Host genotype and genetic diversity shape the evolution of a novel bacterial infection

Pathogens continue to emerge from increased contact with novel host
species. Whilst these hosts can represent distinct environments for
pathogens, the impacts of host genetic background on how a pathogen
evolves post-emergence are unclear. In a novel interaction, we
experimentally evolved a pathogen (Staphylococcus aureus) in populations
of wild nematodes (Caenorhabditis elegans) to test
whether host genotype and genetic diversity affect pathogen
evolution. After 10 rounds of selection, we found that pathogen virulence
evolved to vary across host genotypes, with differences in host
metal ion acquisition detected as a possible driver of
increased host exploitation. Diverse host populations selected for the
highest levels of pathogen virulence, but infectivity was constrained,
unlike in host monocultures. We hypothesize that population
heterogeneity might pool together individuals that contribute
disproportionately to the spread of infection or to enhanced
virulence. The genomes of evolved populations were sequenced, and
it was revealed that pathogens selected in distantly-related host
genotypes diverged more than those in closely-related host
genotypes. S. aureus nevertheless maintained a broad
host range. Our study provides unique empirical insight into the
evolutionary dynamics that could occur in other novel infections of
wildlife and humans.