Endosymbionts are smaller symbiotic partners living inside a host organism, establishing endosymbiosis. The symbionts may live within the host's cells (intracellular) or outside cells (extracellular) in multicellular hosts. A symbiotic relationship may be parasitic (one partner harming the other), commensal (usually not affecting each other) or beneficial (unilaterally or mutually). Examples of intracellular endosymbionts include bacteria and single‐celled algae living inside free‐living cells or cells of various multicellular organisms. It is believed that cellular organelles such as mitochondria and chloroplasts arose within eukaryotic cells by a series of intracellular endosymbioses. Some bacteria live in various marine invertebrates and in insects as extracellular endosymbionts. Many of both intracellular and extracellular endosymbionts form obligate endosymbioses, in which neither the endosymbionts nor their hosts can live without the other. Recent genetic studies have shown that lateral gene transfers occur between symbionts and hosts. In some cases, the transferred symbionts’ genes are expressed in the progeny of multicellular hosts.

Key Concepts:

  • Endosymbiosis is a wide‐spread phenomenon in nature.

  • Endosymbionts may live inside host cells or outside cells in multicellular hosts.

  • Hosts provide suitable habitat to established endosymbionts.

  • Cellular organelles arose within eukaryotic cells by a series of intracellular endosymbioses.

  • Genes are transferred laterally between endosymbionts and hosts.

  • Transferred endosymbionts’ genes may be expressed in the hosts and their progenies.

Keywords: endosymbionts; symbiosis; symbiosomes; symbiont–host relationships; cell components; cell variation; gene transfers

Figure 1.

A schematic diagram to show steps for the ingestion of potential bacterial symbionts and the formation of symbiosomes (Sy). Following phagocytic ingestion into phagosomes (Pg) of would‐be endosymbionts (Bact), most of the bacteria are digested within phagolysosomes (PL) together with any other digestible food organisms (FO). Nondigestible material is excreted as the residual body (RB). Bacteria that survive are sequestered in symbiosomes in which they multiply. Nuc, nucleus; G, Golgi complex; PM, plasma membrane.

Figure 2.

A schematic diagram to show possible interactions among genomes of the host nucleus (Nuc), a cell organelle (Mit, mitochondrion) and a symbiont (S) residing within a symbiosome (Sym).

Figure 3.

A Transmission electron micrograph of an amoeba's symbiosome containing rod‐shaped bacterial endosymbionts (B), some of which are cut in cross‐sections. SM, symbiosome membrane. Bar, 0.5 μm.



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Jeon, Kwang W(Aug 2011) Endosymbionts. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000390.pub3]