Can bystander effects of metals in Daphnia magna be mediated by DNA methylation?

The bystander phenomenon can be described as the detection of effects in non-exposed cells/individuals when these come in contact with exposed ones. Bystander literature has focused mostly on the detection of effects rather than understanding the underlying mechanisms, thus these remain largely unknown from a molecular perspective. Epigenetic mechanisms play a major role in biological responses to environmental stressors, with limited evidence suggesting their involvement in the mechanisms behind bystander effects as well. To gain further insight on this specific topic, we performed bystander experiments with Daphnia magna and an essential (copper) and non-essential (cadmium) metal. In short, metal-exposed organisms were left to co-habit with bystander (not previously exposed) organisms in blank medium for further assessment of gene-specific DNA methylation changes in both groups. In exposed organisms, methylation changes mostly occurred in genes involved in general stress responses and in the offset metal toxicity and oxidative stress. DNA methylation changes were also detected in bystander organisms for both metals, and bystander epigenetic responses targeted specifically the Alpha-ketoglutarate-dependent dioxygenase alkB and Tetraspanin 47 F genes, therefore relating to the cellular signalling of metal stress and the coping with metal-induced DNA damage. Taken together, these insights into the molecular mechanisms underpinning bystander effects contribute to a better understanding of their ecological and ecotoxicological consequences.