A strong priority effect in the assembly of a specialized insect-microbe symbiosis

Microbial community assembly is determined in part by interactions between taxa that colonize ecological niches available within habitat patches. The outcomes of these interactions, and by extension the trajectory of community assembly, can display priority effects - dependency on the order in which taxa first occupy these niches. The underlying mechanisms of these phenomena vary from system to system and are often not well resolved. Here, we characterize priority effects in colonization of the squash bug ( Anasa tristis ) by bacterial symbionts from the genus Caballeronia , using pairs of strains that are known to strongly compete during host colonization, as well as strains that are isogenic and thus functionally identical. By introducing symbiont strains into individual bugs in a sequential manner, we show that within-host populations established by the first colonist are extremely resistant to invasion, regardless of strain identity and competitive interactions. By knocking down the population of an initial colonist with antibiotics, we further show that colonization success by the second symbiont is still diminished even when space in the symbiotic organ is available and physically accessible for colonization. We propose a paradigm in which resident symbionts exclude subsequent infections by manipulating the host environment, partially but not exclusively by eliciting tissue remodeling of the symbiont organ. Importance Host-associated microbial communities underpin critical ecosystem processes and human health, and their ability to do so is determined in turn by the various processes that shape their composition. While natural selection acts on competing genotypes and species during community assembly, the manner by which selection determines the trajectory of community assembly can differ depending on the sequence by which taxa establish within that community. We document this phenomenon, known as a priority effect, during experimental colonization of a North American insect pest, the squash bug Anasa tristis , by its betaproteobacterial symbionts in the genus Caballeronia . Our study demonstrates how stark, strain-level variation can emerge in specialized host-microbe symbioses simply through differences in the order by which strains colonize the host. Understanding the mechanistic drivers of community structure in host-associated microbiomes can highlight both pitfalls and opportunities for the engineering of these communities and their constituent taxa for societal benefit.