Quantitative DNA-methylation in Daphnia magna and effects of multigeneration Zn exposure

Little research on the epigenetic phenomenon of DNA methylation has been performed with invertebrates. However, a few studies have shown that effects of transient chemical exposure can be transferred to non-exposed generations through epigenetic inheritance. If this is a wide-spread phenomenon it may have major consequences for current ecological risk assessments. The presence of DNA methylation in Daphnia magna has only very recently been demonstrated, although not quantified. In the present study, the overall content of 5-methyl-2'-deoxycytidine (5 mdC) in the DNA of D. magna was determined. Additionally, the effect of transgenerational Zn exposure on the 5 mdC content was assessed. An optimized LC-MS-MS method proved to be very suitable for measuring overall levels of 5 mdC in relatively small samples of D. magna DNA. The average [5 mdC]/[dG] ratio ranged from 0.13% to 0.81%. A slight but significant decrease in DNA methylation was found in the offspring (F1) of Zn exposed daphnids in the parental generation. However, this effect did not propagate into the next generation (F2). The presence of DNA methylation in D. magna and the finding that different exposure histories can entail different levels of methylation, open the way for further investigations on the implications of epigenetics in aquatic toxicology.