The littoral zone of Lake Constance is the main area of contact for many fish species and their macroparasites. This is due to sophisticated host-parasite interactions that can include parasite changes between hosts belonging to different trophic levels like macrozoobenthos, fish or piscivorous birds or mammals. In this project we will study the macroparasite community of two littoral fish species, perch and ruffe. We will investigate the local parasite community of fish sampled from sites that differ in wave exposition, structural complexity, current velocity and trophic state. To assess the seasonal dynamic of the macroparasite community, some sites will be sampled at different seasons of the year. Additionally we will investigate differences between sites in the occurrence and prevalence of some selected intermediate hosts (freshwater snails) and final hosts (piscivorous birds) of parasite species that were found in perch and ruffe. Parasite species diversity and infection grade will be compared with the individual major histo-compatibility classII (MHII) allele setting. MHII genes belong to the most variable gene families in the vertebrate genome and are involved in the acquired immune response against foreign antigens from parasites and pathogens. We will investigate, if differences in the local parasite community are related to differences in the MHII allele composition of local fish populations. This could indicate local adaptations of the host's immune system to its parasites, leading to genetic divergence between populations. For the genetic analyses, we will use specific MHII primers for perch that we have developed in a previous study. To test the hypothesis that MHII allele composition influences parasite infection, we will artificially infect parasite free perch that express different numbers of MHII alleles, using a combination of 2-4 different parasite species. Six months post infection, we will assess the success of parasite infection and correlate it with the individual MHII allele setting and different selected condition parameters.