Changes in gene regulation are associated with the evolution of resistance to a novel parasite.

Introduction: Host-parasite interactions are ubiquitous and are important drivers of host diversification and evolution. In particular, host immune systems are frequent targets of parasite-driven selection. The resulting rapid evolution of immune genes is usually framed as an ongoing 'arms race' between a co-evolving pair of host and parasite species. However, immune evolution may often be driven by the acquisition of a new and unfamiliar parasite. For instance, when marine populations of threespine stickleback ( Methods: We compared the transcriptomic responses of lab-reared sticklebacks from three populations of stickleback with varying cestode susceptibilities when exposed to several immune stimuli (alum, cestode protein, or a control injection). Results: The resulting changes in expression reveal strong evidence of shared and population-specific responses during the evolution of defense against a new parasite. Our investigation highlights the roles of several key immunological processes in underlying a general physiological response to tissue damage (fibrosis) and the importance of regulating fibrosis as a necessary step for its co-option into defense against Conclusion: Our results provide strong evidence that changes in gene regulation and increased negative feedback to mitigate immunopathology are essential steps in the evolutionary co-option of an existing pathway to defend against a new parasite infection.