Distinct roles of two RDL GABA-receptors in fipronil action in the diamondback moth (Plutella xylostella)

The phenylpyrazole insecticide, fipronil, blocks insect RDL {gamma}-aminobutyric acid (GABA) receptors, thereby impairs inhibitory neurotransmission. Some insect species, such as the diamondback moth (Plutella xylostella), possess more than one Rdl gene. The involvement of multiple Rdls in fipronil toxicity and resistance remain largely unknown. In this study, we investigated the roles of two Rdl genes, PxRdl1 and PxRdl2, from P. xylostella in the action of fipronil. Expressed in Xenopus oocytes, PxRDL2 receptors were 40-times less sensitive to fipronil than PxRDL1. PxRDL2 receptors were also less sensitive to GABA compared to PxRDL1. Knockout of the fipronil-sensitive PxRdl1 gene reduced the potency of fipronil by 10 fold, whereas knockout of the fipronil-resistant PxRdl2 gene enhanced the potency of fipronil by 4.4 fold. Furthermore, in two fipronil-resistant diamondback moth field populations, the expression of PxRdl2 was elevated by 3.7-fold and 4.1-fold, respectively compared to a susceptible strain, whereas the expression of PxRdl1 was comparable among the resistant and susceptible strains. Collectively, our results indicate antagonistic effects of PxRDL1 and PxRDL2 on the fipronil action in vivo and suggest enhanced expression of fipronil-resistant PxRdl2 potentially a new mechanism of fipronil resistance in insects.