Endosymbiont

An endosymbiont or endobiont is an <a href="/facts/Organism/j8JkL09u">organism</a> that lives within the body or cells of another organism. Typically the two organisms are in a <a href="/facts/Mutualism_(biology)/29hCDDS2">mutualistic</a> relationship. Examples are <a href="/facts/Nitrogen-fixing/TeLoBpWS">nitrogen-fixing</a> <a href="/facts/Bacteria/wC8xSCsU">bacteria</a> (called <a href="/facts/Rhizobia/xGRcYg5h">rhizobia</a>), which live in the <a href="/facts/Root_nodule/qUFHMpsq">root nodules</a> of <a href="/facts/Legume/x6s6gVHY">legumes</a>, single-cell <a href="/facts/Algae/U1ftVKU3">algae</a> inside <a href="/facts/Coral_reef/xzeK6yAp">reef-building</a> <a href="/facts/Coral/j7fBLrpw">corals</a>, and bacterial endosymbionts that provide essential nutrients to <a href="/facts/Insect/IgDNvnUL">insects</a>.
Endosymbiosis played key roles in the development of <a href="/facts/Eukaryotes/SkwyPLMA">eukaryotes</a> and plants. Roughly 2.2 billion years ago a <a href="/facts/Promethearchaeota/APBNYtnH">Promethearchaeota</a> absorbed a <a href="/facts/Bacterium/wC8xSCsU">bacterium</a> through <a href="/facts/Phagocytosis/eBrv2XKV">phagocytosis</a>, that eventually became the <a href="/facts/Mitochondria/ErHqrdJ1">mitochondria</a> that provide energy to almost all living <a href="/facts/Eukaryote/SkwyPLMA">eukaryotic</a> cells. Approximately 1 billion years ago, some of those cells absorbed <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacteria</a> that eventually became <a href="/facts/Chloroplasts/fo8U3yTx">chloroplasts</a>, <a href="/facts/Organelles/vhDFnGbs">organelles</a> that produce energy from sunlight. Approximately 100 million years ago, a lineage of amoeba in the genus <a href="/facts/Paulinella/WU3J0qYj">Paulinella</a> independently engulfed a cyanobacterium that evolved to be functionally synonymous with traditional chloroplasts, called chromatophores.
Some 100 million years ago, <a href="/facts/UCYN-A/czCDXolw">UCYN-A</a>, a nitrogen-fixing bacterium, became an endosymbiont of the marine alga <a href="/facts/Braarudosphaera_bigelowii/ypsknc0e">Braarudosphaera bigelowii</a>, eventually evolving into a <a href="/facts/Nitroplast/xnaWC2jh">nitroplast</a>, which fixes nitrogen. Similarly, <a href="/facts/Diatom/x2oEcP1P">diatoms</a> 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.
Symbionts are either obligate (require their host to survive) or facultative (can survive independently). The most common examples of obligate endosymbiosis are <a href="/facts/Mitochondria/ErHqrdJ1">mitochondria</a> and <a href="/facts/Chloroplast/fo8U3yTx">chloroplasts</a>; however, they do not reproduce via <a href="/facts/Mitosis/ExxrSUs8">mitosis</a> in tandem with their host cells. Instead, they replicate via <a href="/facts/Fission_(biology)/9Ed5vUr6">binary fission</a>, a replication process uncoupled from the host cells in which they reside. Some human parasites, e.g. <a href="/facts/Wuchereria_bancrofti/lyjnk9Fr">Wuchereria bancrofti</a> and <a href="/facts/Mansonella_perstans/Q4yqkX4k">Mansonella perstans</a>, thrive in their intermediate insect hosts because of an obligate endosymbiosis with <a href="/facts/Wolbachia/trqJvTHu">Wolbachia</a> spp. They can both be eliminated by treatments that target their bacterial host.

Endosymbiont open-in-new

Endosymbiont