Hosts do not simply outsource pathogen resistance to protective symbionts
We show that aphid pest populations with high prevalence of protective bacteria have strong intrinsic immunity, contrary to the expectations.
Photo description: A) Pea aphid (Acyrthosiphon pisum), infected with specialist fungal pathogen Pandora neoaphidis. B) Pea aphids form host plant associated populations, and in those populations intrinsic resistance and resistance provided by protective symbiotic bacteria are positively correlated.
Microbial symbionts commonly protect their hosts from natural enemies, but it is unclear how protective symbionts influence the evolution of host immunity to pathogens. One possibility is that ‘extrinsic’ protection provided by symbionts allows hosts to reduce investment in ‘intrinsic’ immunological resistance mechanisms. We tested this idea using pea aphids (Acyrthosiphon pisum) and their facultative bacterial symbionts that increase host resistance to the fungal pathogen Pandora neoaphidis. The pea aphid taxon is composed of multiple host plant associated populations called biotypes, which harbor characteristic communities of symbionts. We found that biotypes that more frequently carry protective symbionts have higher, rather than lower, levels of intrinsic resistance. We describe a simple conceptual model of the interaction between intrinsic and extrinsic resistance and suggest that our results may be explained by selection favoring both the acquisition of protective symbionts and enhanced intrinsic resistance in habitats with high pathogen pressure. Such combined protection is potentially more robust than intrinsic resistance alone.
Hrček J., Parker B., McLean A., Simon J.-C., Mann C.M., Godfray, H. (2018) Hosts do not simply outsource pathogen resistance to protective symbionts. Evolution in press : DOI: 10.1111/evo.13512