Impact of environmental stress on aphid clonal resistance to parasitoids: Role of Hamiltonella defensa bacterial symbiosis in association with a new facultative symbiont of the pea aphid.

Resistance to endoparasitoids in aphids involves complex interactions between insect and microbial players. It is now generally accepted that the facultative bacterial symbiont Hamiltonella defensa of the pea aphid Acyrthosiphon pisum is implicated in its resistance to the parasitoid Aphidius ervi. It has also been shown that heat negatively affects pea aphid resistance, suggesting the thermosensitivity of its defensive symbiosis. Here we examined the effects of heat and UV-B on the resistance of A. pisum to A. ervi and we relate its stability under heat stress to different facultative bacterial symbionts hosted by the aphid. For six A. pisum clones harboring four different facultative symbiont associations, the impact of heat and UV-B was measured on their ability to resist A. ervi parasitism under controlled conditions. The results revealed that temperature strongly affected resistance, while UV-B did not. As previously shown, highly resistant A. pisum clones singly infected with H. defensa became more susceptible to parasitism after exposure to heat. Interestingly, clones that were superinfected with H. defensa in association with a newly discovered facultative symbiont, referred to as PAXS (pea aphid X-type symbiont), not only remained highly resistant under heat stress, but also expressed previously unknown, very precocious resistance to A. ervi compared to clones with H. defensa alone. The prevalence of dual symbiosis involving PAXS and H. defensa in local aphid populations suggests its importance in protecting aphid immunity to parasitoids under abiotic stress.