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

Host-parasite interactions are ubiquitous and are important drivers of diversification and evolution. Host immune systems in particular 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. But, often immune evolution may be driven by the acquisition of a new and unfamiliar parasite. For instance, when marine populations of threespine stickleback (Gasterosteus aculeatus) colonized freshwater lakes [~]12,000 years ago, they encountered and evolved resistance to a freshwater-restricted cestode Schistocephalus solidus. We compared the transcriptomic responses of lab-reared stickleback from three populations with varying cestode susceptibility (naive marine, susceptible lake, and resistant lake), when exposed to several immune stimuli. The resulting changes in expression reveal strong evidence for shared and population-specific responses during 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 regulation of this fibrosis as a necessary step for its co-option into parasite defense. Furthermore, we highlight changes in expression of fibrosis-associated genes which facilitate faster and more targeted deployment of this defense against parasites; fish from resistant populations have higher and more consistent expression of fibrosis genes, which experience strong negative feedback in response to cestode stimuli only. Combined, our results provide strong evidence that changes in gene regulation and increased negative feedback to mitigate immunopathology are essential steps in the evolution of novel parasite defense.