Does the X-type symbiont protect the pea aphid (Acyrthosiphon pisum) from parasitism?

Heritable facultative symbionts (HFS) are common in insects and other arthropods. By definition HFS are not strictly required for host survival and reproduction, however they often confer important conditional benefits, including resistance to parasitoids. The pea aphid, Acyrthosiphon pisum, has arisen as a model organism for studying effects of infection with HFS due to its asexual reproduction combined with the ability to manipulate their symbionts. Pea aphids have eight known HFS, and individual aphids are often infected with multiple HFS species. Despite being a model for HFS studies, the effects of some common HFS species are not well studied. For example, PAXS (Pea Aphid X-Type Symbiont) is widespread in North American populations and prior reports suggest it may play a role in rescuing parasitoid protection conferred by another HFS, Hamiltonella defensa. However, studies using controlled genetic backgrounds are needed to confirm roles for PAXS. Interestingly, PAXS is typically found in natural populations co-infecting hosts with H. defensa. Here, using microinjection and selective curing of HFS with antibiotics, we created experimental aphid lines that were infected with 1) both PAXS and H. defensa, 2) H. defensa only, 3) PAXS only, and 4) uninfected with HFS. All lines shared aphid and Buchnera (obligate symbiont) genotypes.  We then conducted parasitism assays, using the dominant pea aphid parasitoid, Aphidius ervi, under normal culture conditions and when exposed to heat shock to determine the role of PAXS, acting alone or in concert with H. defensa, on the aphid protective phenotype