Endosymbiont

An endosymbiont or endobiont^[1] is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualistic relationship. 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.^[2][3]

Endosymbiosis played key roles in the development of eukaryotes and plants. Roughly 2.2 billion years ago an archaeon absorbed a bacterium through phagocytosis, that eventually became the mitochondria that provide energy to almost all living eukaryotic cells. Approximately 1 billion years ago, some of those cells absorbed cyanobacteria that eventually became chloroplasts, organelles that produce energy from sunlight.^[4] Approximately 100 million years ago, a lineage of amoeba in the genus Paulinella independently engulfed a cyanobacteria that evolved to be functionally synonymous with traditional chloroplasts, called chromatophores.^[5]

Some 100 million years ago, UCYN-A, a nitrogen-fixing bacterium, became an endosymbiont of the marine alga Braarudosphaera bigelowii, eventually evolving into a nitroplast, which fixes nitrogen.^[6] Similarly, diatoms in the family Rhopalodiaceae have cyanobacterial endosymbionts, called spheroid bodies or diazoplasts, which have been proposed to be in the early stages of organelle evolution.^[7][8]

Symbionts are either obligate (require their host to survive) or facultative (can survive independently).^[9] The most common examples of obligate endosymbiosis are mitochondria and chloroplasts, which reproduce via mitosis in tandem with their host cells. Some human parasites, e.g. Wuchereria bancrofti and Mansonella perstans, thrive in their intermediate insect hosts because of an obligate endosymbiosis with Wolbachia spp.^[10] They can both be eliminated by treatments that target their bacterial host.^[11]