Evolutionary genetics of a defensive facultative symbiont of insects: exchange of toxin-encoding bacteriophage

The facultative endosymbiont of aphids, Hamiltonella defensa , kills parasitoid wasp larvae, allowing aphid hosts to survive and reproduce. This protection may depend on toxins that are encoded by the genomes of H. defensa and of its bacteriophage (APSE). Strains of H. defensa vary in degree of protection conferred upon Acyrthosiphon pisum (pea aphid). Although H. defensa is known to undergo some horizontal transmission among aphid maternal lineages, divergence, recombination, and population structure in H. defensa and APSE have not been characterized. We performed a multilocus sequence analysis of 10 bacterial and five phage loci for strains isolated from A. pisum and other aphid species. The H. defensa chromosome was found to be largely clonal, allowing us to generate a well-resolved H. defensa strain phylogeny. In contrast, APSE chromosomes undergo recombination and numerous H. defensa strains have probably lost the phage. Within a set of H. defensa strains that are indistinguishable on the basis of chromosomal genes or restriction digests of chromosomal fragments, loss of APSE is associated with decreased protection, strongly suggesting that APSE-encoded genes contribute to the defensive phenotype. Thus, homologous recombination of APSE genes and sexual transmission of symbionts and phage are likely factors influencing the exchange of ecologically important genes among symbionts. Although H. defensa has been lost, transferred and gained within A. pisum , one subclade of H. defensa appears to be universal within a subclade of the aphid genus Uroleucon , suggesting a transition from facultative, horizontal transmission to strictly vertical inheritance.     

Phage loss and the breakdown of a defensive symbiosis in aphids

Terrestrial arthropods are often infected with heritable bacterial symbionts, which may themselves be infected by bacteriophages. However, what role, if any, bacteriophages play in the regulation and maintenance of insect–bacteria symbioses is largely unknown. Infection of the aphid Acyrthosiphon pisum by the bacterial symbiont Hamiltonella defensa confers protection against parasitoid wasps, but only when H. defensa is itself infected by the phage A. pisum secondary endosymbiont (APSE). Here, we use a controlled genetic background and correlation-based assays to show that loss of APSE is associated with up to sevenfold increases in the intra-aphid abundance of H. defensa. APSE loss is also associated with severe deleterious effects on aphid fitness: aphids infected with H. defensa lacking APSE have a significantly delayed onset of reproduction, lower weight at adulthood and half as many total offspring as aphids infected with phage-harbouring H. defensa, indicating that phage loss can rapidly lead to the breakdown of the defensive symbiosis. Our results overall indicate that bacteriophages play critical roles in both aphid defence and the maintenance of heritable symbiosis.