Endosymbiotic theory
The endosymbiont theory (Greek ἔνδον endo, inside 'and συμβίωσις Symbiosis, living together ' ) states that eukaryotes arose by the fact that prokaryotic precursor organisms have entered into a symbiosis. Thus ( archaea possibly) are chemotrophe and phototrophic bacteria from other prokaryotic cells have been phagocytosed and thereby become endosymbionts. Later, the endosymbionts to organelles in their host cells have developed. The complexes from the host cells and organelles present therein are eukaryotes. The cell organelles that carry many characteristics of prokaryotes, today, are mitochondria and plastids. Complex plant, animal and human cells thus have thus originated in the merger of prokaryotes ( see Figure ). However, there are also eukaryotes without such organelles is being discussed whether these cell components were phylogenetically secondarily lost. Eukaryotes without such organelles can operate either cellular respiration or photosynthesis.
History
The idea of the endosymbiont theory was first published by botanist Andreas Franz Wilhelm Schimper in 1883 that tried thus to explain the origin of chloroplasts. The hypothesis was taken up again in 1905 by the Russian Konstantin Sergeyevich Merejkowski evolutionary biologists. But it was only in 1967 with the publication of Lynn Margulis was known.
Core statement
In simple terms, the theory that was added in the course of the development of life unicellular organisms by another single-celled organisms and have become part of the cell of a higher organism as incurred. In the course of evolution as always emerged more complex organisms. Also, components of human cells originally go back to unicellular organisms that have become a part of the cells.
Explanation
The endosymbiont theory assumes that mitochondria and plastids have evolved from independent prokaryotic organisms. In the course of the evolutionary process, these single-celled organisms have entered into an endosymbiosis with another cell, that is, they live in their host cell for mutual benefit. Even today, one can observe that amoeboid protozoa ( ie, those "soft" with a membrane ) take cyanobacteria without digesting them.
The interaction of the two cellular organisms has then developed in the course of evolution to a mutual dependence in which neither of the parties could survive longer without the other, that is, there was a symbiosis. This is called endosymbiosis. The dependence goes so far that the organelles have lost parts of their ( no longer needed ) genetic material or the corresponding genes have been partially integrated into the nuclear genome. Single protein complexes in organelles, such as ATP synthase, so composed partly of nuclear encoded, in part encoded in the mitochondrial subunits.
Analyzes of genomes suggest that plastids from cyanobacteria descended, while mitochondria are descended from aerobic proteobacteria. This form of endosymbiosis between a eukaryote and a prokaryote is called primary endosymbiosis. Arose the organelle by the inclusion of a eukaryote, which has already experienced a primary Endosymbioseereignis, this is known as secondary endosymbiosis.
Primary Plastids are surrounded by two envelope membranes that correspond to the two membranes of the recorded cyanobacterium, while resulting in phagocytosis therearound lying original third diaphragm is no longer present. There are three types of primary plastid and thus three lines of autotrophic organisms:
- The unicellular algae of the Glaucocystaceae possess plastids, which are the cyanobacterium in many ways very similar and are often referred for this reason as " cyanelles "
- Red algae possess " rhodoplasts " called plastids, which still bear the antenna structure of the cyanobacteria ( phycobilisomes ).
- The plastids of green algae and higher plants represent the most developed plastids and carry a wide variety of antenna complexes. The green plastids of algae and higher plants are called chloroplasts.
Secondary plastids have three or even four envelope membranes. There is no known case in which a recording of a Glaucophyten would have led to a secondary endosymbiosis. In contrast, an abundance of groups of organisms that have received a red alga, and they have reduced in varying degrees exists. Some authors assume that this event has occurred only once in evolution, and thus define the monophyly of Chromalveolata. This group includes the brown algae, yellow-green algae, golden algae, cryptophytes, haptophyte ( calcareous algae ), and the Apicomplexa (eg malaria parasite Plasmodium ).
The secondary endosymbiosis between green algae and eukaryotes is known. Thus it is assumed that the Euglenozoa and Chlorarachniophyta added independently primary endosymbionts in itself.
There are some protozoa that no mitochondria ( and no plastids ) have ( " Archezoa "). Initially it was assumed that they were primitive and evolved directly from the primitive host cell of the endosymbionts. This is probably wrong, because their DNA contains sequences that are unique mitochondrial origin. Probably all amitochondriaten eukaryotes have secondarily lost their mitochondria.
Evidence
- Corals, some shells, the worm Convoluta roscoffensis but, for example, aphids live in symbiosis with algae or bacteria that live inside the cells of their hosts. The endosymbiotic bacteria of aphids acceleration of evolutionary rates are accompanied with observed gene losses and an increase in the AT content of the DNA, as it is also found in cell organelles.
- The roots of some plants live in symbiosis with nitrogen-fixing bacteria ( rhizobia ).
- The Hydra ( Hydra viridissima ) can take up by endocytosis Zoochlorellen and operate with the help of photosynthesis.
- In dinoflagellates different stages can be found: Kleptoplastiden, complex rhodoplasts and tertiary endosymbiosis, which are due to the ingestion of cryptophytes or haptophytes ( Haptophyta ), a group of marine algae. The tertiary endosymbiosis between calcareous algae and dinoflagellates was detected in the species Gymnodinium breve, Gymnodinium galatheanum and Gyrodininium aureolum.