Abstract
Intracellular effects exerted by phytochemicals eliciting insect growth-retarding responses during vector control intervention remain largely underexplored. We studied the effects of Zanthoxylum chalybeum Engl. (Rutaceae) (ZCE) root derivatives against malaria (Anopheles gambiae) and arbovirus vector (Aedes aegypti) larvae to decipher possible molecular targets. We report dose-dependent biphasic effects on larval response, with transient exposure to ZCE and its bioactive fraction (ZCFr.5) inhibiting acetylcholinesterase (AChE) activity, inducing larval lethality and growth retardation at sublethal doses. Half-maximal lethal concentrations (LC50) for ZCE against An. gambiae and Ae. aegypti larvae after 24-h exposure were 9.00 ppm and 12.26 ppm, respectively. The active fraction ZCFr.5 exerted LC50 of 1.58 ppm and 3.21 ppm for An. gambiae and Ae. aegypti larvae, respectively. Inhibition of AChE was potentially linked to larval toxicity afforded by 2-tridecanone, palmitic acid (hexadecanoic acid), linoleic acid ((Z,Z)-9,12-octadecadienoic acid), sesamin, β-caryophyllene among other compounds identified in the bioactive fraction. In addition, the phenotypic larval retardation induced by ZCE root constituents was exerted through transcriptional modulation of ecdysteroidogenic CYP450 genes. Collectively, these findings provide an explorative avenue for developing potential mosquito control agents from Z. chalybeum root constituents.
Highlights
Intracellular effects exerted by phytochemicals eliciting insect growth-reducing responses during vector control intervention remain largely underexplored
We demonstrate that dysregulation of mosquito larval nervous coordination upon exposure to ZCE root extract and its bioactive fraction (ZCFr.5) retards larvalpupal transitions through transcriptional perturbation of ecdysteroidogenic CYP450 regulatory genes and effector transcription factors
When compared to the negative controls that achieved 100% survival, ZCE significantly reduced larval survival rates (An. gambiae, F(5,24) = 105.5, p < 0.001; Ae. aegypti, F(5,24) = 314.4, p < 0.001) but slightly higher doses were required to match the activity of neem oil that was included as positive control
Summary
Previous studies have demonstrated inhibition of An. gambiae, Ae. aegypti, Culex pipiens, and Ae. albopictus mosquito larval development by Zanthoxylum plant extracts [39,40,41,42,43]. While juvenile hormone (JH) expression levels decrease during the last larval instar to allow 20E-induced transformation into pupal stages, we established that the expression of JH biosynthetic rate-limiting enzyme, JH acid O-methyltransferase (JHAMT), in ZCFr.5-treated larvae remained relatively higher compared to that of controls (Fig. 4C-D). This finding could suggest low levels of circulating hemolymph 20E to suppress JH and further underscoring the observed larval growth retardations and precocious pupations. These findings confirmed that the larval treatment that targeted AChE activity perturbed ecdysteroidogenic pathway associated genes delaying larval-pupal transitions and inducing retardant phenotypes
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