Keywords: 
• prochloraz /
• carbendazim /
• zebrafish /
• joint toxicity

Abstract: Although the aquatic organisms are exposed to multiple chemicals more often than single chemical, the combined toxicity are not considered by the present pesticide regulations during risk assessment. Prochloraz and carbendazim are used widely in vegetables and fruits in China. In order to evaluate the combined toxicity of prochloraz and carbendazim, zebrafish was taken as the subject organism, and the acute toxicity and the influence on its key genes of the thyroid axis of zebrafish larvae were determined by static method and real-time fluorescence quantitative PCR method. The results of acute toxicity tests showed, the 96 h-LC₅₀ values of prochloraz and carbendazim on zebrafish embryo were 8.41 mg·L^-1 and 0.81 mg·L^-1 respectively, which were medium and high toxicity. The combination of prochloraz and carbendazim presented antagonism effects during 24-96 h. The results of gene expression indicated that prochloraz inhibited the expression of thyroid-stimulating hormone releasing hormone (CRH), thyroid-stimulating hormone (TSH), thyroid hormone receptor (TRα), and deiodinase (D1, D2) genes and there were no significant changes in the expression of thyroid hormone transporter (TTR) gene. Carbendazim inhibited the expression of CRH, TSH, D1 and D2 genes. Combined exposure (low and medium dose) inhibited the expression of CRH, TSH, D1 and D2 genes in zebrafish. Compared with single agent, the high dose combined group enhanced the expression of TSH, D1 and D2 genes, and the medium dose combined group promoted the expression of TRα gene. In conclusion, the combined effects of prochloraz and carbendazim could influence the early development of zebrafish larvae, and there were differences in gene interference on thyroid axis of zebrafish compared with single dose. Therefore, the results suggested that the combined effects should be considered during the risk assessment of pesticide management.