What is Endosymbiosis?
'''Endosymbiosis''' occurs when a [[Symbiosis|symbiont]] lives inside the body or the [[Cell (biology)|cell]]s of another organism. It is a very widespread phenomenon in living things.<ref name=Buchner/> Examples are: *[[Rhizobia]]: [[nitrogen]]-fixing [[bacteria]] which live in [[root]] nodules on plants of the [[pea]] family. *Singly-celled [[foram]]s which include a single-celled [[alga]] inside the cell. This is 'facultative', which means they may or may not do it. A more far-reaching version is ''idioplastic'' endosymbiosis. Here, the foram consumes the alga, but keeps its [[chloroplast]]s in working order. *[[Green algae]] inside [[Ocean|marine]] [[polychaete]] worms. *Single-celled algae inside [[Coral reef|reef]]-building [[coral]]s. *[[Gut flora]] (often [[flagellate]]s) inside the digestive systems of [[insect]]s and [[vertebrate]]s which digest plant [[cellulose]]. These are 'obligate' symbionts, which means the host must have them.<ref name=Buchner>Buchner P. 1965. ''Endosymbioses of animals with plant microorganisms''. Wiley N.Y.</ref> *[[Lichen]]: [[fungus]]+[[alga]] or [[bacterium]]. *[[Mycorrhiza]]: fungal [[hypha]]e and [[root]]s of [[tree]]s. == Eukaryote theory == '''Endosymbiont theory''' is the idea that [[eukaryote]] [[cells]] arose in [[evolution]] by the fusion of previously free-living [[protist]]s ([[prokaryote]]s).<ref>King R.C. Stansfield W.D. & Mulligan P.K. 2006. ''A dictionary of genetics'', 7th ed. Oxford.</ref><sup>p143</sup> The theory was first suggested by the [[Russia]]n botanist Konstantin Mereshkovsky in 1905.<ref>Mereshkovsky C 1905. Über Natur und Ursprung der Chromatophoren im Pflanzenreiche. ''Biol Centralbl'' '''25''': 593–604.</ref> The nature of [[lichen]]s had been worked out;<ref>de Bary, Antoine 1879. ''Die Erscheinung der Symbiose''. Strassburg.</ref> [[mitochondria]] and [[chloroplast]]s had been seen in cells,<ref>Schimper A.F.W. 1883. Über die Entwicklung der Chlorophyllkörner und Farbkörper. ''Bot. Zeitung'' '''41''': 105–14, 121–31, 137–46, 153–62.</ref> a