Possible Enzymatic Mechanism Underlying Chemical Tolerance and Characteristics of Tolerant Population in Scapholeberis Kingi

Abstract

To determine the potential effects of pesticides on aquatic organisms inhabiting a realistic environment, we explored the characteristics and mechanisms of chemical tolerance in Scapholeberis kingi. We established a chemical-tolerant population via continuous exposure to pirimicarb, an acetylcholinesterase (AChE) inhibitor, and examined the effects of pirimicarb concentration on the intrinsic growth rates (r) of tolerant cladocerans. We also explored the association between r and feeding rate and tested the involvement of antioxidant enzymes [peroxidase (PO) and superoxide dismutase] and AChE in pirimicarb sensitivity. S. kingi was continuously exposed to sublethal pirimicarb concentrations (0, 2.5, 5, and 10 µg/L) for 15 generations and changes (half maximal effective concentration at 48 h, 48 h-EC50) in chemical sensitivity were investigated. In the F14 generation, the sensitivity of the 10 µg/L group was three times lower than that of the control group, suggesting the acquisition of chemical tolerance. Moreover, r was significantly and negatively correlated with 48 h-EC50, suggesting a fitness cost for tolerance. Surprisingly, there was no significant correlation between r and feeding rate. Our generalized linear model indicated that elevated PO activity may be related to chemical tolerance. Therefore, oxidative stress regulation may be involved in the acquisition of chemical tolerance in cladocerans. These findings will help elucidate the characteristics and mechanisms of chemical tolerance in aquatic organisms inhabiting a realistic environment.