Fig. 3

Cross section of Lake Alexandrina, showing depth-specific habitat zones and hypothesized gene flow (redrawn from King et al. (2009)). Data from this lake show that sexual snails are more common in the shallow-water habitats, and asexual snails are more common in the deep-water habitats. Infection by trematode worms is also more common in the shallow water. Because parasite larvae must be ingested by ducks and because ducks forage mainly in the shallow water (<3 meters), we hypothesized that parasite genes would more likely be recycled in the shallow water. Infections in the deep water would be out of the coevolutionary loop. Hence, we predicted that local parasites would be more infective to shallow-water hosts than deep-water hosts