Host control of symbiont transmission: The separation of symbionts into germ and soma

Obligate, vertically transmitted symbionts occur in many species. Hosts often have elaborate developmental processes and specialized organs to control the reproduction and transmission of their symbionts. Control mechanisms divide the symbionts into reproductive, germ-line lineages and nonreproductive, somatic lineages. This germ-soma distinction favors reduced competition among symbionts and fewer virulent effects on the host. Observations suggest a repeated evolutionary trend toward host control of symbionts and division of symbionts into germ and soma. Theory predicts that host control evolves only in response to particular mechanisms of symbiont competition, for example, when symbionts disrupt host development.