The study of ecological immunology, which examines variations in parasite infectivity and host immunity as a result of ecological and evolutionary processes, is one of the fastest growing fields of organismic biology. Parasitism may explain various biologi cal phenomena, such as the widespread occurrence of sexual versus asexual reproduction, sexual selection, the evolution of virulence, and the maintenance of genetic diversity. Pathogens and infectious diseases have also become a major threat to wildlife co nservation and endangered species, by influencing host genetic diversity and altering species composition. Yet this very same coevolution-driven genetic diversity may buffer natural populations against widespread epidemics. Understanding the genetic and mo lecular basis of rapid, antagonistic host-parasite coevolution is of great importance, given that the vast majority of invertebrate species rely exclusively on innate immunity and do not possess an acquired immune system. The genetic approach is based on t wo conflicting models: the gene-for-gene (GFG) model of plant pathologists and the matching-alleles (MA) model of invertebrate zoologists. Using genetically-identifiable clones of Daphnia magna and its microparasite Pasteuria ramosa, we will perform a seri es of cross-infection experiments to estimate whether infection success is inherited by few genes with strong effects, or as a quantitative trait with many genes being involved. We will also look for host-mediated selection on parasites, to assess whether infection patterns correspond to the traits under selection during natural selection. These experiments will allow us to distinguish between the GFG and the MA models. The molecular approach relies on the interactions between host receptors and parasite el icitors. Using monoclonal antibodies to detect adhesin molecules (polypeptides), we will use adhesin variation as a measure of parasite virulence and polymorphism across isolates.
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