Epigenetic modifications as a molecular mechanism for transgenerational effects in Daphnia exposed to ionising radiation

International audience; In order to predict long term consequences of pollution on wildlife, toxic effects have to be assessed on a multigenerational scale. DEBtox models were recently developped to analyze effects of alpha and gamma radiation over several generations of Daphnia magna. A transgenerational damage compartement which level is transmitted from parent to offspring, was used to account for changes in effect severity observed among generations. However, the nature of molecular mechanisms involved in the transmission of radio-induced damage accross generations are poorly understood. In this context, the role of epigenetic processes was recently investigated in D. magna, as part of the transgenerational response to external Cs-137 gamma irradiation (at dose rates of 6.5 µGy/h and 41.3 mGy/h). In this aim, we studied DNA methylation changes after a 25-day chronic exposure of a parental generation (F0) and their potential inheritance by subsequent recovering generations, including F2 (exposed as germ cells in F1 embryos) and F3 (the first truly unexposed generation). Effects on survival, growth and reproduction were examined in all generations and DNA was extracted for whole genome bisulfite sequencing. Results showed that effects on reproduction were significant in generation F0 at the high dose rate and absent in subsequent generations (F1, F2 and F3). In contrast, we detected significant DNA methylation changes in every generation independent of dose rate. Some of these changes were shared between dose rates and among generations. Involved genes included gene families and gene functions which were previously shown to play roles during exposure to ionising radiation. Common DNA methylation changes detected between generations F2 and F3 clearly showed that epigenetic modifications were transmitted to unexposed generations, most likely through the germline.