KNOTT, K.E.*; PUURTINEN, M.; VAN OOIK, T.; HYT�NEN, M.; KAITALA, V.; University of Jyv�skyl�, Finland: Parasite infection, heterozygosity and selfing in the hermaphroditic snail Lymnaea stagnalis

We studied the relationship between heterozygosity and infection by trematode parasites in eight populations of Lymnaea stagnalis. Within natural populations, prevalences of trematode infection were high, from 37% to 94%, and there was no relationship between infection status and individual heterozygosity at microsatellite loci. Additionally, among populations, parasite prevalence was not correlated with population genetic diversity or population selfing rate. We further studied the relationship between heterozygosity and parasite infection in a controlled experimental set-up, in which self- and cross-fertilized snails were exposed to an allopatric population of parasites in semi-natural conditions. In the experiment, selfed offspring were no more susceptible to trematode infection than outcrossed offspring, a result in line with observations from the natural populations. However, population genetic diversity was a significant factor determining infection probability; snails from less heterozygous populations were more likely to be infected than snails from more heterozygous populations. The lack of a relationship between parasite infection and heterozygosity in natural populations can be explained by the probable high spatial and temporal variation in parasite exposure both within and among populations. Local genetic adaptation of parasites to their snail intermediate hosts may be hampered by high gene flow in the parasite populations.