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; however, they do not reproduce via mitosis in tandem with their host cells. Instead, they replicate via binary fission, a replication process uncoupled from the host cells in which they reside.^[10][11] Some human parasites, e.g. Wuchereria bancrofti and Mansonella perstans, thrive in their intermediate insect hosts because of an obligate endosymbiosis with Wolbachia spp.^[12] They can both be eliminated by treatments that target their bacterial host.^[13]