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    • br Disclosures br Introduction The

    2022-04-11


    Disclosures
    Introduction The brown planthopper, Nilaparvata lugens (Stål) (Hemiptera: Delphacidae), is a classic insecticide-induced resurgent pest throughout Asian rice-growing regions (Chelliah and Heinrichs, 1980). The resurgence of N. lugens induced by commonly used chemical agents has been well documented (Azzam et al., 2009; Ling et al., 2011; Wang et al., 1994; Yin et al., 2008), including fenvalerate, triazophos (TZP), detamethrin, methamidophos, and fungicide jinggangmycin (Bao et al., 2009; Jiang et al., 2012; Zhang et al., 2014; Zhu et al., 2014). Our previous work have described that several genes modulate N. lugens fecundity by probably operating in sugar homeostasis. For instance, Ge et al. (Ge et al., 2015) demonstrated that the sugar transporter gene 6 (Nlst6), a facilitative glucose/fructose transporter, mediates uptake of sugars necessary for N. lugens females and the symbiont maintenance and reproduction. Another pyruyvate kinase (PYK) gene functions in the glycolytic pathway, responsible for regulating the balance between glycolysis and gluconeogenesis in globefish (Takifugu rubripes) (Ohta et al., 2003). Knockdown of Nlst6 and NlPYK led to reduced number of eggs laid by females as well as ovarian protein content (Ge et al., 2015; Ge et al., 2017). These findings implyed that glycometabolism plays an important role in the fecundity of N. lugens. Hexokinase (Hex, EC 2.7.1.1), the gateway Cap Firefly Luciferase mg of glucose metabolism, irreversibly catalyzes the ATP-dependent phosphorylation of glucose, yielding ADP and glucose-6-phosphate (G-6-P). This reaction is the initial step in glycogen synthesis, glycolytic and pentose phosphate pathways (Alaryahi et al., 2014; Calmettes et al., 2013; Morris et al., 2006). In Drosophila melanogaster, flight muscle Hexokinase A (Hex-A) is essential hexokinase isozyme and the most conserved (Jayakumar et al., 2007). The other three hexokinases in D. melanogaster are expressed in the fat body (HexC) and in the testis (Hex-t1 and Hex-t2) (Duvernell and Eanes, 2000; Jayakumar et al., 2001). Fraga et al. (2013) found that hexokinase (HexA) expressed only in the embryonic tissue of red flour beetle Tribolium castaneum and Tc-HexA1 RNAi resulted in embryonic lethality as well as reduced number of eggs laid. In Helicoverpa armigera, the depletion of Hex-1 activity reduced metabolic activity, cell viability and retarded pupal development (Lin and Xu, 2016). These mounting evidences suggest that Hexokinase (Hex) may involve in insect fecundity and development via regulation of glucose and energy metabolism. Herein, we postulated that Hex-1 may mediate TZP-enhanced female fecundity by acting in glycometabolism and protein sythesis, notably, the Vg accumulation. Using RNA interference (RNAi) technique in an oral delivery manner, Hex-1 of TZP-treated adult females was depleted for the gene function study. The following are outcomes of experiments designed to test our hypothesis.
    Materials and methods
    Results
    Discussion In this study, we examined the TZP-enhanced fecundity with a particular emphasis on N. lugens Hex-1. Extensive researches concerning mammalian hexokinase revealed that HK activity is predominantly localized in the mammalian muscle (dos Santos et al., 2010; Wilson, 1995) and type I hexokinase is the most predominant in mammalian central nervous system (CNS) (Lawrence et al., 1984; Wilkin and Wilson, 1977). In terms of insect hexokinase, Bombyx mori Muscle hexokinase was virtually all type I, which was present in almost all tissues. Testis and Malphighian tube hexokinase consisted of both types I and II. Midgut contained types I, II and IV, whereas fat body tissue possessed types I, III and IV (Yanagawa, 1978). In the malarial vector Anopheles stephensi, HK-2 and HK-3 were present during all the stages of development whereas HK-1 was absent at all adult stages. Tissue distribution indicated that HK was concentrated in the thorax and its activity increased during larval growth, was maximal in the last instar, and generally static during adult life, declining in senescence (Gakhar and Nagpal, 1996). Tadano et al. (Tadano, 1987) detected HK-1 in three body parts, head, thorax, and abdomen of mosquito Aedes togoi adults. The level of expression for both DM1 and DM2 (two hexokinase isoenzyme sequences) was found to be uniform in 2nd and 3rd instar larvae, pupal and adult stages of Drosophila (). Our expression patterns showed that the highest expression level of Hex-1 in brain (Fig. 1A), which was in line with the findings in mammals (Lawrence et al., 1984; Wilkin and Wilson, 1977), indicating its evolutionarily conserved role in organismal CNS. In comparison with other insects, N. lugens Hex-1 was ubiquitous in various tissues (Fig. 1A) and its expression increased over nymph growth, peaking in the final instar, declining during adult life (Fig. 1B), which was similar to Anopheles stephensi (). Interestingly, the transcript level of Hex-1 considerably declined from 5th instar stage to 1 DPE (Fig. 1B), suggesting it may also modulate insect moulting process.