ADAPTATION BY A PARASITIC TREMATODE TO LOCAL POPULATIONS OF ITS SNAIL HOST.

In each of two reciprocal cross-infection experiments, a digenetic trematode (Microphallus sp.) was found to be significantly more infective to snails (Potamopyrgus antipodarum) from its local host populations. This gives strong evidence for local adaptation by the parasite and indicates that there is a genetic basis to the host-parasite interaction. It is suggested that the parasite should be able to track common snail genotypes within populations and, therefore, that it could be at least partially responsible for the persistence of sexual subpopulations of the snail in those populations that have both obligately sexual and obligately parthenogenetic females.