Endosymbiosis
<p >An <b>endosymbiont</b> is any <a href="http://en.wikipedia.org/wiki/Organism" title="Organism" >organism</a> that lives within the body or cells of another organism, i.e. forming an <b>endosymbiosis</b> (<a href="http://en.wikipedia.org/wiki/Greek_language" title="Greek language" >Greek</a>: ἔνδον <i>endon</i> "within", σύν <i>syn</i> "together" and βίωσις <i>biosis</i> "living"). Examples are <a href="http://en.wikipedia.org/wiki/Nitrogen-fixing" title="Nitrogen-fixing" class="mw-redirect" >nitrogen-fixing</a> <a href="http://en.wikipedia.org/wiki/Bacteria" title="Bacteria" >bacteria</a> (called <a href="http://en.wikipedia.org/wiki/Rhizobia" title="Rhizobia" >rhizobia</a>) which live in <a href="http://en.wikipedia.org/wiki/Root_nodules" title="Root nodules" class="mw-redirect" >root nodules</a> on <a href="http://en.wikipedia.org/wiki/Legume" title="Legume" >legume</a> roots, single-celled <a href="http://en.wikipedia.org/wiki/Algae" title="Algae" >algae</a> inside reef-building <a href="http://en.wikipedia.org/wiki/Corals" title="Corals" class="mw-redirect" >corals</a>, and bacterial endosymbionts that provide essential nutrients to about 10–15% of insects.
Many instances of endosymbiosis are obligate; that is, either the endosymbiont or the host cannot survive without the other, such as the <a href="http://en.wikipedia.org/wiki/Siboglinidae" title="Siboglinidae" >gutless marine worms</a> of the <a href="http://en.wikipedia.org/wiki/Genus" title="Genus" >genus</a><i><a href="http://en.wikipedia.org/wiki/Riftia" title="Riftia" class="mw-redirect" >Riftia</a></i>, which get nutrition from their endosymbiotic bacteria. </p>
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The most common examples of obligate endosymbiosis are <a href="http://en.wikipedia.org/wiki/Mitochondria" title="Mitochondria" class="mw-redirect" >mitochondria</a> and <a href="http://en.wikipedia.org/wiki/Chloroplasts" title="Chloroplasts" class="mw-redirect" >chloroplasts</a>. Some human parasites, e.g. : <i>Wucherichia bancrofti</i> and <i>Mansonella perstans</i> thrive in their hosts because of an obligate endosymbiosis with <i>Wolbachi spp.</i>. They can both be eliminated from their host by treatments that target this bacterium. However, not all endosymbioses are obligate. </p>
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Also, some endosymbioses can be <a href="http://en.wikipedia.org/wiki/Symbiosis" title="Symbiosis" >harmful</a> to either of the organisms involved.
It is generally agreed that certain <a href="http://en.wikipedia.org/wiki/Organelle" title="Organelle" >organelles</a> of the <a href="http://en.wikipedia.org/wiki/Eukaryote" title="Eukaryote" >eukaryotic</a> cell, especially <a href="http://en.wikipedia.org/wiki/Mitochondria" title="Mitochondria" class="mw-redirect" >mitochondria</a> and <a href="http://en.wikipedia.org/wiki/Plastid" title="Plastid" >plastids</a> such as <a href="http://en.wikipedia.org/wiki/Chloroplast" title="Chloroplast" >chloroplasts</a>, originated as bacterial endosymbionts. This theory is called the <a href="http://en.wikipedia.org/wiki/Endosymbiotic_theory" title="Endosymbiotic theory" >endosymbiotic theory</a>, and was first articulated by the <a href="http://en.wikipedia.org/wiki/Russia" title="Russia" >Russian</a> <a href="http://en.wikipedia.org/wiki/Botanist" title="Botanist" class="mw-redirect" >botanist</a> <a href="http://en.wikipedia.org/wiki/Konstantin_Mereschkowski" title="Konstantin Mereschkowski" >Konstantin Mereschkowski</a> in 1905.<sup id="cite_ref-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Endosymbiosis#cite_note-0" >[1]</a></sup></p>