Codner, P. | Search bioRxiv

bioRxiv2026

Constitutive and inducible fibrosis explain immune variation among threespine stickleback populations

Understanding how immune variation arises in natural populations requires disentangling the relative contributions of host genetic differences, environmental variation, and parasite effects, which is rarely possible in wild systems. Threespine stickleback populations vary in their use of intraperitoneal fibrosis as a defense against the helminth parasite Schistocephalus solidus, providing a natural system to study the genetic and ecological drivers of immune variation. We combined a 46-lake field survey with common garden experiments on 20 representative populations exposed to multiple parasite genotypes to test whether population differences in fibrosis persist under controlled conditions and whether they depend on parasite genotype or lake ecology. Fibrosis variation was strongly heritable, with both constitutive and inducible components persisting under common garden conditions. In contrast, parasite genotype had only a weak effect on fibrosis responses. Moreover, inducible fibrosis covaried with lake environmental conditions, with populations from more eutrophic-like lakes exhibiting stronger responses than those from more oligotrophic-like lakes. Together, these results reveal ecologically structured divergence in heritable immune responses among natural populations.

evolutionary biology↗

bioRxiv2025

Heritable and environmental influences on innate immunity across three populations of threespine stickleback (Gasterosteus aculeatus)

Environmental variation plays a key role in immune development and function; factors such as pathogen exposure history, seasonality, and resource availability all affect an individuals immune phenotype. However, the relative contributions of heritable and non-heritable factors remain unclear for most immune phenotypes. We used three populations of threespine stickleback (Gasterosteus aculeatus) with heritable differences in immune function to investigate the relationship between immunity, genetic divergence, and the environment. To test for environmental effects on immunity, fish were raised in tanks with different flow rates (continuous or intermittent). After long-term acclimation to one tank environment, subsets of adult fish were moved to the alternate flow regime and allowed to acclimate for eighteen weeks. We then measured the effects of starting environment, transfer between environments, and final environment across several immune parameters. Fish population and treatment both significantly affected immune function. Stickleback from a population previously found to display the highest heritable levels of innate immunity displayed the highest oxidative burst capacity (ROS) regardless of tank environment. However, all fish in intermittent flow tanks (both resident and transfers) tended to have higher ROS production, more granulocytes, and greater spleen mass than in continuous flow. Variation in liver mass was mainly driven by population effects. We also provide limited data suggesting that the two water flow regimes harbor different microbial environments, offering a potential future direction for understanding the proximate connection between tank environment and immune variation. Overall, this work demonstrates that a simple change in water flow dynamics can induce immune flexibility. Perhaps more importantly, it also highlights the need for further research examining how naturally evolved genetic differences and environmental factors individually and jointly influence the magnitude and direction of immune responses.

evolutionary biology↗