Phenoloxidase and the paradox of immunity in the pea aphid Acyrthosiphon pisum

The genome sequence of the pea aphid, Acyrthosiphon pisum, revealed major apparent deficiencies in its immune system, compared to other insects with sequenced genomes. One argument about the role of these deficiencies is that pea aphids are channeling resources away from expensive immune defenses to pursue other needs. This is counterintuitive, since in addition to pathogen defense an immune system may have interactions with the aphid’s symbiotic bacteria, Buchnera aphidicola. Detailed annotation work demonstrates that the phenoloxidase pathway, which is responsible for melanization and generation of reactive quinones with antimicrobial activity, is intact in the pea aphid. Phenoloxidase is constitutively active in pea aphids, with additional enzyme present in an inactive form, and is not inducible by bacterial infection. The presence of a functional immune pathway that is not activated in response to pathogens demonstrates that, while the immune system is not wholly deficient, its role in the pea aphid is not fully understood and deviates from traditional models.