Parasite gene flow in riverine habitats: ascertaining the roles of stream drift, river bifurcations and host dispersal.

Determining the factors that shape parasite gene flow across complex landscapes is central to understanding the coevolutionary process. In rivers, unidirectional currents, stream drift, may facilitate downstream parasite dispersal, while bifurcating branches may cause population subdivision among branches. The generative habitat processes in rivers can potentially interact with host dispersal to determine gene flow within the aquatic ecosystem. We examined the population genetic structure and gene flow of a trematode infecting semi-aquatic snakes to determine the relative contributions of stream drift, river bifurcations and host dispersal in shaping parasite gene flow in three connected riverine ecosystems. We found the strongest population structure immediately below a recently constructed reservoir at the confluence of the two rivers, with mild structure between one out the the three reaches of the river. Patterns of isolation by distance along linear pathways were not uniform, despite similar path network path lengths. We found the strongest evidence for isolation by distance associated with the river bifurcation. The comparison of terrestrial versus within river network dispersal indicates that parasite transmission between branches occurs along river networks. Short-distance terrestrial dispersal however may be important along some linear networks. Our results highlight the complexity of host-habitat interactions shaping parasite gene flow and the need for empirical data from natural systems to develop accurate models of parasite transmission in rivers.