Reduction of <i>Buchnera</i> with rifampicin impairs the density‐dependent induction of winged morph in pea aphid

Wing dimorphism is a critical trait that helps insects better adapt to environments. High population density and poor nutrition are known to induce winged morph in aphids. Buchnera aphidicola, an obligate endosymbiont of aphids, supplies the host with essential amino acids and B vitamins, while the mechanistic basis of Buchnera's role in density-dependent production of winged aphids is largely unclear. Reduction of Buchnera from maternal aphids or the 1st instar nymphs with antibiotic rifampicin reduced the proportion of winged aphids. In contrast, for the 2nd instar nymphs, reduction of Buchnera failed to decline the proportion of winged aphids, suggesting the 1st instar is a critical period for wing plasticity. Genes associated with amino acid transport and insulin signaling were identified by RNA sequencing as differentially expressed genes between rifampicin and dimethyl sulfoxide-treated 1st instar nymphs. Proton-assisted amino acid transporter 1 (PAT1), was down-regulated by rifampicin. Knockdown of PAT1 in 1st instar nymphs reduced the proportion of winged aphids. In addition, reduction of Buchnera with rifampicin up-regulated several marker genes of the insulin signaling pathway, and reduced the phosphorylation level of forkhead transcription factor subgroup O (FOXO), a determinant of wing morph downstream of insulin signaling. Application of FOXO phosphorylation inhibitor LY2780301 decreased the proportion of winged aphids, while the agonist SC79 increased the proportion of winged aphids. These results revealed that reduction of Buchnera sharply declined the proportion of winged aphids, indicating that the vertical transmission of Buchnera could bring on a maternal signal of crowding perception, which was required for the production of winged offspring.