This study experimentally evolved *Staphylococcus aureus* in wild nematode (*Caenorhabditis elegans*) populations to examine the impact of host genotype and genetic diversity on pathogen evolution. Results showed that pathogen virulence varied across host genotypes, potentially driven by differences in host metal ion acquisition. Diverse host populations selected for higher pathogen virulence but constrained infectivity, unlike monocultures. Genome sequencing revealed that pathogens selected in distantly-related host genotypes diverged more than those in closely-related genotypes, yet *S. aureus* maintained a broad host range. The findings offer insights into the evolutionary dynamics of novel infections.

Alice K. E. Ekroth, Michael Gerth, Emily J. Stevens, Suzanne A. Ford, Kayla C. King