Superinfecting symbionts: Interactions between two defensive mutualists and their aphid host, Acyrthosiphon pisum

Worldwide, many economically important insect species are infected with heritable bacterial symbionts that mediate ecological interactions affecting their pest status. Pea aphids (Acyrthosiphon pisum), long a model for symbiosis research, are infected with a diverse assemblage of symbionts affecting heat tolerance, host-plant utilization, and defense against natural enemies, including biocontrol agents. Infection benefits have almost exclusively been studied in aphids infected with a single symbiont, yet field surveys indicate that many individual aphids are infected with two or more symbiont species (i.e. superinfected). Particular co-infections are more or less common than expected by chance, indicating that specific inhibitory or synergistic symbiont-symbiont interactions may affect frequencies. For example, two prevalent symbionts, Hamiltonella defensa, which protects against parasitoids, and the anti-fungal protector Regiella insecticola, only rarely infect the same aphid despite their non-overlapping protective benefits. Moreover, when found together, superinfections often reduce to single infections in lab-held lines, suggesting these two well-described symbionts are ideal for investigating antagonistic interactions between competing mutualists. I have created experimental lines in three aphid genetic backgrounds that are singly infected by either H. defensa or R. insecticola, uninfected entirely, or superinfected. I have used these to (1) investigate costs associated with superinfection, (2) characterize the defensive phenotypes of superinfected aphids, and (3) determined the effects of superinfection on symbiont abundance. The prevalence of superinfections and the importance of bacterial communities, rather than isolated bacterial lines, to animal hosts render a better understanding of superinfection dynamics vital to pest management.