Interactions between vertically transmitted symbionts: cooperation or conflict?

Multiple infections (i.e. the simultaneous infection of a host by multiple microorganisms) have been shown to be a major evolutionary force of eukaryote–prokaryote interactions but have been almost entirely studied in cases of conflicting interactions taking place between horizontally transmitted parasites. Although multiple infections with vertically transmitted symbionts are widespread in nature and especially in invertebrates, the ecological and evolutionary importance of such coexistences remains underexplored and underestimated. By locking the different partners together, vertical transmission creates privileged situations for symbiont–symbiont interactions, especially cooperation and dependence between symbionts, and for the evolution of social behaviours among bacteria. Recent literature illustrates the potential for cooperation or conflict among vertically transmitted symbionts that share the same host and opens new avenues for studying microbial communities.