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Encyclopedia > Endosymbiosis

An endosymbiont (also known as intracellular symbiont) is any organism that lives within cells of another organism, i.e. forming an endosymbiosis (Greek: endo = inner and biosis = living). For instance, some nitrogen fixing bacteria (e.g. in Rhizobium, Sinorhizobium and Bradyrhizobium) live in plants, and several insect species contain obligate bacterial endosymbionts. Several other examples of endosymbiosis exist.


Many instances of endosymbiosis are obligate, i.e. neither the endosymbiont nor the host can survive without the other. One example is represented by gutless marine worms which can not survive without their endosymbiotic bacteria. However, the term is also used when the endosymbiosis is not obligate or even is not beneficial to one of the organisms involved. See symbiosis for further discussion of this issue.


It is generally believed that certain organelles of the eukaryotic cell, especially mitochondria and chloroplasts, originated as bacterial endosymbionts. This theory is known as the endosymbiotic theory.


Bacterial obligate endosymbionts in insects

Among bacterial endosymbionts of insects, the best studied are the pea aphid Acyrthosiphon pisum and its endosymbiont Buchnera sp. APS, and the tsetse fly Glossina morsitans morsitans and its endosymbiont Wigglesworthia glossinidia brevipalpis. As with endosymbiosis in other insects, the symbiosis is obligate in that neither the bacteria nor the insect is viable without the other. Scientists have been unable to cultivate the bacteria in lab conditions outside of the insect. With special nutritionally-enhanced diets, the insects can survive, but are unhealthy, and at best survive only a few generations.


The endosymbionts live in specialized insect cells called bacteriocytes (also called mycetocytes), and are maternally-transmitted, i.e. the mother transmits her endosymbionts to her offspring. In some cases, the bacteria are transmitted in the egg, as in Buchnera; in others like Wigglesworthia, they are transmitted via milk to the developing insect embryo.


The bacteria are thought to help the host by either synthesizing nutrients that the host cannot make itself, or by metabolizing insect waste products into safer forms. For example, the primary role of Buchnera is thought to be to synthesize essential amino acids that the aphid cannot acquire from its natural diet of plant sap. The evidence is (1) when aphids' endosymbionts are killed using antibiotics, they appear healthier when their plant sap diet is supplemented with the appropriate amino acids, and (2) after the Buchnera genome was sequenced, analysis uncovered a large number of genes that likely code for amino acid biosynthesis genes; most bacteria that live inside other organisms do not have such genes, so their existence in Buchnera is noteworthy. Similarly, the primary role of Wigglesworthia is probably to synthesize vitamins that the tsetse fly does not get from the blood that it eats.


The benefit for the bacteria is that it is protected from the environment outside the insect cell, and presumably receives nutrients from the insect. Genome sequencing reveals that obligate bacterial endosymbionts of insects have among the smallest of known bacterial genomes and have lost many genes that are commonly found in other bacteria. Presumably these genes are not needed in the environment of the host insect cell. (A complementary theory as to why the bacteria may have lost genes, Muller's ratchet, is that since the endosymbionts are maternally transmitted and have no opportunity to exchange genes with other bacteria, it is more difficult to keep good genes in all individuals in a population of these endosymbionts.) Research in which a parallel phylogeny of bacteria and insects was inferred supports the belief that the obligate endosymbionts are transferred only vertically (i.e. from the mother), and not horizontally (i.e. by escaping the host and entering a new host).


Attacking obligate bacterial endosymbionts may present a way to control their insect hosts, many of which are pests or carriers of human disease. For example aphids are crop pests and the tsetse fly carries the organism (trypanosome protozoa) that causes African sleeping sickness. Other motivations for their study is to understand symbiosis, and to understand how bacteria with severely depleted genomes are able to survive, thus improving our knowledge of genetics and molecular biology.


References

Obligate bacterial endosymbionts in insects:


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The Serial Endosymbiosis Theory of Eukaryotic Evolution (2294 words)
The term "endosymbiosis" specifies the relationship between organisms which live one within another (symbiont within host) in a mutually beneficial relationship.
This discovery served to demonstrate that endosymbiosis could provide a major mechanism for cellular evolution and explain the introduction of new species (Jeon 1991).
ecent research by Martin and Müller (1998) into the origin of the mitochondrion has led to a new theory of endosymbiosis called the "hydrogen hypothesis." In the current picture of the origin of the eukaryotic cell, the mitochondrion was a "lucky accident" (Vogel 1998).
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