Study Reveals Pesticide Metabolite's Mirror-Image Forms Differ in Maternal Transfer and Thyroid Effects

A new study published in Environmental Chemistry and Ecotoxicology reveals that mirror-image forms of a persistent pesticide metabolite behave differently when transferring from mother fish to their offspring, with significant implications for environmental risk assessment. The research demonstrated that S-o,p'-DDD accumulated preferentially in adult zebrafish and transferred more efficiently to their offspring compared to the R-enantiomer, leading to pronounced developmental defects and endocrine disruption across both generations.

Lead author Lili Niu explained the motivation behind the research: Many pesticides exist in two mirror-image forms, but environmental assessments usually treat them as if they're the same. We wanted to know whether that assumption is actually safe, especially across generations. The researchers fed adult zebrafish diets containing each form of o,p'-DDD for four weeks, then measured chemical accumulation in adults and transfer to developing embryos while tracking hatching success, deformities, survival, and changes in thyroid hormones.

The findings showed that offspring consistently carried even higher chemical levels than their parents, demonstrating highly efficient maternal transfer. The S-enantiomer accumulated 134-176% more in adults and over 100% more in their larvae than its mirror-image R-form. This led to more severe outcomes in the next generation, including increased mortality, malformations, and reduced hatching success in the S-DDD exposed groups. Molecular docking simulations against key thyroid-related proteins provided a mechanistic explanation for this stereospecific toxicity, showing that S-DDD binds more strongly to several proteins involved in producing and regulating thyroid hormones.

What surprised us most was how consistently the S-form caused stronger effects at every level we tested, Niu noted. A small structural difference in the molecule led to very large differences in how much accumulated in the fish, how it affected their hormone system, and how their offspring developed. The team emphasized that understanding enantiomer-specific effects will help improve ecological risk predictions for long-lasting pollutants and support the development of more accurate environmental standards. If we ignore these differences, we risk underestimating long-term harm to wildlife, Niu added. Our work shows that even very low exposure in parents can create meaningful risks for the next generation. The complete study is available at https://doi.org/10.1016/j.enceco.2025.10.021.