Exploring Gene Expression Patterns and Evolutionary Responses in Host-Parasite inteactions: Insights from theSchistocephalus solidus– Threespine stickleback System

ABSTRACT Helminth parasites pose a significant threat to host survival and reproductive success, imposing strong selection pressure on hosts to evolve countermessures (e.g., immune responses and behavioral changes). To gain insights into the underlying mechanisms of host-parasite co-evolution, we examined differences in gene expression in immune tissues of two Alaskan stickleback ( Gasterosteus aculeatus ) populations with varying susceptibility to infection by the cestode Schistocephalus solidus . Our analyses revealed distinct patterns of immune gene expression at the population-level in response to infection. Infected fish from the high infection population displayed signs of immune manipulation by the parasite, whereas this phenomenon was absent in fish from the low infection population. Notably, we found significant differences in immune gene expression between the populations, with uninfected Rocky Lake fish showing up-regulation of innate immune genes associated with inflammation compared to uninfected Walby Lake fish. These findings highlight the divergent evolutionary paths taken by different stickleback populations in their response to the same parasite.