Homogeneity of phage toxin in a tripartite defensive symbiosis

Heritable bacteria are widespread in insects and often confer ecologically important traits, such as protection from natural enemies.  Pea aphids, Acyrthosiphon pisum, are associated with several protective symbionts, including Hamiltonella defensa, which protects against the parasitoid Aphidius ervi, but only when the bacterial symbiont carries a temperate bacteriophage (APSE).  Levels of anti-wasp protection vary, possibly due to the presence of different toxin homologs encoded by each of the two variants of APSE previously characterized in North American A. pisum (APSE-2 & -3). We greatly expanded the number and geographic range of A. pisum sampled to investigate the diversity of APSEs. Despite increased sampling, we found that majority of H. defensa-infected A. pisum carried cytolethal distending toxin (CdtB) encoding APSE-2s., despite the fact that APSE-3s confer greater anti-wasp protection. Sequencing revealed the coexistence of two cdtB alleles, and that some APSE-2s have swapped structural genes with non-cdtB-coding APSEs. Further work is needed to explore whether this variation influences the resistance phenotype and why APSE-2s have spread in natural populations at the expense of APSE-3s.