Loss of bacteriophage reduces fitness of aphids infected with a defensive symbiont

In the pea aphid, Acyrthosiphon pisum, infection with the heritable facultative bacterial symbiont, Hamiltonella defensa, protects against the parasitoid wasp, Aphidius ervi, only if the bacteria are themselves infected with APSE bacteriophages. A. pisum harboring H. defensa with phage variant APSE-3, which encodes a YD-repeat toxin, receive near-total protection against A. ervi. While it is clear that APSE-3 is critical to defense, little is known about how it may influence other aspects of the heritable symbiosis. To determine if phage infection influences H. defensa abundance in aphids, we conducted quantitative PCR of single-copy H. defensa genes to estimate bacterial densities. We compared aphids infected with APSE-3-carrying H. defensa to clonal aphids containing the same strain of H. defensa without APSE-3 and found that phage-free aphids had higher titers of H. defensa. APSEs may reduce bacterial copy number, e.g., through lysis, or by influencing tissue tropism or bacterial growth rates. Increases in H. defensa were associated with commensurate decreases in the nutrient-provisioning obligate symbiont Buchnera aphidicola. An overabundance of H. defensa with concomitant reductions in Buchnera potentially reduces the fitness of APSE-free H. defensa-infected aphids. We therefor compared basic fitness parameters between APSE-infected and APSE-free aphids. Phage free aphids, which are H. defensa-rich and B. aphidicola-poor, had a ~40% decrease in lifetime offspring production, delayed onset of first reproduction, and lower mass at adulthood relative to clonal aphids with phage-carrying H. defensa. In addition to providing anti-parasitoid defense, APSEs may play important roles in maintaining the A. pisum-H. defensa symbiosis.