Odor Coding Of Odorant Receptor Neurons And Function Of An Odorant Receptor In Locust Mouthparts

Insects are the largest group of living things in nature,and they have evolved sophisticated olfactory organs to receive a wide range of chemical stimuli.This distinguished ability enables them to detect and discriminate thousands of odor molecules to direct behavior,such as food and mate selection,oviposition site choice,avoid predators.Many evidences propose that the different olfactory organs of a species play different roles.For many insects,the antennae and mouthpart palps are important olfactory organs.Both of them are covered with a variety of chemosensory hairs that house the specialized olfactory sensory neurons(OSNs).However,there are few studies on molecular olfaction mechcanisms of insect mouthpart palps.Here we investigated the odor coding of olfactory receptor neurons in locust maxillary palps,differentiated the funcational types of basiconic sensilla on maxillary palp;We first identified one novel locust(Locusta migratoria)odorant receptor(LmigOR142),and analyzed the differential expression patterns of 11 ORs identified from palp transcriptomes;finally characterized the function of an odorant receptor LmigOR12 in palps.The main results are as following:I.Odor Coding of Olfactory Receptor Neurons in Locust Maxillary PalpsOdor coding relies on the activity of different classes of receptor neurons with distinct response characteristics.The odor coding of olfactory receptor neurons in maxillary palp of locust has been studied in this paper.Extensive extracellular recordings from olfactory receptor neurons in single sensillum revealed that there are 8 funcitonal subtypes of basiconic sensilla on maxialry palps.The sensillum types are intermingled on the surface of the palp,but their distribution is not at random.The neurons exhibit diverse response characteristics,providing the basis for an olfactory code.Some excitatory responses continue beyond the end of odor delivery,but responses to most odors terminate abruptly after the end of odor delivery,with some followed by a period of post stimulus quiescence.?.Different Expression of Odorant Receptors in the Antennae and Palps in L.migratoriaThe main chemosensory organs of locusts consisted of the antennae and the mouthparts(maxillary and labial palps),which are suggested to perform different olfactory functions and molecular mehcanisms underlie which are still unknown.To understand the differences of locust antennae and palps in olfaction molecular mechanisms,the olfactory-gene expression of these two organs were conducted.Notably,a novel odorant receptor,LmigOR142 hase been identified from the antennal transcriptorme.It has 408 amino acid residues with 7 predicted transmembrane domains,and it is specifically expressed in antennae.Additionally,we obtained 11 putative OR gene segments from the palp transcriptome.For the 11 OR genes identified in palp transcriptome,the expression levels of LmigOR12,LmigOR13,LmigOR 14,and LmigOR 18 in the palps were significantly higher than those in the antennae.Whereas,LmigOR15,LmigOR16,LmigOR17,LmigOR19,LmigOR21,LmigOR22 were demonstrated to be expressed in antenna at higher level than they in palps.III.An Odorant Receptor Specifically Expressed in Palps Was Involved in Detection of Three AldehydesOur electrophysiological experiments showed that the palp sresponded remarkably stronger to(E,E)-2,4-heptadienal and(E,E)-2,4-hexadienal than antennae.Besides,we also found that another two odorants,hexanal and E-2-hexenal,elicited stronger absolute values in EPG than in EAG.We speculated that there would be some specific ORs expressed in palps,which are responsible for the detection of these chemicals.Meanwhile,the RT-PCR analysis indicated that LmigOR12 was highly expressed in palps.Thus we presumed that LmigOR12 might be involved in detection to the aldehydes.We found that the responses of EPG of locust nymphs injected with dsRNA of LmigOR 12 to hexanal and E-2-hexenal were significantly reduced in comparison with locust injected with dsRNA of GFP.Interestingly,the response of EPG of locust nymphs injected with dsRNA of LmigOR12 to(E,E)-2,4-heptadienal was significantly lower than that of animals injected with dsRNA of GFP.In turn,no changes of EPGs were detected to(E,E)-2,4-hexadienal between the two dsRNA experimental animals.In contrast,there was no significant difference in EAG responses to hexanal and E-2-hexenal between the LmigOR12 and GFP dsRNA injected locusts.This research lays foundation for the study of locust chemoreception as well as insight for the understanding of insect olfaction evolution.Consequently,the palps could play an important role in speciation through food selection.The palps,therefore,would be a potential area for investigating the specific roles in insect chemoperception in the future.Additionally,the newly identified and characterized olfactory genes might be exploited as potential target for locust control.