Dionisio, F., Gama, J.A. & Carvalho, A.F.P. (2015) Symbiosis between non-transferable plasmids and prokaryotic cells. Reticulate Evolution, Interdisciplinary Evolution Research 3 (ed. by R. Gontier), 191-205. Springer International Publishing, Switzerland.
Plasmids are common in the prokaryotic world, both in bacteria and archaea. Most of these extrachromosomal DNA molecules do not code for essential genes. One may expect that the replication of plasmids and the expression of plasmidic genes impose a fitness cost to their host. Given this cost, and given that plasmid-free cells often arise, it is striking that so many non-transferable plasmids are able to maintain themselves inside prokaryotic cells without being counter-selected in favor of plasmid-free cells. A solution to this paradox would be the evolution of controlling mechanisms to regulate rivalry between plasmids for the stability of these symbiotic relationships. In this chapter, we discuss the evolutionary selective conditions for such mechanisms to evolve.