Mechanisms and Evolution of Heritable Microbial Density in Insect Hosts

Within-host density is a critically important aspect of vertically transmitted symbioses that influences the fitness of both hosts and microbes. I review recent studies of symbiont density in insects, including my laboratory’s work on pea aphids and maternally transmitted bacteria. These studies used systems approaches to uncover the molecular mechanisms of how both hosts and microbes influence symbiont density, and they shed light on whether optimal density is different from the perspective of host and microbial fitness. Mounting empirical evidence suggests that antagonistic coevolution shapes vertically transmitted symbioses even when microbes provide clear benefits to hosts. This is potentially because of differing selective pressures at the host and within-host levels. Considering these contrasting evolutionary pressures will be critically important in efforts to use vertically transmitted symbionts for biocontrol and as lessons from model systems are applied to the study of more complex microbiomes.