Intracellular bacterial symbiosis in the genus Sitophilus: the 'biological individual' concept revisited.

Eukaryotic cells, as genetic entities, most often involve several physically associated genomes that direct the metabolic cell equilibrium. In the coleopteran insects of the genus Sitophilus, in addition to the nucleus and the mitochondrial genomes, two other intracellular bacterial genomes belonging to the alpha and the gamma groups of Proteobacteria are also present. Coexisting with the eukaryotic host cell genomes, they intervene in the physiology and reproduction of the host. They are both transmitted vertically to the progeny and exhibit different levels of symbiont integration in insects. Their coexistence within a eukaryotic cell system illustrates the genetic complexity of animal tissue and questions the concept of the 'biological individual'.