proposed (in a footnote) that green plants had arisen from a symbiotic union of two organisms.
Ivan Wallin extended the idea of an endosymbiotic origin to
mitochondriaIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5–10 micrometers in diameter...
in the 1920s. These theories were initially dismissed or ignored. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts (for example studies by Hans Ris), combined with the discovery that plastids and mitochondria contain their own DNA (which by that stage was recognized to be the hereditary material of organisms) led to a resurrection of the idea in the 1960s.
The endosymbiotic hypothesis was popularized by
Lynn MargulisLynn Margulis is an American biologist and University Professor in the Department of Geosciences at the University of Massachusetts Amherst...
. In her 1981 work
Symbiosis in Cell Evolution she argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic
spirochaeteSpirochaetes belong to a phylum of distinctive Gram-negative bacteria, which have long, helically coiled cells...
s that developed into eukaryotic
flagellaA flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion...
and
ciliaA cilium is an organelle found in eukaryotic cells. Cilia are tail-like projections.There are two types of cilia: motile cilia and non-motile, or primary, cilia, which typically serve as sensory organelles...
. This last idea has not received much acceptance, because flagella lack DNA and do not show ultrastructural similarities to prokaryotes. See also
Evolution of flagellaThe evolution of flagella is of great interest to biologists because the three known varieties of flagella each represent an extremely sophisticated cellular structure that requires the interaction of many different finely-tuned systems.-The eukaryotic flagellum:There are two competing groups of...
.
According to Margulis and
SaganDorion Sagan is an American science writer, essayist, and theorist. He has written and co-authored many books on evolution, most recently Notes from the Holocene and Into the Cool, co-authored with Eric D.Schneider, on the subject of non-equilibrium thermodynamics.Sagan is the son of astronomer...
, "Life did not take over the globe by combat, but by networking" (i.e., by cooperation).
The possibility that
peroxisomePeroxisomes are organelles from the microbody family and are present in almost all eukaryotic cells. They participate in the metabolism of fatty acids and many other metabolites. Peroxisomes harbor enzymes that rid the cell of toxic peroxides...
s may have an endosymbiotic origin has also been considered, although they lack DNA.
Christian de DuveChristian René de Duve is an internationally acclaimed cytologist and biochemist. De Duve was born in Thames Ditton, Surrey, Great Britain, as a son of Belgian immigrants. They returned to Belgium in 1920...
proposed that they 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 they may be formed
de novo, contradicting the idea that they have a symbiotic origin (Gabaldón et al. 2006).
It is also believed that these endosymbionts transferred some of their own DNA to the host cell's nucleus during the evolutionary transition from a symbiotic community to an instituted eukaryotic cell. This hypothesis is thought to be possible because it is known today from scientific observation that transfer of DNA occurs between prokaryotic species, even if they are not closely related. Prokaryotes can take up DNA from their surroundings and have a limited ability to incorporate it into their own genome.
Evidence
Evidence that mitochondria and plastids arose from ancient endosymbiosis of bacteria is as follows:
- New mitochondria and plastids are formed only through a process similar to binary fission
Binary fission, or prokaryotic fission, is the form of asexual reproduction and cell division used by all prokaryotic and some eukaryotic organisms...
. In some algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in...
, such as EuglenaEuglena is a genus of unicellular protists, of the class Euglenoidea of the phylum Euglenozoa . They are single-celled organisms. Currently, over 1000 species of Euglena have been described. Marin et al. revised the genus to include several species without chloroplasts, formerly classified as...
, the plastids can be destroyed by certain chemicals or prolonged absence of light without otherwise affecting the cell. In such a case, the plastids will not regenerate.
- They are surrounded by two or more membranes
A biological membrane or biomembrane is an enclosing or separating amphipathic layer that acts as a barrier within or around a cell. It is almost invariably a lipid bilayer, composed of a double layer of lipid molecules and proteins that may constitute close to 50% of membrane...
, and the innermost of these shows differences in composition from the other membranes of the cell. The composition is like that of a prokaryotic cell membrane.
- Both mitochondria and plastids contain DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
that is different from that of the cell nucleus and that is similar to that of bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
(in being circular in shape and in its size).
- DNA sequence analysis and phylogenetic estimates suggests that nuclear DNA contains genes that probably came from plastids.
- These organelles' ribosome
Ribosomes are complexes of RNA and protein that are found in all cells. The ribosome is part of the mechanism that translates the DNA sequence into the protein sequence. Ribosomes from bacteria, archaea and eukaryotes , have significantly different structure and RNA...
s are like those found in bacteria (70s).
- Proteins of organelle origin, like those of bacteria, use N-formylmethionine as the initiating amino acid.
- Much of the internal structure and biochemistry of plastids, for instance the presence of thylakoid
A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. The word "thylakoid" is derived from the Greek thylakos, meaning "sac". Thylakoids consists of a thylakoid membrane surrounding a thylakoid lumen...
s and particular chlorophyllChlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from the Greek χλωρός and φύλλον...
s, is very similar to that of cyanobacteriaCyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria = blue)...
. Phylogenetic estimates constructed with bacteria, plastids, and eukaryotic genomes also suggest that plastids are most closely related to cyanobacteria.
- Mitochondria have several enzymes and transport systems similar to those of prokaryotes.
- Some proteins encoded in the nucleus are transported to the organelle, and both mitochondria and plastids have small genomes compared to bacteria. This is consistent with an increased dependence on the eukaryotic host after forming an endosymbiosis. Most genes on the organellar genomes have been lost or moved to the nucleus. Most genes needed for mitochondrial and plastid function are located in the nucleus. Many originate from the bacterial endosymbiont.
- Plastids are present in very different groups of protist
Protists , are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy...
s, some of which are closely related to forms lacking plastids. This suggests that if chloroplasts originated de novo, they did so multiple times, in which case their close similarity to each other is difficult to explain.
- Many of these protists contain "secondary" plastids that have been acquired from other plastid-containing eukaryotes, not from cyanobacteria directly.
- Among the eukaryotes that acquired their plastids directly from bacteria (known as Primoplantae), the glaucophyte
The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater microscopic algae. Together with the red algae and Viridiplantae they form the Archaeplastida...
algae have chloroplasts that strongly resemble cyanobacteria. In particular, they have a peptidoglycanPeptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria, forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid residues...
cell wall between their two membranes.
- Mitochondria and plastids are just about the same size as bacteria.
Secondary endosymbiosis
Primary endosymbiosis involves the engulfment of a bacterium 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 (for a review see McFadden 2001).
One possible secondary endosymbiosis in process has been observed by Okamoto & Inouye (2005). The heterotrophic protist
HatenaHatena is a one celled organism described in 2006. The Hatena is a flagellate, and can resemble a plant at one stage of its life, in which it carries a photosynthesizing alga inside itself, or an animal, acting as predator in another stage of its life...
behaves like a predator until it ingests a green alga, which loses its flagella and cytoskeleton, while
Hatena, now a host, switches to photosynthetic nutrition, gains the ability to move towards light and loses its feeding apparatus.
The process of secondary endosymbiosis left its evolutionary signature within the unique topography of plastid membranes. Secondary plastids are surrounded by three (in euglenophytes and some dinoflagellates) or four membranes (in haptophytes, heterokonts, cryptophytes, and chlorarachniophytes). The two additional membranes are thought to correspond to the plasma membrane of the engulfed alga and the phagosomal membrane of the host cell. The endosymbiotic acquisition of a eukaryote cell is represented in the cryptophytes; where the remnant nucleus of the red algal symbiont (the
nucleomorphNucleomorphs are small, reduced eukaryotic nuclei found in certain plastids. So far, only two groups of organisms are known to contain a nucleomorph: the cryptomonads of the supergroup Chromista and the chlorarachniophytes of the supergroup Rhizaria. The nucleomorphs support the endosymbiotic...
) is present between the two inner and two outer plastid membranes.
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 is still debated.
Problems
- Neither mitochondria nor plastids can survive in oxygen or outside the cell, having lost many essential genes required for survival. The standard counterargument points to the large timespan that the mitochondria/plastids have co-existed with their hosts. In this view, genes and systems that were no longer necessary were simply deleted, or in many cases, transferred into the host genome instead. (In fact these transfers constitute an important way for the host cell to regulate plastid or mitochondrial activity.)
- The transfer of genes from mitochondria and plastids to the “host genome” or cell nucleus raises a further problem: why were all genes not transferred? In other words, why do any genes at all remain in mitochondria and plastids? This problem is addressed by the CoRR Hypothesis
The CoRR hypothesis states that the location of genetic information in cytoplasmic organelles permits regulation of its expression by the reduction-oxidation state of its gene products....
, which proposes that genes and respiratory chain proteins are Co-located for Redox Regulation.
- A large cell, especially one equipped for phagocytosis, has vast energetic requirements, which cannot be achieved without the internalisation of energy production (due to the decrease in the surface area to volume ratio as size increases). This implies that, for the cell to gain mitochondria, it could not have been a primitive eukaryote, but instead a prokaryotic cell. This in turn implies that the emergence of the eukaryotes and the formation of mitochondria were achieved simultaneously.
- Genetic analysis of small eukaryotes that lack mitochondria shows that they all still retain genes for mitochondrial proteins. This implies that all these eukaryotes once had mitochondria. This objection can be answered if, as suggested above, the origin of the eukaryotes coincided with the formation of mitochondria.
These last two problems are accounted for in the
Hydrogen hypothesisThe hydrogen hypothesis is a model proposed by William F. Martin and Miklós Müller in 1998 that describes a possible way in which the mitochondrion arose as an endosymbiont within a prokaryote , giving rise to a symbiotic association of two cells from which the first eukaryotic cell could have...
.
See also
- Hatena
Hatena is a one celled organism described in 2006. The Hatena is a flagellate, and can resemble a plant at one stage of its life, in which it carries a photosynthesizing alga inside itself, or an animal, acting as predator in another stage of its life...
- Lichen
Lichens are composite organisms consisting of a symbiotic association of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
- Symbiogenesis
Symbiogenesis is the merging of two separate organisms to form a single new organism. The idea originated with Konstantin Mereschkowsky in his 1926 book Symbiogenesis and the Origin of Species, which proposed that chloroplasts originate from cyanobacteria captured by a protozoan...
- Transfer of mitochondrial and chloroplast DNA to the nucleus
As a logical conclusion of the endosymbiotic theory, since modern-day mitochondrial and chloroplast genomes do not contain a full set of housekeeping genes, and lack many that other descendants of their speculative ancestors share, there must have been a loss of genes...
- Viral Eukaryogenesis
Viral eukaryogenesis is the hypothesis that the cell nucleus of eukaryotic life forms evolved from a large DNA virus in a form of endosymbiosis within an archaea cell, being a form of symbiogenesis...
(hypothesis that the cell nucleus originated from endosymbiosis).
- Protobiont
Protobionts are systems that are considered to have possibly been the precursors to prokaryotic cells. If RNA is trapped inside, the system can use the RNA or select for it....
- Numt
Numt is an abbreviated term for “nuclear mitochondrial DNA”, which describes any transfer or “transposition” of cytoplasmic mitochondrial DNA sequences into the separate nuclear genome of a eukaryotic organism...
- Hydrogen hypothesis
The hydrogen hypothesis is a model proposed by William F. Martin and Miklós Müller in 1998 that describes a possible way in which the mitochondrion arose as an endosymbiont within a prokaryote , giving rise to a symbiotic association of two cells from which the first eukaryotic cell could have...
External links