CLINAL VARIATION FOR LOCAL ADAPTATION IN A HOST-PARASITE INTERACTION

Using a reciprocal cross-infection experiment, we detected clinal vari ation in the relative ability of trematodes to infect snails taken fro m the same habitat. Snails (Potamopyrgus antipodarum) collected from t hree habitats in a New Zealand lake were exposed to a digenetic tremat ode (Microphallus sp.) collected from infected snails from the same th ree locations, The habitats were arranged according to depth, from sha llow (the shore bank habitat: less than 1 m deep), to intermediate (th e Isoetes habitat: 1.5-3 m), to deep (the Elodea habitat: 4-6 m). Snai ls were assayed for infection at two points in the development of the parasite: 4-months post infection (the blastocercariae stage), in whic h the parasite was not yet infective to the final hosts (ducks), and 7 -months post infection (the metacercariae stage), in which the worms w ere fully encysted and competent for transmission to the final host. A t 7-months post infection, the results showed that shallow-water paras ites were significantly more infective to shallow-water hosts, but dee p-water parasites were about equally infective to snails from all thre e habitats. This result suggests that gene flow by the parasite is pri marily from shallow to deep habitats, and that recycling of the parasi te and host-parasite coevolution is primarily in shallow water. Such r ecycling would be consistent with our observations of foraging by the definitive host of the parasite, and may explain the maintenance of hi gh frequencies of sexual individuals around the shallow-water margins of the lake.