The subunit gene Ld alpha 1 of nicotinic acetylcholine receptors plays important roles in the toxicity of imidacloprid and thiamethoxam against Leptinotarsa decemlineata

摘要

Nicotinic acetylcholine receptors (nAChRs) are pentameric ACh-gated ion channels. It is believed that nAChRs composed of different subunits may vary in their function and toxicological characteristics. Neonicotinoids are activators of nAChRs and important insecticides that are extensively used for crop protection and resistance has been developed by some pests. They are also major insecticides for the control of Leptinotarsa decemlineata, which is a destructive defoliator pest that invaded the Xinjiang region of China in the 1990s. However, little is known about the constitution or subunits of the target in this pest. In this study, the full-length cDNAs encoding four new nAChR subunits (named Ld alpha 3, Ld alpha 6, Ld alpha 10, and Ld beta 1) were cloned from L. decemlineata. These genes encode 822-, 753-, 672-, and 759-amino acid proteins, respectively, which share typical features of insect nAChRs subunits and closely resemble the corresponding subunits of the nAChRs from Tribolium castaneum. Temporal and spatial expression analyses showed that these genes, as well as the previously identified Ld alpha 1, Ld alpha 2, and Ld alpha 8 genes, are widely expressed in all developmental stages, including eggs, larvae of various instars, pupae, and adults. All genes monitored were expressed at higher levels in the head than in the thorax and abdomen, except for Ld alpha 10. Dietary ingestion of double-stranded RNA bacterially expressed for Ld alpha 1 (dsLd alpha 1) significantly reduced the mRNA level of Ld alpha 1 in treated larvae and adults by 48.0% and 78.6%, respectively. Among the non target genes, Ld alpha 3, Ld alpha 9, and Ld beta 1 were significantly up-regulated in larvae. A toxicity bioassay showed that dsLd alpha 1 treatment greatly decreased the sensitivity to imidacloprid and thiamethoxam in adults. The larval susceptibility to thiamethoxam but not to imidacloprid was also reduced because of the lower down-regulation of Ld alpha 1. Thus, our results suggest that Ld alpha 1 encodes a subunit of a functional nAChR that mediates the toxicity of imidacloprid and thiamethoxam against L. decemlineata and that the down-regulation of Ld alpha 1 might be an important mechanism for resistance and/or tolerance of L. decemlineata to neonicotinoids. (c) 2015 Elsevier Inc. All rights reserved.