Secondary Endosymbiosis

∞ generated and posted on 2016.12.16 ∞

Possession or acquisition as well as relatively long-term intracellular retention by a cell of an additional, especially eukaryotic cell.

Endosymbiosis is the long-term existence of one cell living inside of another cell while with Secondary Endosymbiosis there is a second cell involved; specifically, Secondary Endosymbiosis is the long-term existence of one cell living inside of another cell which in turn has a third cell living inside of it, where these two cells together are living inside of the third, larger cell, which in turn were acquired simultaneously as a pair (and typically consist of a cyanobacterium that is living inside of a eukaryotic algae which in turn is acquired, as a pair, by an unrelated eukaryotic cell). Not to be confused with serial endosymbiosis!!!

This occurs particularly in terms of acquisition by a eukaryotic cell of a second, photosynthetic cell which then serves as a chloroplast equivalent for the larger, host cell.

Though secondary endosymbiosis is not strictly serial endosymbiosis, in fact both secondary and serial endosymbiosis typically if not necessarily always occur in tandem. That is, a eukaryotic cell that acquires a second eukaryotic cell as an endosymbiont almost certainly already possesses an endosymbiont, particularly a mitochondrion.

Figure legend: Secondary endosymbiosis in eukaryotic algae. The original plastid was a free-living cyanobacterium. Secondary endosymbiosis is the acquisition of a descendant of the original cyanobacterium-acquiring alga. Not shown are mitochondria and other cell details other than nuclei- and plasma membrane-equivalents as found in association with plastids. Note that endosymbiont nuclear genomes are not as prominent as suggested in the figure (where the point is more that these organisms, at a minimum, once had these various nuclear genomes rather than that they remain a prominent aspect of such plastids.

Note that while secondary endosymbiosis tends to imply serial endosymbiosis, serial endosymbiosis does not necessarily imply secondary endosymbiosis.

See also tertiary endosymbiosis.