Molecular and metabolic mechanisms of transgenerational effects in Daphnia exposed to radionuclides

International audienceUnderstanding how radioactive contaminants affect wildlife species at every level of biological organization is a major challenge in radioecology. Mechanistic links among observed effects are necessary to predict consequences for survival, growth and reproduction which are critical for population dynamics. With its short life cycle, the cladoceran Daphnia magna is particularly suitable to address toxicity over several generations, on a much more relevant time scale to natural biota. Multigenerational toxicity tests were conducted with depleted uranium (U), americium-241 (Am-241) and cesium-137 (Cs-137), inducing respectively a chemical toxicity, an internal alpha radiotoxicity and an external gamma radiotoxicity. Experimental results showed in all cases that toxic effects on survival, body size, fecundity increased in severity across generations, demonstrating that measured effects in one generation might not be representative of toxicity in offspring generations, and ultimately of population response.Reduction in body size and fecundity induced by depleted U, Cs-137 and Am-241 were analyzed using DEBtox models with respectively internal concentration, external gamma dose rate and internal alpha dose rate as dose metrics. For each radionuclide, a combination of several modes of action was necessary to explain observed effects. A damage compartment with hereditary damage level was introduced in order to explore possible modes of action associated with the increase in toxicity across generations. Modelling was performed using a Bayesian framework, in order to quantify uncertainty in parameter estimations and model predictions. Studies of DNA alterations, using a qPCR technique, confirmed that molecular damage was accumulated in daphnids exposed to depleted U and Cs-137 and transmitted to their progeny, as a possible underlying mechanism causing the increase in effect severity over generations. Further studies of DNA methylation recently investigated the role of epigenetic processes in the transmission of effects from parents exposed to Cs-137 to their unexposed progeny