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 APSE. Here we use a controlled genetic background and correlation-based assays to show that loss of APSE produces up to seven- fold 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.

APSElossaphidfitnessTable2Fitness data (time to first reproduction, fresh weight at time of first reproduction, and offspring production by cohorts of four aphids) in Acyrthosiphon pisum clonal lines sharing the same aphid genotype and Hamiltonella defensa strain, with (a1a-5a+) and without (a1a-5aX) bacteriophage APSE.phageAPSElossbacterialtitresfigure1Quantitative PCR data for Hamiltonella defensa dnaK and APSE gene P28 in experimental lines sharing the same aphid genotype (1a, 1b and 1d) and genotypically diverse clonal lineages (1c).correctedphageAPSElossbacterialtitresfigure1.xlsx