Temporal expression profiles of Daphnia magna genes after ibuprofen exposure. Shown are the temporal expression profiles of D. magna (<24 hours old) genes after 2 to 48 hours of exposure to ibuprofen (mean ± standard error). Gene expression was measured using quantitative PCR. Dotted and solid lines represent control and exposed (80 mg/l ibuprofen) expression, respectively. Target gene expression was calculated using DART-PCR  and normalized to a geNorm  estimated normalization factor based on the geometric mean of Act (actin), GAPDH (glyceraldehyde-3-phosphate dehydrogenase) and UBC (ubiquitin conjugating enzyme). Exposed expression levels are shown relative to controls at corresponding duration of exposure (note the different y-axes). The temporal 'gray zones' generally reveal fluctuating expression patterns perhaps reflecting homeostatic instability. Asterisks (*) denote a significant (P < 0.05, Student's t-test) difference from controls. The Lip (which encodes triacylglycerol lipase), CTP (choline-phosphate cytidylyltransferase), Ltb4dh (leukotriene B4 12-hydroxydehydrogenase) and COX (cycloxygenase) genes are related to lipid metabolism, whereas RXR (retinoid × receptor), JHE (juvenile hormone esterase), DmagVTG1 (vitellogenin 1), VMO1 (vitelline outer layer membrane protein 1), and Cht (chitinase) are associated with signal transduction and endocrine functions.