The heritable bacterial symbiont Hamiltonella defensa protects the aphid Acyrthosiphon pisum from attack by the parasitoid Aphidius ervi by killing wasps as they develop within living aphids. Using a controlled genetic background, we have found that H. defensa must carry a toxin-encoding bacteriophage (a virus that infects bacteria), called APSE to produce the protective phenotype. Further, we found that phage loss occurs repeatedly in laboratory-held strains of H. defense-infected A. pisum, resulting in greatly increased susceptibility to parasitism in each instance. This combination of experimental and correlation-based evidence indicates that the phage APSE endows the bacterial symbiont with benefits that extend to the animal host. The localization of these factors to a mobile genetic element provides a mechanism to move ecologically important traits, such as defense against parasitic wasps within and among host lineages.  

Results/Conclusions

In addition to endowing H. defensa with factors that likely contribute its spread within host population, bacteriophages also have the potential to influence the aphid-bacterial symbiosis in other ways. For example, in a common aphid background, we have found that titers of phage-carrying H. defensa are lower across multiple time points, than those of the same H. defensa strain that is phage-free. This suggests that the lytic action of these phages may play a role in regulating symbiont densities within aphid hosts. We are also finding significant ‘lifestyle’ differences among different H. defensa strain and phage-variant combinations, with, for example, some exhibiting high levels of phage integration into the H. defensa chromosome and others showing little integration, yet maintaining high genomic copy number. Thus, in addition to bringing in novel genes that benefit both the bacterial and animal hosts, phage infection may influence this symbiosis in diverse ways.