The endosymbiotic theory ( ancient Greek ἔνδον éndon 'inside' and συμβίωσις symbíōsis ' living together') states that eukaryotes emerged from an endosymbiosis of prokaryotic precursor organisms . According to this, chemotrophic and phototrophic bacteria were ingested by archaea , in which they developed into cell organelles of their host cells, including mitochondria and plastids . However, there are also eukaryotes that neither do cell respiration nor photosynthesis and have no such organelles, whereby it is assumed that these cell components have subsequently been lost.
The idea of the endosymbiotic theory was first published by the botanist Andreas Franz Wilhelm Schimper in 1883, who tried to explain the origin of chloroplasts . The hypothesis was taken up again around 1905 by the Russian evolutionary biologist Konstantin Mereschkowski , in 1922 by Ivan Wallin and in 1924 by Boris Koso-Poljanski . But it wasn't until 1967 with the publication of Lynn Margulis that she became better known.
The endosymbiotic theory assumes that mitochondria and plastids have developed from independent prokaryotic organisms. In the course of the evolutionary process , these unicellular organisms have entered into an endosymbiosis with another cell, which means that they live in their host cell for mutual benefit. Even today one can observe that amoeboid protozoa (ie those with a “soft” membrane ) ingest cyanobacteria without digesting them.
The interaction of the two cellular organisms then developed into a mutual dependency in the course of evolution , in which neither partner could survive without the other, that is, a symbiosis developed . This is called endosymbiosis . The dependency goes so far that the organelles have lost parts of their (no longer required) genetic material or the corresponding genes have been partially integrated into the core genome. Individual protein complexes in the organelles, such as B. ATP synthase , are composed partly of nuclear-coded, partly of mitochondrially coded subunits.
Analyzes of the genomes indicate that plastids are derived from cyanobacteria , while mitochondria are derived from aerobic α-proteobacteria ( Rickettsiales ). This form of endosymbiosis between a eukaryote and a bacterium is called primary endosymbiosis. If the cell organelle was created through the ingestion of a eukaryote who has already experienced a primary endosymbiosis event, this is referred to as secondary endosymbiosis.
Primary plastids are surrounded by two enveloping membranes, which correspond to the two membranes of the ingested cyanobacterium, while the original third membrane formed around it during phagocytosis is no longer present. There are a total of four lines of primary plastids and therefore of autotrophic primary endosymbionts:
- the glaucophyte line:
the unicellular algae of the Glaucophyta (syn. Glaucocystaceae) have plastids that are very similar to the cyanobacterium in many respects and are therefore often referred to as " cyanelles " or "cyanoplasts", sometimes also as "muroplasts",
- the rhodophyte line:
red algae (wiss. Rhodophyta) have plastids called “ rhodoplasts ”, which still carry the antenna structure ( phycobilisomes ) of the cyanobacteria.
- the chloroplastid line:
The plastids of the viridiplantae (syn. chloroplastida, green algae and higher plants ) represent the most developed plastids and carry a large variety of antenna complexes. The green plastids of algae and higher plants are called chloroplasts .
- of the Paulinella line:
The plastids of the amoeboid Paulinella chromatophora ( Euglyphida ) are called chromatophores .
In all of these lines, the cyanobacteria that were once absorbed have been adapted to such an extent that they are no longer viable and have become the organelle, the plastids or chloroplasts. It has long been debated whether the resulting primary chloroplasts originated from a single endosymbiotic event or from several independent events in different eukaryotic lineages. It is now generally accepted that virtually all organisms with primary chloroplasts have a single common ancestor, which arose from a primary endosymbiosis about 600 million to 2 billion years ago. The time taken cyanobacterium was apparently near the present species Gloeomargarita lithophora , this is basal in the family tree of cyanobacteria close to the genus Synechococcus . The alga Cyanophora , a glaucophyte , is one of the most primitive organisms that contain a chloroplast. The exception is Paulinella chromatophora . This apparently comes from an ancestor who, independently and much later - about 90 to 500 million years ago - took in a cyanobacterium of the genus Prochlorococcus (or Synechococcus ).
Secondary plastids have three or even four covering membranes. There is no known case in which ingestion of a glaucophyte resulted in secondary endosymbiosis. In contrast, there is an abundance of groups of organisms that have ingested a red alga and have reduced it to varying degrees. Some authors assume that this event occurred only once in evolution, and so define the monophylum of the chromalveolata . In this group belong brown algae , yellow-green algae , golden algae , Cryptophyceae , haptophyte (calcareous algae), and the Apicomplexa (z. B. Malaria germs can Plasmodium ).
Secondary endosymbioses between eukaryotes and green or red algae (i.e. primary endosymbionts) are also known. It is assumed that the Euglenozoa and the Chlorarachniophyta have absorbed primary endosymbionts independently of each other. Obviously, tertiary endosymbioses also occurred.
Mitochondria and MROs
There are some protozoa ("Archezoa") that have no mitochondria (and certainly no plastids). At first it was assumed that they were primitive and emerged directly from the primitive host cell of the endosymbionts. This is probably wrong. Most of these organisms have organelles with the hydrogenosomes or mitosomes , which apparently either originate from mitochondria or have a common origin with these in the α-proteobacteria. In some cases, their own DNA and ribosomes are still present.
Mitochondria and similar organelles such as hydrogenosomes and mitosomes are therefore classified together as " mitochondria-related organelles" ( English mitochondrion-related organelles , MROs). These also include the anaerobic and DNA- free organelles of Henneguya salminicola (alias H. zschokkei , Myxozoa )
An exception is the genus Monocercomonoides ( Excavata ), which have no organelles from this group. It is believed that these unicellular organisms acquired a cytosolic system through horizontal gene transfer in order to provide the iron-sulfur clusters required for protein synthesis. After that, their mitochondrial organelles were superfluous in all their functions and were lost. In all these cases, the DNA in the cell nucleus contains sequences that are clearly of mitochondrial origin. All amitochondrial eukaryotes have probably lost or transformed their mitochondria secondarily.
- One can observe different stages between symbiosis and endosymbiosis in different living beings today:
- Corals , some mussels , the Convoluta roscoffensis worm, but also aphids , for example, live in symbiosis with algae or bacteria that live inside their hosts' cells. In the case of the endosymbiotic bacteria of the aphids, accelerations of the evolution rates accompanied by gene losses and an increase in the AT content of the DNA are observed, as can also be found in cell organelles.
- The freshwater polyp Hydra viridissima (Green Hydra) can absorb zoochlorella through endocytosis and use them to photosynthesize .
- Some acoelomorpha e.g. B. the genus Waminoa live in symbiosis with zooxanthellae and feed on their photosynthesis products , among other things . Convoluta and Symsagittifera (both Convolutidae ) live in symbiosis with the unicellular green alga Tetraselmis convolutae .
- In general, zooxanthellae are protists that can live as endosymbionts in a number of living things.
- The roots of some plants live in symbiosis with nitrogen-fixing bacteria ( rhizobia ).
- In foraminifera and sponges are red algae present as endosymbionts.
- Noctiluca scintillans takes on green algae of the species Pedinomonas noctilucae ( Pedinophyceae ), which live on inside as endosymbionts.
- In dinoflagellates different stages can be found: Kleptoplastiden , complex rhodoplasts and tertiary endosymbiosis that the absorption of cryptophyceae or calcareous algae ( Haptophyta are due), a group of marine algae. The tertiary endosymbiosis between calcareous algae and dinoflagellates has been proven in the species Gymnodinium breve , Gymnodinium galatheanum and Gyrodinium aureolum .
- The fungus Geosiphon pyriforme (syn. G. pyriformis ) contains endosymbiotic cyanobacteria of the genus Nostoc .
- Aphids (e.g. the pea louse ) can harbor endosymbiotic bacteria of the Buchnera genus (possibly also Regiella , both Enterobacteriaceae ) in special intestinal cells (so-called 'bacteriocytes') , which are passed on to the next generation via the eggs. Two endosymbiotic bacteria are found nested in the intestinal cells of moth scale insects ( Aleyrodoidea ). In lubricating lice of the species Planococcus citri was even a nested 'Endosymbiosis secondary found.
- In terms of their structure, plastids and mitochondria are prokaryotes: no cell nucleus, circular DNA , the DNA is not associated with histones , but condensed by so-called HLPs ( analogy ), size corresponds to small bacteria. They make their own proteins using a prokaryotic protein biosynthesis apparatus. Their ribosomes are similar to those of bacteria, not those of the host cell (≤ 70-S instead of 80-S ribosomes). The mRNA of the two organelles does not have the 5'-cap sequence typical for eukaryotes and the polyadenylation is also missing. The cyanelles of the glaucophyta are even surrounded by a thin bacterial cell wall. Like cyanobacteria, red algae and glaucophyta use phycobilins to capture photons in photosynthesis.
- The DNA sequences of the mitochondria are similar to those of the α-proteobacteria , while plastid DNA sequences are placed in the cyanobacteria family tree. A comparison with the host DNA indicates no origin of the organelles from the host.
- Primary plastids and mitochondria are surrounded by double membranes , whereby, according to the hypothesis, the outer one was added when the bacterium was "swallowed". The inner corresponds to that of bacteria (occurrence of cardiolipin , no cholesterol; also occurrence of transmembrane proteins (β-barrel proteins), which are only found in the membranes of bacteria and cell organelles), the outer to that of eukaryotes.
- The best evidence for secondary endosymbiosis can be found in the Chlorarachniophyceae , amoeba belonging to the Cercozoa , and the Cryptophyceae, an independent class of algae. Both groups of algae contain complex plastids with four covering membranes. Between the two outer and the two inner envelope membranes is the periplastid space with a nucleomorph , a greatly reduced eukaryotic cell nucleus with three linear small chromosomes and eukaryotic 80-S ribosomes . Genome sequencing and phylogenetic analyzes showed that the nucleomorph and plastid of the Chlorarachniophyceae can be traced back to a secondary endosymbiosis with a green alga, whereas the complex plastid of the Cryptophyceae can be traced back to a secondary endosymbiosis with a red alga. The nucleomorph genome of the Chlorarachniophycee Bigelowiella natans and of the Cryptophycee Guillardia theta is / was completely sequenced. Since starch synthesis takes place in the cytoplasm in red algae and not in the plastids as in green algae and land plants , the presence of starch in the periplastid space of the Cryptophycea also suggests a secondary endosymbiosis.
- Mitochondria and plastids multiply through division and are distributed to daughter cells when the host cell divides. They do not arise de novo , i.e. that is, they cannot be regenerated by the cell if they are accidentally lost.
- The membrane-bound ATPases of bacteria and organelles (such as mitochondria) are closely related to one another, as are those of archaea and the eukaryotes themselves. There is only a more distant relationship between these two groups. Horizontal gene transfer is suspected in small groups of bacteria and archaea with the 'wrong' ATPases .
- Chemical evolution
- Evolution theory
- Compartmentalization hypothesis
- Ectosymbiosis , especially microbial E.
- Kappa organisms
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