Ecological importance of a bacteriophage to the pea aphid, Acyrthosiphon pisum

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. While the specific protective phenotypes attributed to some APSE variants have been investigated, little is known about the role of any APSE haplotype in other aspects of the symbiosis. To determine if phage infection influences H. defensa abundance in aphids, we conducted quantitative PCR of a single-copy H. defensa gene 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 decreases in the nutrient-provisioning obligate symbiont Buchnera aphidicola. An overabundance of H. defensa with concomitant reductions in Buchnera is associated with a reduction in the fitness of APSE-free H. defensa-infected aphids. Phage-free aphids, which have higher H. defensa densities and lower B. aphidicola densities relative to phage-infected clones, 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. The known phage haplotypes vary in both intensity of defensive phenotype and stability of infection, so we are engaged in further identification, sequencing, and characterization of APSE variants to more accurately depict phage-bacterium-aphid interactions in the field.