Many interspecific interactions are shaped by coevolution. Transmission mode is thought to influence opportunities for coevolution within symbiotic interactions. Vertical transmission maintains partner fidelity, increasing opportunities for coevolution, but horizontal transmission may disrupt partner fidelity, potentially reducing opportunities for coevolution. Despite these predictions, the role of coevolution in the maintenance of horizontally transmitted symbioses is unclear. Leveraging a tractable insect–bacteria symbiosis, we tested for signatures of pairwise coevolution by assessing patterns of host–symbiont specialization. If pairwise coevolution defines the interaction, we expected to observe evidence of reciprocal specialization between hosts and their local symbionts. We found no evidence for local adaptation between sympatric lineages of Anasa tristis squash bugs and Caballeronia spp. symbionts across their native geographic range. We also found no evidence for specialization between three co-localized Anasa host species and their native Caballeronia symbionts. Our results demonstrate generalist dynamics underlie the interaction between Anasa insect hosts and their Caballeronia symbionts. We predict that selection from multiple host species may favor generalist symbiont traits through diffuse coevolution. Alternatively, selection for generalist traits may be a consequence of selection by hosts for fixed cooperative symbiont traits without coevolution.

Mutualism depends on the alignment of host and symbiont fitness. Horizontal transmission can readily decouple fitness interests, yet horizontally transmitted mutualisms are common in nature. We hypothesized that pairwise coevolution and specialization in host-symbiont interactions underlies the maintenance of cooperation in a horizontally transmitted mutualism. Alternatively, we predicted selection by multiple host species may select for cooperative traits in a generalist symbiont through diffuse coevolution. We tested for signatures of pairwise coevolutionary change between the squash bug Anasa tristis and its horizontally acquired bacterial symbiont Caballeronia spp. by measuring local adaptation. We found no evidence for local adaptation between sympatric combinations of A. tristis squash bugs and Caballeronia spp. across their native geographic range. To test for diffuse coevolution, we performed reciprocal inoculations to test for specialization between three Anasa host species and Caballeronia spp. symbionts isolated from conspecific hosts. We observed no evidence of specialization across host species. Our results demonstrate generalist dynamics underlie the interaction between Anasa insect hosts and their Caballeronia spp. symbionts. Specifically, diffuse coevolution between multiple host species with a shared generalist symbiont may maintain cooperative traits despite horizontal transmission.