Norgestrel exposure accelerated the abnormal occurrence of germinal vesicle breakdown and triggered damage to cellular structure and function in oocytes of the Pacific oyster (Crassostrea gigas)

Progestins are ubiquitous in aquatic environments, with adverse effects on aquatic organisms' reproduction even at extremely low concentrations (0.8 ng/L). As female germ cells, oocytes play an irreplaceable role in organismal reproductive processes. However, the toxic mechanism of norgestrel (NGT), a typical progestogen, on oocytes of aquatic invertebrates remains unclear. This study thus investigated the toxic effects and mechanisms of 10 ng/L and 1000 ng/L NGT exposure (30 minutes) on oocytes of Crassostrea gigas from histological and molecular biological perspectives. Results showed that NGT exposure promoted abnormal germinal vesicle breakdown (GVBD), accompanied by cell membrane damage and vacuolation. Activation of the GHSR1-AKT-H2A cascade signaling pathway might be a potential driver of GVBD induction. Meanwhile, reduced expression of the cathepsin L gene suggested that while NGT promotes abnormal GVBD, it may also impair oocyte developmental capacity. Additionally, NGT disrupted metabolic processes including glycerophospholipid metabolism, mitochondrial energy metabolism, and autophagy—changes potentially linked to NGT-induced cell membrane damage and mitochondrial dysfunction. Notably, the 1000 ng/L NGT group exerted stronger toxic effects than the 10 ng/L NGT group, a difference possibly associated with reduced antioxidant capacity and detoxification function in the high-concentration group. Correlation analysis revealed that oocytes exist a non-dose-dependent conserved biological response to NGT exposure, with key genes such as H2A and their highly correlated metabolites serving as potential 'epigenetic-metabolic' dual markers. In conclusion, this study demonstrates that NGT exposure accelerates abnormal GVBD in oocytes, accompanied by histological damage and metabolic dysfunction. These findings provide theoretical basis and new targets for assessing the environmental toxicity and ecological risks of progestins.