Local Versus Allopatric Parasites: Acanthocephalans From Different Watersheds Are Associated With Different Infection Strategies in Definitive Host

Understanding how infection‐related traits vary across parasite populations and host stages is key to predicting host–parasite dynamics in complex life cycles. We tested whether the geographic origin of the acanthocephalan parasite Pomphorhynchus laevis influences infection success in both its intermediate ( Gammarus pulex ) and definitive ( Squalius cephalus ) hosts. Parasite eggs from two watersheds (Seine and Saône) were used to infect naïve gammarids and subsequent larval stages of the parasites were used to infect naïve fish (from the Seine). Parasites from the Saône showed significantly higher infection success and virulence in gammarids than those from the Seine, including reduced survival even in exposed but uninfected individuals, revealing strong population‐level differences in infection traits. When analyses were restricted to low infection intensities, differences in gammarid survival persisted, indicating that these effects were not solely due to parasite load. Fish in the Seine basin were more successfully infected by local parasites, but differences relative to parasites from the Saône remained modest. Overall, our results highlight that population‐level variation in parasite performance can generate asymmetric infection patterns across host stages, emphasizing the importance of considering the full life cycle in studies of parasite evolution and host–parasite dynamics.