The development of genetically inherited resistance to zinc in laboratory-selected generations of the earthworm Eisenia fetida

The capacity of species to adapt both physiologically and genetically to contaminants may allow populations to persist in polluted environments. Such ‘adaptation’ can have important implications for risk assessment, since it may mean that prediction based on extrapolation of toxicity studies with naı̈ve populations may prove invalid for long-term contaminated sites. To investigate the evolution of zinc resistance in Eisenia fetida, worms from a previously unexposed population (parent) were selected and reared over two generations (F1, F2) while exposed to zinc in the laboratory. Relative sensitivities of unexposed and selected generations were then compared by exposing parent, F1 and F2 individuals to zinc in contact filter papers tests. Calculation of effect concentrations from this work indicated differences in sensitivity to zinc for successive generations, with higher toxicity values (LC50, LC90, LC99) found for the selected worms. The increases in resistance found for F1 and F2 worms were confirmed in a discriminating dose study. In addition to comparing the sensitivities of the parent, F1 and F2 generations for zinc, toxicity tests were also conducted with copper to assess if there was evidence of cross-resistance between the two metals. Results indicated similar increases in resistance to copper to those found for zinc. Mechanisms underlying the increased metal resistance were studied in toxicokinetic experiments. Results indicated no clear trends between the three generations indicating that physiological responses, other than differences in kinetic parameters, are responsible for the increased resistance found in the selected worms.