Symbiont community assembly in an insect-microbial mutualism

Many insects, including serious pests of crops and livestock, rely on mutualistic interactions with specific microbial symbionts for essential physiological functions. Thus, insects have evolved elaborate mechanisms that guarantee the acquisition of beneficial microbes either from the environment or through direct parental transmission. This raises the possibility that targeting symbiont function and transmission may represent a novel biological control strategy for insect pests. However, from the microbial perspective, colonization of a new host is not guaranteed, and which microbes are able to colonize the host shapes the final composition, and possibly function, of symbiont communities.

The squash bug, Anasa tristis De Geer (Hemiptera: Coreidae), acquires bacterial symbionts of the genus Caballeronia from the environment every generation. Previous work has demonstrated that Caballeronia infections associated with squash bugs tend to be dominated by one or a few taxa, but the roles of microbial competition, host choice, and even random chance in shaping symbiont composition in individual insects is unclear. Here, we demonstrate neutral and non-neutral competition among symbionts for host colonization using strains isolated from wild insects. Although some strains appear capable of inhibiting others in laboratory cultures, indicative of environmental or within-host selection, bottlenecks during symbiont acquisition by bugs also generate low diversity in symbiont communities. Our findings illuminate the processes that might affect symbiont transmission in nature, which poses both opportunities and limitations for the efficacy of symbiont-mediated pest control.