Clash of Clades: Intraspecific Variation in Regiella insecticola, a Protective Endosymbiont

Bacterial symbionts are ecologically important microbes that have been shown to manipulate many aspects of their hosts’ physiology. Past work in model systems of host-symbiont interactions has highlighted the great diversity of symbiotic partners, especially in arthropods. Using pea aphids, Acyrthosiphon pisum, I show that population diversity among the bacterial partner cannot be explained by phenotypic differences at the host-level alone. Regiella insecticola provides A. pisum resistance to the entomopathogenic fungi Pandora neoaphidis. Regiella consists of two divergent clades that can be identified by sequence homology. I found that Regiella strains belonging to Clade 2, but not Clade 1, significantly downregulate key aphid immune genes responsible for defense from bacterial infection, and establish at higher symbiont titer within the host. Both existing at a higher titer and downregulating host immune gene expression might increase the cost to harbor Clade 2 Regiella strains, but I found that resistance to P. neoaphidis infection is not correlated with Regiella Clade. I survey a wild population of pea aphids from Knoxville, Tennessee, USA and show that Regiella Clade 2 is prevalent in the wild population compared to Regiella Clade 1. I then examine symbiont interactions within the host by challenging an established infection of a Regiella strain with an injection of a strain from the other clade. I found that the coinfections resolved differentially based on the original infecting strain, where the Clade 2 Regiella is retained more often than the Clade 1 Regiella overall. I therefore suggest that intra-host interactions favor Clade 2 Regiella. Our findings are important because they show that both host-level and intra-host interactions contribute to variation found within symbiont species. Further, I suggest that mutualists can evolve phenotypes that significantly antagonize the host if that phenotype provides an advantage over other symbiotic partners in the population.