Rapid adaptation of Brachionus dorcas (Rotifera) to tetracycline antibiotic stress

•Monogonont populations can rapidly develop increased tolerance to OTC.•Exposure to OTC selects for clones with higher growth rates and lower mictic ratios.•Complete recovery of population growth rates illustrates an example of evolutionary rescue.•OTC selection results in two fitness costs and three fitness gains.•The fitness consequence of OTC selection differs with the measured fitness variables. Although natural populations can rapidly adapt to selection pressures, the fitness consequences of selection are controversial. In this study, a selection experiment was conducted with replicate populations of Brachionus dorcas that were exposed to two sublethal concentrations (26.8 and 78.3 mg/L) of oxytetracycline (OTC), followed by two common garden experiments (population growth and life table experiments). During the 102-day (approximately 36 asexual generations) selection experiment, a markedly increased growth rate but a significantly decreased mictic ratio over time in the populations exposed to OTC when compared to the control populations suggested that the former adapted to the selection pressures and that a trade-off exists between asexual and sexual reproduction. The high and stable population growth rates after 90 days of OTC selection illustrate an example of evolutionary rescue. After 102 days of selection, OTC-selected populations showed higher population growth rates than the control populations when exposed to OTC, indicating significantly increased tolerance. OTC-selected populations showed a lower average growth rate, longer average generation time and life expectancy at hatching, and higher average net reproduction rate and proportion of mictic offspring than the control populations in the absence of OTC, which indicate that OTC selection results in two fitness costs and three fitness gains and that the effect of OTC selection on fitness differs with the measured fitness variables. Both the evolutionary potential of populations under the stress of higher concentrations of OTC and the fitness costs and gains of selection in the absence of OTC indicate that past exposures to pollutants cannot be neglected when evaluating the effects of current stressors on natural populations.