Coevolutionary hotspots and coldspots for host sex and parasite local adaptation in a snail-trematode interaction

Reciprocal adaptation between interacting species may occur in some regions (coevolutionary 'hotspots') and not others ('coldspots'). In a previous study, we found hotspots and coldspots along a continuous depth gradient in two different New Zealand lakes. Specifically, we found that Microphallus sp. trematodes were locally adapted to Potamopyrgus antipodarum snails collected from shallow-water margins of the lakes, but not to snails collected from deep-water habitats. As sexual snails were more common in the shallow water, and asexual snails more common in the deep water, the results were also consistent with the Red Queen hypothesis, which predicts that sex should be favored in environments with coevolving parasites. Here, we repeated our earlier experiment to determine whether the results are robust over time (two years) and space (three lakes). We also tested whether our measure of parasite local adaptation was sensitive to parasite dose. Our results suggest that shallow-water habitats are temporally stable coevolutionary hotspots, and that the pattern is spatially robust over three lake populations. We also found that, while parasite dose affects the magnitude of local adaptation, it does not obscure the signature of local adaptation in this snail-trematode system.