Immune responses elicited by social aphid soldier attacks

To protect their colonies from natural enemies, social insects have evolved morphological and behavioral traits for defense. Although most small sap-feeding insects are vulnerable when they occur in sedentary groups, the social aphid Pemphigus obesinymphae has a soldier caste of nymphs that successfully defends the gall. P. obesinymphae soldiers swarm invaders and pierce them with needle-like stylets that are adapted for plant-feeding. By an unknown mechanism, soldiers attack fly larvae, leading to systemic melanization and death. We hypothesized that aphid soldiers induce widespread, systemic reactions in the fly immune system that cause morbidity, and therefore underlie effective defense. To better characterize the immune response of P. obesinymphae victims, real-time PCR was used to assess differentially expressed immune factors in attacked larval victims. Genes in the Toll, IMD and melanization pathways were quantified. P. obesinymphae attack resulted in the upregulation of genes in the Drosophila melanization pathway in particular. P. obesinymphae may secrete effector molecules or agents during attack that mimic septic injury. Sociality in aphids, which depends crucially on defense, may thus involve the evolution of biochemical and behavioral transitions that bridge successful attack of plants to that of animals.