The evolution of parasite host range in genetically diverse host populations

Parasites vary enormously in their host range. Why are some parasites specialists and others generalists? We tested the hypothesis that genetically diverse host populations select for parasites with broader host ranges than genetically homogeneous host populations. To do so, we selected for increased killing ability of the bacterial parasite Serratia marcescens in populations of the host Caenorhabditis elegans that were either diverse (50% mix of genotypes N2 and LTM1) or homogenous (100% N2 or LTM1). We found mixed support for the hypothesis. After 20 generations of selection, parasites selected in diverse host populations maintained a broad host range, as shown by the retention of high killing ability of a novel host genotype. Parasites selected in diverse populations killed N2 hosts equally well as parasites selected in homogenous populations of N2. However, N2-selected parasites lost killing ability against the novel host, consistent with the evolution of narrow host ranges in homogenous environments. In contrast, parasites selected in homogenous LTM1 populations did not specialize: they did not increase in their killing ability on LTM1 and did not lose killing ability against the novel host. Our results argue that the evolution of host range depends upon both the identity and diversity of hosts that a parasite encounters.