| Daphnia is a commonly used test organism in toxicology, ecology, and evolution. HR96 is a CAR/PXR/VDR ortholog in invertebrates, and a promiscuous endo- and xenobiotic nuclear receptor involved in acclimation to toxicants. Daphnia HR96 is activated by chemicals such as atrazine and linoleic acid (LA) (n-6 fatty acid), and inhibited by triclosan and docosahexaenoic acid (DHA) (n-3 fatty acid). HR96 is also a key regulator of Magro, Niemann Pick genes, and other genes involved in cholesterol and triacylglycerol uptake, metabolism, and transport. We hypothesized that Daphnia magna exposed to HR96 activators will increase their lipid resources and in turn either develop a tolerance towards starvation or demonstrate increased reproduction relative to untreated animals or animals exposed to HR96 inhibitors, and this phenotype would be associated with the retention or production of certain lipids. Exposure of late adolescent daphnids to fatty acids (DHA and LA) reduced the starvation survival of their offspring. However, DHA exposed juvenile daphnids typically survived following reproduction during starvation indicating greater nutrient stores and overall health of these animals than the other treated groups. Interestingly, the HR96 inhibitor, triclosan, significantly increased offspring survival compared to all other groups except the untreated daphnids. Additional experiments demonstrated that juveniles exposed directly to triclosan also showed increased survival but did not mature and therefore never reproduced. Lipidomic analysis comparing starved adults and neonates showed that neonates have relatively more sphingomyelins and relatively less phosphatidylcholine than adults. The HR96 inhibitors, DHA and triclosan significantly increased sphingomyelins, and DHA down-regulated magro and a sphingomyelinase; two HR96-regulated genes crucial in producing sphingosine from the sphingomyelins. It is interesting to speculate that a drop in sphingosine, a key developmental lipid, may be associated with greater longevity and slow/no development. In conclusion, fatty acids and toxicants can cause sublethal effects by altering the resource allocation of lipids and in turn perturbing starvation survival, reproduction, and juvenile development. |
