Functional Analysis Reveals Glutamate and Gamma-Aminobutyric Acid-Gated Chloride Channels as Targets of Avermectins in the Carmine Spider Mite

The carmine spider mite, Tetranychus cinnabarinus (Boisduval), is one of the most important pests in the agricultural industry. Avermectins, as efficient acaricides, have been extensively used in the mite control. The targets for avermectins in mammals, nematodes, and insects had already been identified as gamma-aminobutyric acid and glutamate chloride channels. However, the targets for avermectins in mites are still not clear. Here, we report the cloning of 5 full-length glutamate-gated chloride channel genes (TcGluCls) and 3 gamma-aminobutyric acid chloride channel genes (TcGABACls) and their expression levels in different developmental stages in T. cinnabarinus. All 8 genes were successfully silenced in 2 sets using RNAi. One set was made of 5 TcGluCls, and the other set was composed of 3 TcGABACls. Compared with the control, after gene silencing the mortalities of mites in both sets decreased significantly when treated with 2 concentrations of abamectin, lethal concentration (LC30) and LC70. Functional expression in Xenopus laevis oocytes and electrophysiological assays showed that these candidate targets are mediated by L-glutamate and GABA, respectively. The electrophysiological assays showed all 5 TcGluCls are sensitive to abamectin and ivermectin. Nevertheless, 3 TcGABACls are unresponsive to the same doses of these 2 acaricides. Our data suggested that TcGluCls are targets for abamectin and ivermectin. TcGABACls could not be activated by abamectin or ivermectin under current experimental conditions. However, the result of RNAi indicated that downregulation of TcGABACls decreased acaricidal activity of abamectin, suggesting that TcGABACls could not be excluded completely as targets for abamectin in mites.