<b>Beyond symbiosis:</b><b> </b><b><i>Buchnera</i></b><b><i> </i></b><b>density is below optimum for fitness of its aphid host,</b><b> </b><b><i>Rhopalosiphum padi</i></b>

Symbiotic microbes often underpin insect nutrition and performance, yet evolutionary forces shaping endosymbiont density within hosts remain poorly resolved. In aphids, the obligate bacterium Buchnera aphidicola supplies essential nutrients, but high symbiont loads may impose costs. Here, we use a global pest of cereal crops, the bird cherry–oat aphid, Rhopalosiphum padi, to explore how variation in Buchnera density relates to fitness variation. While the antibiotic rifampicin caused large declines in Buchnera density and fitness in R. padi, tetracycline caused only a partial reduction in Buchnera density and fecundity followed by rapid recovery. We then measured Buchnera density, body length, and fitness (fecundity and intrinsic rate of increase) for hundreds of individuals across experiments and generations and applied the Lande–Arnold phenotypic selection framework to estimate directional, quadratic, and correlational selection. Buchneradensity experienced consistently positive directional selection, further exacerbated by tetracycline treatment; individuals with relatively higher symbiont densities had a higher fitness. However, selection was also weakly concave, with fitness levels plateauing at high densities, consistent with diminishing returns and/or increasing symbiont maintenance costs. Body length was under positive directional selection, and correlational selection on body length and Buchnera density was observed in one experiment. Quadratic selection on Buchnera appeared stronger in apterous than alate morphs which had a narrower phenotypic range of symbiont densities. Together, these results demonstrate how within-clone variation in symbiont density translates into fitness differences, raising questions about why the population mean density of Buchnerain R. padi is lower than expected under optimal fitness.