Are 20-hydroxyecdysone and related genes potential biomarkers of sublethal exposure to lipid-altering contaminants?

In Daphnia magna, 20-hydroecdysone (20E) is the main molting hormone and its metabolism is of interest to identify new biomarkers of exposure to contaminants. The present study aimed to (i) assess baseline levels of 20E and transcription levels of four related-genes (shade, neverland, ultraspiracle, and ecdysteroid receptor); and (ii) evaluate effects in D. magna after 21 days of exposure to fenarimol (anti-ecdysteroid) and a mixture of gemfibrozil and clofibric acid (lipid-lowering drugs) at sublethal concentrations. Endpoints included transcription of the target genes and quantification of 20E, mortality, and reproduction of daphnids. Baseline results showed that average responses were relatively similar and did not vary more than 2-fold. However, intra-day variation was generally high and could be explained by sampling individuals with slightly different stages of their development. Exposure tests indicated a significant decrease in daphnid reproduction following chronic exposure to a concentration of 565 μg/L of fenarimol. However, no difference was observed between the control and exposed groups for any of the investigated genes, nor for the levels of 20E after 21 days of exposure. Following exposition to gemfibrozil and clofibric acid at 1 μg/L, no changes were observed for the measured parameters. These results suggest that changes in transcription levels of the target genes and concentrations of 20E may not be sensitive endpoints that can be used as biomarkers of sublethal exposure to the target compounds in D. magna. Measuring multiple time points instead of a single measure as well as additional molecular endpoints obtained from transcriptomic and metabolomic studies could afford more insights on the changes occurring in exposed daphnids to lipid-altering compounds and identify efficient biomarkers of sublethal exposure.