Do aphid soldiers elicit an immune response in victims?

Defense is key to the formation and maintenance of groups, but most small sap-feeding insects are not well-equipped to defend themselves. The social aphid Pemphigus obesinymphae, however, has nymphal soldiers that defend the colony from invaders by piercing them with their stylets. The mechanism by which is poorly understood. In the lab, soldiers will attack a surrogate invader (Drosophila larvae), resulting in death of the larvae. After attack by P. obesinymphae, systemic melanization in the Drosophila larvae is observed. This response is likely due to the overactivation of immune-regulatory pathways. To further quantify this observation, we used real-time PCR to compare expression of immune genes before and after attack by aphid soldiers. We focused on genes of the Toll, IMD, and melanization pathways and identified specific immune factors that were differentially expressed in attacked larvae. We found that attack by P. obesinymphae activated immune regulatory genes in the Drosophila melanization pathways. This suggests that the soldier attacks are eliciting an immune response in Drosophila larvae. Of the transcripts overexpressed in larvae attacked by aphids, some were significantly upregulated compared to pierced controls. These data indicate that in addition to the physical injury of piercing, P. obesinymphae soldiers may be introducing an effector molecule that leads to an upregulation of genes in the melanization pathway.  Using mutant Drosophila lines with knock-outs in the melanization pathway, we were able to partially recover survivorship of Drosophila larvae after attack by aphid soldiers.  This implies that overactivation of the melanization pathway is partial responsible for the death of the Drosophila larvae.