"The study of parasitism is important for human welfare but also addresses basic scientific questions cutting across biological disciplines. Traditionally, studies of host-parasite interactions have used various scenarios of how genotypes of the two parties interact based on well-developed theory. Yet, one of the most striking observations is the huge variation in resistance systems across organisms. We lack an appreciation of the diversity of these resistance systems and how they affect, in turn, the parasites. Here, it is proposed to investigate the hypothesis that variation in gene expression is a key element that defines an alternative, flexible and highly adaptable resistance system. The project attempts to unify genomic studies of defence mechanisms with questions of evolutionary ecology of host-parasite interactions. Therefore, the study focuses on an ecologically well-studied system of hosts (Bombus spp.) and their prevalent trypanosome infections (Crithidia). In this system, a highly genetically polymorphic parasite is kept in check by a host with seemingly conserved immune effectors (e.g. anti-microbial peptides, AMPs). Their expression varies depending on the host-parasite pairing, suggesting that variation of the synergistic mixture of expressed defence elements might be crucial and could affect parasite population structure in turn. In the project, experiments and cutting-edge molecular methods will be applied to a natural host-parasite system."