Strict gut symbiont specificity in Coreoidea insects governed by interspecies competition within Caballeronia strains.

Host-bacteria symbioses are specific and transgenerationaly stable. In hosts that acquire their symbionts from the environment, selective mechanisms are required to identify beneficial partners among environmental microorganisms. In Coreoidea stinkbugs, which house environmentally acquired symbionts in the midgut, bacterial competition shapes symbiont specificity whereby Caballeronia strains consistently outcompete other bacteria. Here, we show that competition within the gut also occurs among Caballeronia strains themselves, driving specificity at a finer taxonomic scale. The stinkbugs Riptortus pedestris and Coreus marginatus, when reared on the same soil sample, preferentially selected for α- and β-subclade Caballeronia, respectively. In a gnotobiotic infection system, representative strains from the α-, β-, and γ-subclades can independently colonize the midgut of both insect species in monoculture. However, in pairwise co-culture infections, each host exhibits selectivity for either α- or β-subclade strains, consistent with patterns observed in the soil inoculation experiment. In R. pedestris, we further find that both priority effects and displacement mechanisms shape interspecies competition outcomes. At the molecular level, metabolic capabilities, resistance to antimicrobial peptides, and chemotactic behavior determine symbiont competitive success. In R. pedestris, the reproductive fitness benefits conferred by the symbiosis align with the observed strain specificity in the tested strain panel, suggesting a functional link between symbiont selection and host fitness, despite these processes occurring at distinct stages of the symbiotic relationship. Our findings highlight that the gut in Coreoidea species constitutes a multifactorial, species-specific selective environment that contributes to the colonization of the symbiotic midgut region by the best-adapted Caballeronia strain.