Examining the role of parasites in limiting unidirectional gene flow between lake and river sticklebacks

1. Parasites are important selective agents with the potential to limit gene flow between host populations by shaping local host immunocompetence. 2. We report on a contact zone between lake and river three-spined sticklebacks (Gasterosteus aculeatus) that offers the ideal biogeographic setting to explore the role of parasite-mediated selection on reproductive isolation. A waterfall acts as a natural barrier and enforces unidirectional migration from the upstream river stickleback population to the downstream river and lake populations. 3. We assessed population genetic structure and parasite communities over four years. In a set of controlled experimental infections, we compared parasite susceptibility of upstream and downstream fish by exposing laboratory-bred upstream river and lake fish, as well as hybrids, to two common lake parasite species: a generalist trematode parasite, Diplostomum pseudospathaceum and a host-specific cestode, Schistocephalus solidus. 4. We found consistent ge...