Interactions of the pea aphid secondary symbiont Hamiltonella defensa and its bacteriophage, APSE

The bacterial symbiont Hamiltonella defensa infects many hemipteran insects, including the pea aphid (Acyrthosiphon pisum).  Recent studies establish that H. defensa is itself infected by a bacteriophage named APSE which together benefit aphids by providing resistance to certain species of parasitoid wasps.  However, data also suggest that H. defensa can adversely affect aphid fitness and that APSE infection attenuates these fitness costs.  Here we expand on these results by establishing an in vitro study system in which we experimentally study H. defensa growth and function in the presence and absence of APSE.   Our results indicate that H. defensa growth does not fully depend on the presence of insect cells and several lines of experimental data indicate this dependency is due to nutritional factors.  We also show that APSE variants differentially interact with H. defensa with one strain exhibiting low-level replication while another persists only as an integrated provirus.  Such ‘lifestyle’ differences between phage variants potentially influence the function and regulation of the symbiosis in ways important to the biology of the aphid host.