Strong neutral genetic differentiation in a host, but not in its parasite.

The genetic diversity and population structure of a parasite with a complex life cycle generally depends on the dispersal by its most motile host. Given that high gene flow is assumed to hinder local adaptation, this can impose significant constraints on a parasite's potential to adapt to local environmental conditions, intermediate host populations, and ultimately to host-parasite coevolution. Here, we aimed to examine the population genetic basis for local host-parasite interactions between the eye fluke Diplostomum lineage 6, a digenean trematode with a multi-host life cycle (including a snail, a fish, and a bird) and its second intermediate host, the three-spined stickleback Gasterosteus aculeatus L. We developed the first microsatellite primers for D. lineage 6 and used them together with published stickleback markers to analyse host and parasite population structures in 19 freshwater lakes, which differ in their local environmental characteristics regarding water chemistry and Diplostomum abundance. Our analyses suggest that one parasite population successfully infects a range of genetically differentiated stickleback populations. The lack of neutral genetic differentiation in D. lineage 6, which could be attributed to the motility of the parasite's definitive host as well as its life cycle characteristics, makes local host-parasite co-adaptations seem more likely on a larger geographical scale than among the lakes of our study site. Our study provides a suitable background for future studies in this system and the first microsatellite primers for a widespread fish parasite.