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

A recent study published in Environmental Chemistry and Ecotoxicology has revealed significant differences in how mirror-image forms of a pesticide metabolite behave when transferring from mother fish to their offspring, with important implications for environmental risk assessment. The research focused on o,p'-DDD, a persistent pesticide metabolite that exists in two chiral forms known as enantiomers - molecules that are mirror images of each other but cannot be superimposed, much like left and right hands.

Lead author Lili Niu explained the motivation behind the research, noting that while many pesticides exist in two mirror-image forms, environmental assessments typically treat them as identical. The team investigated whether this assumption holds true, particularly across multiple generations. Their findings demonstrate that the S-enantiomer of 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.

The experimental approach involved feeding adult zebrafish diets containing each form of o,p'-DDD for four weeks. Researchers then measured chemical accumulation in the adults and their developing embryos, while tracking hatching success, deformities, survival rates, and changes in thyroid hormones essential for healthy growth. The results showed that offspring consistently carried higher chemical levels than their parents, indicating highly efficient maternal transfer. Specifically, the S-enantiomer accumulated 134-176% more in adults and over 100% more in their larvae compared to the R-form.

These accumulation differences translated to more severe outcomes in the next generation. The S-DDD exposed groups showed increased mortality, higher rates of malformations, and reduced hatching success. To understand the mechanism behind these effects, the research team used computer-based molecular docking simulations to examine how each form interacts with key proteins involved in producing and regulating thyroid hormones. These simulations revealed that S-DDD binds more strongly to several thyroid-related proteins, providing a mechanistic explanation for its greater biological impact.

The study's findings, detailed in the publication available at https://doi.org/10.1016/j.enceco.2025.10.021, suggest that evaluating only racemic mixtures of chiral pesticides may significantly underestimate real-world environmental hazards. The research demonstrates that even small structural differences in pesticide molecules can lead to substantial differences in accumulation patterns, hormonal effects, and developmental outcomes across generations. This work has important implications for improving ecological risk predictions for long-lasting pollutants and supports the development of more accurate environmental standards that account for enantiomer-specific effects in pesticide regulation and environmental monitoring.