Theory on the evolution of niche width argues that resource heterogeneity selects for niche breadth. For parasites, this theory predicts that parasite populations will evolve, or maintain, broader host ranges when selected in genetically diverse host populations relative to homogeneous host populations. To test this prediction, we selected the bacterial parasite Serratia marcescens to kill Caenorhabditis elegans in populations that were genetically heterogeneous (50% mix of two experimental genotypes) or homogeneous (100% of either genotype). After 20 rounds of selection, we compared the host range of selected parasites by measuring parasite fitness (i.e. virulence, the selected fitness trait) on the two focal host genotypes and on a novel host genotype. As predicted, heterogeneous host populations selected for parasites with a broader host range: these parasite populations gained or maintained virulence on all host genotypes. This result contrasted with selection in homogeneous populations of one host genotype. Here, host range contracted, with parasite populations gaining virulence on the focal host genotype and losing virulence on the novel host genotype. This pattern was not, however, repeated with selection in homogeneous populations of the second host genotype: these parasite populations did not gain virulence on the focal host genotype, nor did they lose virulence on the novel host genotype. Our results indicate that host heterogeneity can maintain broader host ranges in parasite populations. Individual host genotypes, however, vary in the degree to which they select for specialization in parasite populations.

Host susceptibility to parasites can vary over space and time. Costs associated with the maintenance of host defence are thought to account for a portion of this variation. Specifically, trade-offs wherein elevated defence is maintained at the cost of fitness in the absence of the parasite may cause levels of host defence to change over time and differ between populations. In previous studies, we found that populations of the host nematode, Caenorhabditis elegans, evolved greater levels of parasite avoidance and resistance against the bacterial parasite, Serratia marcescens. Here, we passaged these host populations either in the presence or absence of the parasite to test for a cost of elevated host defences. After 16 generations, we found that elevated levels of host defence were maintained during evolution in both the presence and absence of the parasite. Further, this maintenance of defence was not the result of limited standing genetic variation, but rather the absence of a measurable cost associated with defence. Therefore, costs associated with host defence may not broadly account for differences in host susceptibility across space and time.

An updated phylogeny of the clade of toxic Lepiota species is presented, and new insights in the distribution of L. brunneoincarnata and L. subincarnata are given. Lepiota brunneoincarnata is widespread in Europe and temperate Asia, and L. subincarnata is now known from Asia, Europe, and North America. Morphological and anatomical descriptions are provided for these two species based on material from the western Himalayan forests in Pakistan, where they are reported for the first time.

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.

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