The evolution of constitutively active humoral immune defenses in Drosophila populations under high parasite pressure

Both constitutive and inducible immune mechanisms are employed by hosts for defense against infection. Constitutive immunity allows for a faster response, but it comes with an associated cost that is always present. This trade-off between speed and fitness costs leads to the theoretical prediction that constitutive immunity will be favored where parasite exposure is frequent. We selected populations of Drosophila melanogaster under high parasite pressure from the parasitoid wasp Leptopilina boulardi. With RNA sequencing, we found the evolution of resistance in these populations was associated with them developing constitutively active humoral immunity, mediated by the larval fat body. Furthermore, these evolved populations were also able to induce gene expression in response to infection to a greater level, which indicates an overall more activated humoral immune response to parasitization. The anti-parasitoid immune response also relies on the JAK/STAT signaling pathway being activated in muscles following infection, and this induced response was only seen in populations that had evolved under high parasite pressure. We found that the cytokine Upd3, which induces this JAK/STAT response, is being expressed by immature lamellocytes. Furthermore, these immune cells became constitutively present when populations evolved resistance, potentially explaining why they gained the ability to activate JAK/STAT signaling. Thus, under intense parasitism, populations evolved resistance by increasing both constitutive and induced immune defenses, and there is likely an interplay between these two forms of immunity. Author SummaryImmune defenses can be induced after infection or they may be constitutively active, even in uninfected individuals. As constitutive immunity is a more rapid response, theory predicts that it will be favored when animals frequently encounter parasites. When we subjected populations of Drosophila melanogaster to high rates of parasitization from its natural parasite, Leptopilina boulardi parasitoid wasps, we indeed observed that the immune response became constitutively active. Uninfected insects had an activated humoral immune response and produced cytokine-secreting immune cells that were normally induced after infection. However, we also found that these populations evolved a greater induced response. This included a greatly increased cytokine response after infection, suggesting that the constitutive activation of some aspects of the immune system may allow a greater induced response in other tissues.