Evaluating coevolution in a horizontally transmitted mutualism

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.