Intraspecific variation in symbiont density in an insect–microbe symbiosis

Many insects host vertically transmitted microbes, which can confer benefits to their hosts but are costly maintain and regulate. A key feature of these symbioses is variation: for example, symbiont density vary among genotypes. However, the evolutionary forces maintaining this variation remain unclear. We studied in using pea aphid (Acyrthosiphon pisum) bacterium Regiella insecticola, a that protect its against fungal pathogens. found relative varies both between two phylogenetic clades “biotypes.” Higher infections correlated with stronger survival costs, has little effect on protection provides fungi. Instead, we some genotypes, dramatic decline precedes loss infection. Together, our data suggest optimal infection likely different from perspective microbial fitness. might prevent by high within-host densities, do not appear benefit higher numbers may be advantaged losing symbionts certain environments. The standing observed natural populations could therefore maintained antagonistic coevolutionary interactions symbiotic microbes.