Increased Virulence and Large-Scale Reduction in Genome Size of Tetraploid Evolved in Nematode Hosts.

is an opportunistic fungal pathogen of humans, yet the within-host dynamics of infection are not clear. While is commonly diploid, it exhibits a range of ploidies, including tetraploidy. Previous work found that tetraploid populations exhibited rapid adaptation and significant genome instability when evolved . Host immune function alters the rate and magnitude of virulence evolution, but the effects of the host immunity on tetraploid populations are unclear. Here, we tested the effects of the host immunity on genome stability and virulence evolution of tetraploid using experimental evolution. We selected for increased virulence within either immunocompetent or immunocompromised hosts. After nine passages we observed a response to selection for increased virulence. Both populations exposed to either immunocompetent or immunocompromised hosts increased virulence after passage through hosts. However, the populations passaged through immunocompetent hosts under selection exhibited unique temporal dynamics, a rapid increase in virulence and then subsequent loss of virulence. Most populations exhibited genome size reduction within six passages, however populations exposed to immunocompetent hosts exhibited the most rapid transition to ~diploid. Therefore, we found that tetraploids rapidly increase in virulence and decrease genome size within host environments. Further, the combination of selection for greater virulence in the presence of immunocompetent hosts results in major virulence fluctuations and genome size changes. Thus, host immunity significantly impacts the evolutionary trajectories of tetraploid .