Theoretically the expression profile of GluCl
Theoretically, the expression profile of GluCl variants was related to their physiological roles. It was reported that all the three variants (GluCl A, GluCl B, GluCl C) were highly expressed in head tissue of L. striatellus and M. domestica, and the expression level of GluCl A was significantly higher than GluCl B and GluCl C (Wu et al., 2017; Kita et al., 2014). Furthermore, LsGluCl A showed high expression level in L. striatellus leg while MdGluCl C was highly expressed in M. domestica leg. In this study, the three CsGluCl variants were predominantly expressed in nerve cord and LiCl what tissues of C. suppressalis. CsGluCl C also had high expression level in cuticle. While CsGluCl A and CsGluCl B showed similar temporal expression patterns, the expression levels of CsGluCl C dramatically increased in late pupae and early adult stages. These results indicated that CsGluCl A and CsGluCl B may function in neural transmission, while CsGluCl C has multiple physiological roles in C. suppressalis. RNAi technologies have been applied in the field of insect toxicology to identify or validate genes encoding insecticide target proteins (Kim et al., 2015). In this study, both injection and oral delivery of dsGluCl lead to <50% RNAi efficiencies, which is consistent with relatively low RNAi efficiencies observed in Lepidopteran species including C. suppressalis (Wang et al., 2017; Su et al., 2016; Xu et al., 2018). Interestingly, the feeding of dsGluCl induced somewhat better RNAi effects than microinjection of dsGluCl. Similar result was also found in RNAi-mediated knock-down of midgut aminopeptidase-N gene in larvae of C. suppressalis (Wang et al., 2017). Considering that CsGluCl showed relatively high expression levels in foregut and hindgut, we speculated that the oral delivery method might result in higher RNAi efficiency of gut-related genes than injection in C. suppressalis. Abamectin (avermectin B1) belongs to the avermectin subfamily of macrocyclic lactones. Several lines of evidence have suggested that GluCls are the primary targets of macrocyclic lactones (Meng et al., 2018; Fuse et al., 2016). In this study, susceptibilities of C. suppressalis larvae to abamectin increased remarkably when the expression of CsGluCl was suppressed. This result was consistent with the up-regulation of GluCl in abamectin resistant Frankliniella occidentalis (Meng et al., 2018). Similarly, suppression of acetylcholinesterase-encoding ace transcript level by RNAi followed by insecticide exposures also increased the insect susceptibility to insecticide in T. castaneum, Diaphorina citri and Blattella germanica (Revuelta et al., 2009; Kishk et al., 2017; Lu et al., 2012). However, the susceptibilities to abamectin were decreased when GluCls were knocked down in Bemisia tabaci and P. xylostella (Shi et al., 2012; Wei et al., 2018). This discrepancy indicates that the mechanisms of toxic action of abamectin might be more complicated than we anticipated. In fact, it has been recently reported that the macrocyclic lactone such as ivermectin exerts a unique triple action (activation, potentiation, and antagonism) on Musca GluCl (Fuse et al., 2016). In addition to the classical neurotransmission function, GluCls possessed various physiological roles in invertebrate. GluCls have been demonstrated to regulate the production and embryogenesis in nematode Brugia malayi (Li et al., 2014), and the movement and feeding in C. elegans (Yates et al., 2003). Insect GluCls were implicated in the flight and waking control in Locusta migratoria (Janssen et al., 2007), the rest and arousal (McCarthy et al., 2011), rhythmic light avoidance (Collins et al., 2012) and inhibitory actions on the olfactory system (Liu and Wilson, 2013) in D. melanogaster, the olfactory learning and memory in A. mellifera (El Hassani et al., 2012; Boumghar et al., 2012; Demares et al., 2014), the biosynthesis of juvenile hormone in Diploptera punctate (Liu et al., 2005; Chiang et al., 2002). A recent study revealed that knockdown of GluCl induced reduced hatch rate in H. zea (Wang et al., 2018). The effects of CsGluCl on insect development were explored in this study. Knockdown of CsGluCl significantly reduced larvae weights and pupation rates in C. suppressalis, and further demonstrated that GluCls play important physiological roles in the development of insects.