Symbiont
Symbiosis is any close and long-term biological interaction between two organisms of different species. In 1879, Heinrich Anton de Bary defined symbiosis as "the living together of unlike organisms". The term is sometimes more exclusively used in a restricted, mutualistic sense, where both symbionts contribute to each other's subsistence. This means that they benefit each other in some way.
Symbiosis is diverse and can be classified in multiple ways. It can be obligate, meaning that one or both of the organisms depend on each other for survival, or facultative, meaning that they can subsist independently. When one organism lives on the surface of another, such as head lice on humans, it is called ectosymbiosis; when one partner lives inside the tissues of another, such as Symbiodinium within coral, it is termed endosymbiosis. Where the interaction reduces both parties' fitness, it is called competition; where just one party's fitness is reduced, it is called amensalism. Where one benefits but the other is largely unaffected, this is termed commensalism. Where one benefits at the other's expense, it is called parasitism. Finally, where both parties benefit, the relationship is described as
mutualistic.
Symbiosis has often driven the evolution of species; mutualism has enabled species for example to colonise new environments. Symbiogenesis is thought to have helped to create the eukaryotes as bacteria were incorporated as mitochondria and chloroplasts within cells. Major co-evolutionary relationships include mycorrhiza, the pollination of flowers by insects, the protection of acacia trees by ants, seed dispersal by animals, nitrogen fixation by bacteria in the root nodules of legumes, and the mutualistic partnership of algae and fungi to form lichens.
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