Monocrotophos (MCP) is an organophosphorus pesticide moderately toxic to fish, and it has significant estrogenic properties in vivo. In this study, zebrafish (Danio rerio) were exposed to 0.001, 0.010, and 0.100 mg/L 40% MCP pesticide in a semi-static manner from fertilization to 40 days post-hatching. Histological analyses were performed to determine whether sex differentiation in zebrafish was affected by MCP, and the mRNA expression levels of genes involved in sexual differentiation were quantified by real-time PCR to clarify the possible mechanism(s) of action. The results revealed a prominent increase in the proportion of females (71%) in the 0.100 mg/L MCP pesticide treatment as well as the presence of one intersex individual in each of the groups exposed to 0.001 and 0.100 mg/L MCP. MCP exposure stimulated forkhead transcription factor gene L2 (foxl2) expression and suppressed doublesex/mab-3 related transcription factor 1 (dmrt1) expression, indirectly leading to elevated gonadal aromatase (cyp19a1a) gene expression, which should promote phenotypic feminization. In addition, MCP treatment increased the transcription of brain aromatase (cyp19a1b), resulting in an indirect impact on sexual differentiation. The results from this investigation can be used for risk and hazard assessment of MCP pesticide.
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