Evolution of drug-tolerant nematode populations in response to density reduction

Resistance to xenobiotics remains a pressing issue in parasite treatment and global agriculture. Multiple factors may affect the evolution of resistance, including interactions between life-history traits and the strength of selection imposed by different drug-doses. We experimentally created replicate selection lines of free-living Caenorhabditis remanei exposed to Ivermectin at high and low doses to assess whether survivorship of lines selected in drug-treated environments increased, and if this varied with dose. Additionally, we maintained lines where mortality was imposed randomly to control for differences in density between drug-treatments and to distinguish between the evolutionary consequences of drug-treatment vs ecological processes due to changes in density-dependent feedback. After 10 generations we exposed all of the selected lines to high-dose, low-dose and drug-free environments to evaluate evolutionary changes in survivorship as well as any costs to adaptation. Both adult and juvenile survival was measured to explore relationships between life-history stage, selection regime and survival. Intriguingly, both drug-selected and random-mortality lines showed an increase in survivorship when challenged with Ivermectin; the magnitude of this increase varied with the intensity of selection and life-history stage. Our results suggest that interactions between density-dependent processes and life-history may mediate evolved changes in susceptibility to control measures.