Functional bioassays suggest phage-infected Hamiltonella defensa confer resistance to parasitism in the pea aphid by directly killing wasp embryos

The bacterial symbiont Hamiltonella defensa infects diverse aphid species, including the pea aphid (Acyrthosiphon pisum).  Prior studies in A. pisum established that H. defensa is often infected by a phage named APSE and that phage-infected H. defensa benefit aphids by providing resistance to the parasitoid wasp, Aphidius ervi. In contrast, the underlying mechanism(s) of resistance remains unclear.  In this study, in vivo assays established the precise timing of A. ervi embryonic development in A. pisum and showed that wasp embryos die at specific times when developing in aphids containing H. defensa infected by APSE3 or APSE2.  We then used an in vitro assay to show that APSE-infected H. defensa kill A. ervi at the same embryonic stage that mortality occurs in vivo, whereas phage-free H. defensa do not.  Overall, these findings suggest that resistance to parasitism in A. pisum is due to APSE-infected H. defensa producing factors that directly kill A. ervi.