endosymbiosis
(noun)
 A condition of living within the body or cells of another organism.
Examples of endosymbiosis in the following topics:
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Phylogeny of the Eukarya
- Later endosymbiosis led to the spread of plastids in some lineages.
 
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Microbes and the Origin of Life on Earth
- When one organism actually lives inside the other it's called endosymbiosis.
 
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Mutualism vs. Symbiosis
- Endosymbiosis: a relationship in which one of the symbiotic species lives inside the tissue the other.
 
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Cyanobacteria
- According to the endosymbiotic theory, chloroplasts in plants and eukaryotic algae have evolved from cyanobacterial ancestors via endosymbiosis.