Reception of Human Odorants and Their Chemical Antagonists in the Yellow Fever Mosquito, Aedes aegypti
Type of DegreePhD Dissertation
Entomology and Plant Pathology
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The yellow fever mosquito Aedes aegypti (Linnaeus) (Diptera: Culicidae) is a vector of several tropical diseases, including yellow fever, dengue fever, Zika fever, and chikungunya. Mosquitoes use multiple cues such as CO2, skin odor, and body heat for host seeking, identifying their hosts on the basis of their specific skin odorant profile. Around 340 compounds have been isolated from human skin emanations, but which of these compounds can be detected by Ae. aegypti and which contribute to host seeking remain largely unknown. Unfortunately, Ae. aegypti has developed resistance to DEET, the most common active ingredient in insect repellents; DEET-based insect repellents are also probably more toxic to humans than the non-DEET-based products now being registered. Hence, new chemical compounds are needed to protect humans from Ae. aegypti mosquitoes. In this study, 103 commercially available human odorants were examined in the antennal olfactory sensilla of female Ae. aegypti using single sensillum recording (SSR). Results showed that Ae. aegypti only responded to certain human odorants, including aldehydes, alcohols, aliphatics/aromatics, ketones, amines, and heterocyclics. Carboxylic acids on the panel did not elicit any responses in any of the sensilla types, but the SBTII, GP, and SST sensilla responded to most of the odorants detected by Ae. aegypti, with different types of sensilla exhibiting different selectivity and sensitivity. Both olfactory receptor neurons ‘A’ and ‘B’ in the trichoid sensilla contributed to the human odor sensation, and aldehydes were more likely to be discriminated by Ae. aegypti than aliphatics/aromatics. The sensillar responses of both non-blood fed and blood-fed female mosquitoes to the human odorants detected by Ae. aegypti were then investigated at 24-36, 48-60, and 72-84 hours post blood meal. Results indicated that Ae. aegypti with a blood meal showed compromised sensitivity to certain aldehydes, alcohols, aliphatics/aromatics, ketones, and amines at one or multiple time points, suggesting that these odorants may be important for Ae. aegypti host seeking. Next, the responses of antennal olfactory sensilla of female Ae. aegypti to 48 plant-derived chemical compounds were studied. Results demonstrated that different types of sensillum exhibited different response patterns to the chemical compounds tested. The SST2 sensillum evoked inhibitory responses to several compounds, including eucalyptol, citronellal, and α-terpinene. When eucalyptol was applied together with other odorants (i.e. 4,5-dimethylthiazole, cyclohexanone or 2-methyl-2-thiazoline) in the SST2 sensilla, the excitatory responses elicited by the odorants were reduced dramatically with increased dose of eucalyptol from 1 µg to 10 µg. These results indicated that eucalyptol might be used as confusant to protect humans from mosquitoes. Furthermore, the responses of Ae. aegypti odorant receptors to the salient human odorants were characterized using Xenopus oocyte expression system and two-electrode voltage clamp. The oocytes expressing AaOR13, AaOR15, or AaOR55 together with the co-receptor AaOrco yielded strong to moderate responses to the human odorants tested but very weak responses to the chemical compounds that elicited inhibitory responses in SST2 sensilla. When a human odorant (benzaldehyde, p-cresol, or sulcatone) and a chemical compound (α-terpinene or citronellal) were delivered simultaneously to the oocytes that expressed AaOR13, AaOR15, or AaOR55 together with AaOrco, the current oocyte response to an odorant + a chemical compound was significantly weaker than that to an odorant alone. The antagonistic effects of α-terpinene and citronellal on the reception of human odorants suggested that these chemicals may be good alternates to protect humans from Ae. aegypti, though the potency needs to be verified by future behavioral studies.