Strain diversity of Hamiltonella defensa, a protective symbiont of pea aphids

Aphids are frequently infected with one or more of nine heritable facultative symbionts (HFS) that confer a range of conditional benefits. Within symbiont species, strain differences can create additional genetic and phenotypic variation. For example, the pea aphid, Acyrthosiphon pisum, is commonly infected with the HFS Hamiltonella defensa which protects the aphid from attack by parasitoids. This protective phenotype varies with bacterial genotype and the presence and type of bacteriophages called APSEs (Acyrthosiphon pisum secondary symbionts). Here, we used a combination of strain-specific primers and sequencing of strain typing loci to characterize the genetic diversity of Hamiltonella and APSE across several field seasons of two locations. We found surprisingly high levels of strain diversity of each player, as well as combinations of Hamiltonella and APSE. Varying in selection pressure from enemies and other sources may maintain this variation, which in turn, may contribute to diversity in defenses.