Microbial warfare and the evolution of symbiosis

Cooperative symbionts enable their hosts to exploit a diversity of environments. A low genetic diversity (high relatedness) between the symbionts within a host is thought to favour cooperation by reducing conflict within the host. However, hosts will not be favoured to transmit their symbionts in costly ways that increase relatedness, unless this also provides an immediate fitness benefit to the host. We suggest that costly antimicrobial warfare, with compounds such as bacteriocins, could provide a relatively universal reason for why hosts would gain a benefit from increasing the relatedness between bacterial symbionts. We theoretically test this hypothesis with a simple illustrative model that examines whether hosts should manipulate relatedness, and an individual-based simulation, where host control evolves in a structured population. We find that hosts can be favoured to manipulate relatedness, to reduce conflict between symbionts via this immediate reduction in symbiont warfare.