Parasites Mediate Condition-Dependent Sexual Selection for Local Adaptation in a Natural Insect Population

ABSTRACT Condition-dependent sexual selection has been suggested to reduce mutation load, and sexual selection might also accelerate local adaptation and promote evolutionary rescue through several ecological and genetic mechanisms. Research on condition-dependent sexual selection has mainly been performed in laboratory settings, while data from natural populations are lacking. One ecological factor that can cause condition-dependent sexual selection is parasitism. Here, we quantified ectoparasite load ( Arrenurus water mites) in a natural population of the common bluetail damselfly ( Ischnura elegans ) over 15 years. We estimated parasite-mediated sexual selection in both sexes and investigated how parasite resistance and tolerance changed over time and influenced population density. Parasites reduced mating success in both sexes, but the effects were stronger in males than in females. The male fitness advantage of carrying fewer parasites was higher under experimental low-density conditions than under high-density field conditions, suggesting that male-male competition could reduce parasite-mediated sexual selection. We further show that population density declined during the study period, while parasite resistance and male fitness tolerance (fecundity) increased, suggestive of increasing local adaptation against parasites and ongoing evolutionary rescue. We suggest that condition-dependent sexual selection can facilitate population persistence and promote evolutionary rescue by increasing local adaptation against parasites.