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Concept
Endosymbiont
Summary
An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον endon "within", σύν syn "together" and βίωσις biosis "living".) Examples are nitrogen-fixing bacteria (called rhizobia), which live in the root nodules of legumes, single-cell algae inside reef-building corals and bacterial endosymbionts that provide essential nutrients to insects.
The history behind the concept of endosymbiosis stems from the postulates of the endosymbiotic theory. The endosymbiotic theory (symbiogenesis) pushes the notion of bacteria exclusively living in eukaryotic organisms after being engulfed by them. This is popular with the concept of organelle development observed with eukaryotes. Two major types of organelle in eukaryotic cells, mitochondria and plastids such as chloroplasts, are considered to be obtained from bacterial endosymbionts.
There are two main types of symbiont transmissions. In horizontal transmission, each new generation acquires free living symbionts from the environment. An example is the nitrogen-fixing bacteria in certain plant roots. Vertical transmission takes place when the symbiont is transferred directly from parent to offspring. An example is pea aphid symbionts. Also, it is possible for both to be involved in a mixed-mode transmission, where symbionts are transferred vertically for some generation before a switch of host occurs and new symbionts are horizontally acquired from the environment. Other examples include Wigglesworthia nutritional symbionts of tse-tse flies, or in sponges. When a symbiont reaches this stage, it begins to resemble a cellular organelle, similar to mitochondria or chloroplasts.
Many instances of endosymbiosis are obligate; that is, either the endosymbiont or the host cannot survive without the other, such as the gutless marine worms of the genus Riftia, which obtain nutrition from their endosymbiotic bacteria.
Official source
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