Evolutionary and ecological perspectives on epidemiological traits in helminth infections of sticklebacks
The interaction of an organism with its environment is a hallmark of life and pre-requisite for natural selection. Among the strongest evolutionary processes is the interaction between hosts and parasites. Their antagonistic co-evolution of host defence and parasite exploitation is shaped through both genotypes, local environmental conditions, and their potential for plastic responses. However, the relative contribution of these effects is often unclear. Here, I aimed to find answers to the questions how and why epidemiological traits vary among populations by using hosts and parasites from geographically distinct and ecologically divergent populations. I used three-spined sticklebacks (Gasterosteus aculeatus) as vertebrate model organisms to study defence mechanisms against helminth parasites. Helminths are of exceptional interest because they can have complex immune modulatory effects on their hosts. This phenomenon is already applied in clinical settings, where helminths, their ova, or their products are used to treat autoimmune or inflammatory disorders (helminth therapy). Nevertheless, many questions on the specificity of the host-helminth interaction have yet to be answered. Incorporating evolutionary and ecological perspectives, I found that different strains of the same cestode species (Schistocephalus solidus) had profoundly different effects on divergent G. aculeatus types. This effect was linked to the co-evolutionary history and ecology of G. aculeatus and S. solidus. The infection outcome was largely determined by effects of host and parasite genotypes, while interaction effects were generally weak and only evident over the scale of continents. Moreover, parasite-induced phenotypic plasticity transcended host genetic differences. By revealing the dominant effect of the parasite and the relative importance of induced plasticity, this thesis advances our understanding about the role of each partner in a host-parasite interaction. I propose to incorporate such evolutionary and ecological perspectives in future research of helminth therapy.