Can metal stress induce transferable changes in gene transcription in Daphnia magna?

DNA methylation has recently been reported in Daphnia magna, which indicates the possible presence of epigenetic mechanisms regulating gene expression in this species. As such, effects of transient chemical exposure could be transferred through epigenetic inheritance to non-exposed generations. In this study, in the Zn-exposed daphnids, a large number of genes were found to be differentially transcribed, amongst which transcription and translation related genes (downregulated), genes associated with oxidative stress (upregulated) and different types of metabolism-related genes (mostly upregulated). In the two subsequent generations of non-exposed daphnids, a considerable number of differentially regulated genes were observed, indicating an effect of Zn-exposure in the non-exposed progeny. However, none of the differentially transcribed genes observed in the Zn-exposed generation were regulated in the same direction in both non-exposed subsequent generations. The exposure of D. magna to a sublethal Zn concentration for one generation did not result in a stable transgenerational epigenetic effect with consequences for reproductive output nor was a stably epigenetically inheritable effect observed on the transcription of any of the studied genes. An important observation was the large number of genes that were differentially transcribed between different control generations with no pre-exposure history. These genes were not considered in the analysis of the effect of Zn exposure on gene transcription. This differential regulation between subsequent control generations was attributed to possible differences in synchronization of the molting and reproductive cycle of the daphnids in the different generations. This finding is of major importance for the interpretation and design of future microarray experiments with adult Daphnia.