Background. Xeno-oestrogens have negative effects on the endocrine systems of wildlife including freshwater fish. Pendimethalin is a herbicide found in the environment which exerts some oestrogenic action in vivo. Dwarf medaka (known also as Thai medaka), Oryzias minutillus Smith, 1945, inhabit the natural aquatic environments. Dorsal fin of this species is a secondary sex character controlled by sex hormones and assumes as a sensitive bioindicator for testing of oestrogenic chemicals. In this study, we aimed to examine the pendimethalin effects on hormone receptor expressions and dorsal fin biometrics in adult Thai medaka. Materials and Methods. Pendimethalin effects on the androgen receptor (AR) and estrogen receptor (ER) β expressions in dorsal fins were determined by semi-quantitative RT-PCR. The fin biometry was presented as the values (%) of the dorsal fin length (DFL) divided by the standard length (SL). Results. In males, AR levels decreased when the fish were exposed to 0.1 µg · mL–1 pendimethalin for 60 days and were exposed to 1 µg · mL–1 for 30 and 60 days. In females, a 60-day treatment with 1 µg · mL–1 caused AR levels to decrease. Conversely, in males, ERβ levels increased after 30 and 60 days of treatment with 1 µg · mL–1. In females, ERβ levels increased after 30 days of treatment with 1 µg · mL–1 and 60 days of treatment with 0.1 µg · mL–1. In males, values of DFL/SL% decreased after exposure to 1 µg · mL–1 for 60 days. Additionally, we observed 4 testis-ova among 30 gonads from males treated with 1 µg · mL–1 for 60 days. Conclusion. Our results suggest that pendimethalin may interfere with endocrine processes via hormone receptors, leading to the feminisation of dorsal fins and gonads in Thai medaka
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