Genetic slippage after sex maintains diversity for parasite resistance in a natural host population.

Although parasite-mediated selection is a major driver of host evolution, its influence on genetic variation for parasite resistance is not yet well understood. We monitored resistance in a large population of the planktonic crustacean over 8 years, as it underwent yearly epidemics of the bacterial pathogen . We observed cyclic dynamics of resistance: Resistance increased throughout the epidemics, but susceptibility was restored each spring when hosts hatched from sexual resting stages.