Caballeronia strain identity and not host-derived origin determines the fitness outcomes in coreid bugs

Thousand of species of true bugs (Hemiptera) participate in a mutualistic relationship with environmentally acquired bacteria in the diverse genus Caballeronia for their nutrition and development. Are all these bacteria functionally equivalent, or do different insects benefit more from different symbiont strains? Previous research in squash bugs (Anasa spp.) has found no specialization among insect populations and species within Anasa. Here, we expand on this work by testing for specialization at the insect genus level. We performed a reciprocal inoculation experiment, infecting leaf-footed bugs (Leptoglossus phyllopus) and squash bugs (Anasa tristis) with one of 10 different Caballeronia strains isolated from both insect genera. We predicted that bugs inoculated with congeneric strains would perform better than those inoculated with heterogeneric strains, indicating host-symbiont specialization at the insect genus level. However, we found little evidence of specialization across host genera. L. phyllopus and A. tristis showed no significant difference in survival to adulthood or adult mass when inoculated with congeneric or heterogeneric strains. However, one Caballeronia strain consistently conferred lower fitness in both insects. Our results demonstrate variation in the quality of the different Caballeronia strains, but this is not driven by host-symbiont specialization at the insect genus level. This work highlights a limitation of environmental symbiont acquisition: the risk of associating with a sub-optimal symbiont strain.