Constitutive and inducible fibrosis explain immune variation among threespine stickleback populations

Population-specific immunity can drive variation in infection outcomes, but studying immune variation in the wild is challenging because exposure histories are unknown. Comparing wild populations with those reared in a common environment can disentangle genetic and environmental drivers of immunity. We applied this approach in freshwater threespine stickleback, where populations vary in their use of intraperitoneal fibrosis to defend against the helminth parasite Schistocephalus solidus . We combined a 46-lake immune survey with a common garden experiment using 20 representative populations to examine variation in fibrosis and infection. Laboratory assays included exposures to live tapeworms and immune challenges with tapeworm proteins and aluminum phosphate (Alum). We found heritable variation in both constitutive fibrosis and inducible fibrosis. Inducible responses to tapeworms were associated with lake environmental conditions, with fish from more eutrophic-like lakes showing stronger fibrosis induction than those from more oligotrophic-like lakes. Together, these results show how integrating wild immune variation with common garden experiments can reveal novel heritable defenses and link their evolution to ecological variation.