Spatial variation in susceptibility to infection in a snail-trematode interaction

Parasites should be better at infecting hosts from sympatric populations th an allopatric populations most of the time (parasite local adaptation). In a previous study of a population of snail parasites (Microphallus sp.) from Lake Alexandrina, New Zealand, we found that Microphallus was more infecti ve to snails (Potamopyrgus antipodarum) in shallow water but not in deep wa ter. Were, we repeated the original study and also monitored the developmen t of the parasite. We found that parasites from shallow water were more inf ective to hosts from shallow water and developed more rapidly in these host s. In contrast, parasites from deep water were not more infective to hosts from deep water and did not develop more rapidly in them. These results sug gest clinal variation in the susceptibility of these snails, with shallow-w ater snails more susceptible than deep-water snails. We offer 2 possible ex planations for these results. First, gene flow in the Microphallus populati on is primarily from shallow to deep water, leading to an asymmetric patter n of local adaptation. Alternatively, snails from shallow water may be more susceptible for reasons independent of gene flow, perhaps due to differenc es in host condition between habitats.