Discovery and evaluation of cadmium-adapted Daphnia pulex genotypes in a region of historical mining reveals adaptation protects the germline from cadmium-induced mutations

Exposure to chemical pollutants can alter the rate, and genome-wide distribution of germline mutations. However, studies measuring the effect of chemical exposure on mutation rate and spectra have not considered the ecological and evolutionary backgrounds of studied genotypes, which could influence the rates and patterns of germline mutations in altered environments, e.g., chemical pollution. Utilizing a study of natural Daphnia pulex populations, we conducted a comprehensive experiment to test our hypothesis that adaptation to chemical pollution also protects the germline from mutagenesis. We, 1) identified Daphnia pulex populations that have adapted to live in mining-devastated regions by increasing their cadmium tolerance. 2) We completed a mutation-accumulation (MA) experiment with an adapted genotype to measure the germline mutation rate in both control conditions and an environmentally relevant cadmium concentration. 3) We compared these MA experiment results to a previously reported, identically designed MA experiment with a nonadapted genotype. We report that patterns of cadmium-induced mutagenesis in the adapted genotype were reversed compared to our previous observations in the nonadapted genotype. Cadmium exposure altered the single nucleotide mutation (SNM) rate in the same genome regions in adapted and nonadapted genotypes, but the rates were changed in opposite directions. Cadmium also altered specific SNM classes in these genotypes in opposite directions. The reversal of mutational trends in the adapted genotype suggests protection against cadmium genotoxicity. We further demonstrate that adapted populations have elevated gene copy-number and expression levels of metallothionein, the protein that protects against cadmium toxicity by binding to cadmium irreversibly.