RNAseq of Acyrthosiphon pisum infected by APV

Insects are associated with numerous heritable bacterial symbionts that provide defense against specific natural enemies, yet little is known about symbiont protection when hosts face multiple threats. In pea aphids (Acyrthosiphon pisum), the facultative endosymbiont Hamiltonella defensa confers protection against the parasitoid, Aphidius ervi, while the related Regiella insecticola protects against the aphid-specific fungal pathogen, Pandora neoaphidis. Pea aphids also experience chronic infections with A. pisum virus (APV), yet little is known about interactions between facultative symbionts, their targeted enemies and specialized viruses. Here we use the pea aphid experimental system to determine whether two common defensive symbionts protect against APV, or if their protective services, or constitutive infection costs, are impacted by APV infection. We observed highly divergent outcomes of APV infection depending on the presence and identity of facultative symbionts. APV imposed large costs on symbiont-free aphids, costs which were elevated in aphids housing H. defensa. In contrast, APV costs were largely eliminated in aphids with R. insecticola, which along with reductions in APV titers indicate that the examined strain improves aphid tolerance to this virus. APV did not influence symbiont-mediated protection against parasitoids or fungal pathogens. We also conducted APV transmission assays to better understand virus infection dynamics. We found that although APV is vertically transmitted at low rates, routine horizontal transmission through food plants, with occasional transfer by parasitoids, is likely the major route of spread. Facultative symbionts did not impede APV acquisition.