Data for: Variation in density, immune gene suppression, and co-infection outcomes among strains of the aphid endosymbiont Regiella insecticola

Many insects harbor heritable microbes that influence host phenotypes.
Symbiont strains establish at different densities within hosts. This
variation is important evolutionarily because within-host density has been
linked to the costs and benefits of the symbiosis for both partners.
Studying the factors shaping within-host density is important to our
broader understanding of host-microbe coevolution. Here we focused on
different strains of Regiella insecticola, a facultative symbiont
of aphids. We first showed that strains of Regiella establish in pea
aphids at drastically different densities. We then found that variation in
density is correlated with the expression levels of two key insect immune
system genes (phenoloxidase and hemocytin), with the suppression of immune
gene expression correlating with higher Regiella density. We then
performed an experiment where we established co-infections of a higher-
and a lower-density Regiella strain, and we showed that the higher-density
strain is better able to persist in co-infections than the lower-density
strain. Together, our results point to a potential mechanism that
contributes to strain-level variation in symbiont density in this system,
and our data suggest that symbiont fitness may be increased by
establishing at higher density within hosts. Our work highlights the
importance of within-host dynamics shaping symbiont evolution.