The Neuronal And Molecular Mechanism Of Olfactory Encoding For Host-plant Volatiles In Helicoverpa Assulta

The insects develop an extremely sensitive olfaction system during the long-term evolution.They can perceive and distinguish majority odors from environment to accomplish various behavioral responses,such as host plant location,oviposition-site selection,mate recognition and natural enemy avoidance.They can detect several compounds by the sensilla located on the antenna of insects.Different sensilla confer different functions.The chemical signal can be coverted into electrical signal by activating odorant receptors(ORs)and further input to the central nervous system(CNS)directly along the axon.Study on the mechanism of insect olfaction,not only can regulate insect behavior but also can get the differences of olfaction between the two closely related species.Helicoverpa assulta were taken as our experimental subject.The single sensillum recording(SSR)technology is used to detect different types of sensilla and identify OSNs on the female and male antennae of H.assulta to different odors.Based on the transcriptome,some odorant receptors are cloned with high expression on antenna between both sexes and functional characterization of several ORs/Orco were conducted in the Xenopus oocyte expression system.Then using the Y-tube to test the effects of odors on the insect behaviors.The main results are as follows:1.The single sensillum recording(SSR)technology is used to test the different types of sensilla and the functional characteristics of OSNs in trichoid sensilla and sensilla bascionic on the female and male antennae of H.assulta to 20 odorants including those from tobacco.Based on the selectivity of electrophysiological responses and different spike amplitudes,three types of trichoid sensilla were found on female antenna,Type A(F-T-A),Type C(F-T-C)and Type D(F-T-D).Type-A,the major one,three OSNs housed in it but only one OSN can be activated by four odors and showed higher response to butyl salicylate.Type-D narrowly response to three odors,including a tobacco volatile-Nerolidol.Four types of sensilla bascionic were found on female antenna,Type A(F-B-A),Type B(F-B-B),Type C(F-B-C)and Type D(F-B-D).However,only two types of sensilla trichoid were found on male antenna,Type A(M-T-A)and Type D(M-T-D).There are four types of sensilla bascionic on male antenna,Type A(M-B-A),Type B(M-B-B),Type C(M-B-C)and Type D(M-B-D),and three types of them,Type A(B-A),Type B(B-B)and Type C(B-C)are with the same responses as that of sensilla bascionic on female antenna.Type-D is different between sexes,which broadly tuned to 10 odors in female but 6 odors in male.2.One Orco and 9 odorant receptors(HassOrco,HassOR17,HassOR24,HassOR27,HassOR40,HassOR41,HassOR42,HassOR43,HassOR44,HassOR55)from H.assulta are cloned.The in vitro function of all the ORs were analyzed by Xenopus oocytes heterologous expression combined with the two-electrode voltage clamp recording system.The results showed that HassOR27/Orco tuned to Methyl salicylate,cis-3-Hexenyl acetate,Methyl benzoate,Ethyl benzoate and Butyl salicylate with higher response to Butyl salicylate.HassOR42/Orco also tuned to Phenylacetaldehyde with stronger response,but less to 2-Phenylethanol and Methyl 4-hydroxybenzoate.HassOR43/Orco robustly responded to Methyl benzoate,Methyl salicylate,Methyl 4-hydroxybenzoate with small response,but exclusively tuned to Methyl 2-methoxy benzoate.HassOR44/Orco obviously responded to Methyl 2-methoxy benzoate.HassOR40/Orco was a narrowly receptor which responded to three odors including a tobacco volatile-Nerolidol.While HassOR17/Orco,HassOR24/Orco,HassOR41/Orco and HassOR55/Orco did not respond to any chemicals tested in this study.3.A specific OSN housed on the sensilla trichodea Type D can be activated by nerolidol,geraniol and geranyl acetate.Based on the Xenopus oocyte results,HassOR40 is a narrowly-tuned receptor which responded to these three odors.So HassOR40 is speculated that expressed on the specific OSN because of their same ligands.H.assulta and H.armigera are two closely related species.The identity of the amino acid sequences of OR40 between the two species is 99.0%.To identify the influence of nerolidol on both sexes of H.assulta,we did the Y-tube behavioral experiment.The results showed that both males and females were attracted to the nerolidol.The females displayed a strong preference to the odor compared with the hexane.An olfactory circuit of 'OR-OSN' of H.assulta detecting nerolidol is clarified.Which can be provided not only to illuminate the mechanism of sensing different host plants but also prevent and control these agricultural pests more efficiency and environmental friendly.