Abstract: 【Aim】 To screen suitable reference genes for studying the expression levels of CYP392E subfamily genes in susceptible (SS), avermectin-resistant (Av-R) and spirotetramat-resistant (Sp-R) strains of Tetranychus urticae. 【Methods】 The mRNA expression stability of six house keeping genes, i. e.,  α-tubulin, β-actin, ELFn, GAPDH, 5.8S rRNA gene and SDHA of T. urticae was detected by quantitative real-time PCR (qRT-PCR), and the relative expression level of CYP392E subfamily reference genes of cytochrome P450 monooxygenases (P450s) was analyzed. 【Results】 ELFn was the most stable reference gene in SS, Av-R and Sp-R strains of T. urticae. When ELFn was used as the reference gene, the expression level of CYP392E7 in Av-R strain (2.18-fold as high as that in SS strain) was significantly higher than that in SS strain, while there was no significant difference between Sp-R strain and SS strain. However, the relative expression levels of other genes did not increase, and even those of CYP392E4, CYP392E9 and CYP392E10 in Av-R strain and CYP392E1 and CYP392E9 in Sp-R strain decreased significantly (P<0.05) (48%, 74%, 65%, 63% and 73% of the expression level in SS strain, respectively). 【Conclusion】 ELFn could be used as an ideal reference in SS, Av-R and Sp-R strains of T. urticae. The significant change of the expression levels of CYP392E4, CYP392E7, CYP392E9 and CYP392E10 in Av-R strain may be related with the resistance of T. urticae to avermectin; CYP392E1 and CYP392E9 in Sp-R strain may also be related with the resistance of T. urticae to spirotetramat.

Key words:  Tetranychus urticae, avermectin, spirotetramat, resistant strain, susceptible strain, cytochrome P450 monooxygenases (P450s), CYP392E subfamily, qRT-PCR, reference gene, expression level