Long-term acquisition of an , microorganism by cells that already possess a long-term intracellular mutualistic microorganism.

Another way of saying this is acquisition by a cell of multiple cells, each serving as with different functions. Presumably these acquisitions occur one after another rather than simultaneously, and thus in rather than in . See also simply endosymbiosis.

Note that this acquisition is other than exploiting the acquired cells directly as but instead, for this process to be considered to be , requires relatively long-term retention of the acquired cells.

For example, serial endosymbiosis was seen with the acquisition of (i.e., chloroplasts) by eukaryotic cells already possessing mitochondria. Contrast, though, with what is known as as well as (though these also can be serial but arguably also "parallel" endosymbiotic events).

Here is step-by-step (re-)explanation of the concept:

(1) Serial means one after another (versus parallel, which means happening at the same time). (2a) Endosymbiosis is one organism living inside of the cell of another organism, particularly (but not always) with a prokaryotic cell living inside of a eukaryotic cell. (2b) Symbiosis, I should add, means that these physically intimate relationships between two different species last for a relatively long time, in this case for many generations. (3) Mitochondria are endosymbionts, descendants of bacteria that took up residence inside of eukaryotic cells (or, instead, what would become eukaryotic cells). (4) Chloroplasts are also endosymbionts, representing cyanobacteria that took residence inside of eukaryotic cells. (5) The most prominent example of serial endosymbiosis, though certainly not the only one, is the acquisition by some ancestor to all eukaryotes of a mitochondrion. This was then followed, in some lineages (i.e., what would be algae) by acquisition of a chloroplast. (6) Thus, a cell that had an endosymbiont (a mitochondrion) acquired another endosymbiont (a chloroplast). That acquisition of one endosymbiont and then another, by the same cell or lineage of cells, is called serial endosymbiosis (and which, actually, is a surprisingly common phenomenon).