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Int J Biol Sci 2014; 10(8):846-860. doi:10.7150/ijbs.9297

Research Paper

Identification of Candidate Olfactory Genes in Chilo suppressalis by Antennal Transcriptome Analysis

Depan Cao1,2#, Yang Liu1#, Jinjin Wei1, Xinyan Liao1, William B. Walker3, Jianhong Li2, Guirong Wang1 Corresponding address

1. State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing, China
2. Laboratory of Pesticide, College of Plant Science & Technology, Huazhong Agricultural University, Wuhan, China
3. Swedish University of Agricultural Sciences, Department of Plant Protection Biology, Chemical Ecology Research Group, Alnarp, Sweden
# These authors contributed equally to this work.

This is an open access article distributed under the terms of the Creative Commons Attribution (CC BY-NC) License. See http://ivyspring.com/terms for full terms and conditions.
How to cite this article:
Cao D, Liu Y, Wei J, Liao X, Walker WB, Li J, Wang G. Identification of Candidate Olfactory Genes in Chilo suppressalis by Antennal Transcriptome Analysis. Int J Biol Sci 2014; 10(8):846-860. doi:10.7150/ijbs.9297. Available from http://www.ijbs.com/v10p0846.htm

Abstract

Antennal olfaction, which is extremely important for insect survival, mediates key behaviors such as host preference, mate choice, and oviposition site selection. In insects, odor detection is mediated by multiple proteins in the antenna, especially the odorant receptors (ORs) and ionotropic receptors (IRs), which ensure the specificity of the olfactory sensory neuron responses. In this study, we identified the olfactory gene repertoire of the rice stem borer, Chilo suppressalis, an economically important agricultural pest, which inflicts great damage to the rice yield in south and east part of Asia, especially in Southern China. By Illumina sequencing of male and female antennal transcriptomes, we identified 47 odorant receptors, 20 ionotropic receptors, 26 odorant binding proteins, 21 chemosensory proteins and 2 sensory neuron membrane proteins. Our findings make it possible for future research of the olfactory system of C. suppressalis at the molecular level.

Keywords: C. suppressali, antennal olfaction, olfactory gene

Introduction

Chemical sensing is critically important to insect survival. For insects, olfaction, which is the primary sensory perception modality, is used to detect odor molecules in the environment. Olfaction guides the insect towards food, mating partners, and oviposition sites and also to facilitate detection of predators and toxic compounds [1]. The antenna is a specialized organ for insect sensing, especially for olfaction. Several types of sensilla, which are specialized hair-like, multi-pore structures, cover the surface of the antennae. Olfactory receptor neurons (ORNs) and auxiliary structures are housed within the antennae, positioned at the sensilla root [2]. For most of the olfactory sensilla, each hosts 1-4 ORNs, which extend their dendrites up into the sensilla and project their axons into the antennal lymph on towards the brain [3]. The ORNs convert ecologically relevant volatile chemicals into an electrical impulse, which is transported to the primary olfactory center of the brain, the antennal lobe [4]. Within the sensilla-ORN structure, a number of gene families have been identified to play active roles in olfaction. These include the odorant binding proteins (OBPs), chemosensory proteins (CSPs), odorant receptors (ORs) ionotropic receptors (IRs), and the sensory neuron membrane proteins (SNMPs).

OBPs are hydrophilic soluble proteins that are secreted by the accessory cells around the ORNs and accumulate in the sensilla lymph [5]. OBPs are thought to be the first proteins that participate in the olfactory signal transduction procedure [6]. It is postulated that as the odor molecules diffuse through pores on sensilla, the soluble OBPs in sensillum lymph fluid selectively bind the liposoluble odor molecules [7] and transport them through the sensillum lymph to the surface of ORN dendrites [8, 9]. OBPs are also thought to be directly involved in the activation of ORx/Orco complex in the recognition of some special odors [10, 11]. Like OBPs, the CSPs are small soluble proteins that are enriched in the sensillum lymph, but also expressing broadly in non-olfactory tissues. The olfactory properties of CSPs are quite clear, for they bind odorant or pheromone compounds [12], but little is known about how CSPs function in olfactory system. Moreover, the broad tissues expression of CSPs implies unknown roles in non-olfactory procedures.

ORs are trans-membrane proteins located in the dendrite membrane of ORNs. Insect ORs are seven-transmembrane domain proteins [13] with a reversed membrane topology (intracellular N-terminus) compared to the G-protein coupled vertebrate ORs [14]. In chemosensory signal transduction process, ORs play a central role as a bio-transducer, facilitating the conversion of the chemical message to an electrical signal. In this system, it is generally thought that in disparate individual OSNs, a single variable, ligand-binding ORx and a highly conserved, non-ligand binding Orco protein make up a stand-alone heteromeric structure that functions as a ligand-gated ion channel [14-17].

IRs make up a recently discovered ionotropic glutamate receptor (iGluR) -like protein family which has been shown to be involved in chemosensation [18]. The insect IRs contain structural regions that are conserved in iGluRs, namely, three transmembrane domains (M1, M2 and M3), a bipartite ligand-binding domain with two lobes (S1 and S2) and one ion channel pore (P). But the conserved iGluR glutamate binding residues in S1 and S2 lobes are not retained in IRs, indicating their atypical binding characters [19]. Unlike the exclusive ORs, two or three IR genes were always co-expressed with one or both of the conserved IR8a and IR25a in one IR-expressing neuron [18]. Furthermore, IRs are thought to be a class of receptors far more ancient than OR families that animals use for sensing chemicals in the surrounding environment; this gene family has an extensive distribution, as it is found in mollusks, annelid and nematodes [20], and it displays a relatively high homology across species [21].

In this study, we sequenced and analyzed Chilo suppressalis adult antennal transcriptomes using Illumina sequencing. Our goals were to identify olfaction-related genes of this pest insect species, which is destructive to the rice farming in China, across Asia and in the Pacific. We report the results including sequencing, gene annotation, GO annotation and specifically, a set of 47 ORs, 20 IRs, 26 OBPs, 21 CSPs and 2 SNMPs.

Results

Transcriptome overview

With utilization of a 90PE RNA-Seq strategy by Illumina HiSeq 2000, about 56.4 million and 58.8 million raw-reads were obtained respectively from the libraries of male and female antenna. After filtering, 53.4 million and 55.3 million clean-reads comprised of 4.8 and 4.9 gigabases were generated for male and female antenna. Assemblies led to the generation of 79,706 and 77,404 unigenes separately for male and female. After merging and clustering, a final transcript dataset was revealed, with 66,560 unigenes consisting of 15,462 distinct clusters and 51,098 distinct singletons. The dataset was 50.63 megabases in size and with a mean length of 761nt and N50 of 1,271nt. 11,849 unigenes were larger than 1,000nt in length, which comprised 17.80% of all unigenes (Table 1).

Through annotation by blastx, 30,232 (45.4%) unigenes matched to known proteins; the remaining unigenes failed to match against any sequence with an e-value < 1e-5 in neither of the nr nor SwissProt databases. Among the annotated unigenes, 70.4% had a best blast match to Lepidopteran sequences, primarily Danaus plexippus (59.2%), and Bombyx mori (7.4%) (Figure 1A). 52.0% of the annotated unigenes showed strong homology, with e-value < 1e-45.

 Table 1 

Assembly summary of C. suppressalis antenna transcriptome

SampleTotal NumberTotal Length(nt)Mean Length(nt)N50Consensus
Sequences
Distinct
Clusters
Distinct
Singletons
ContigFemale130,22944,138,907339569---
Male133,39444,421,350333543---
UnigeneFemale77,40445,204,67558496977,40412,25465,150
Male79,70644,793,75356291679,70611,67068,036
Merge66,56050,635,660761127166,56015,46251,098
 Figure 1 

Annotation summary of C. suppressalis antenna unigenes. (A) Species distribution of unigenes' best-hit annotation term in nr database. (B) Gene ontology classifications of the C. suppressalis unigenes.

Int J Biol Sci Image (Click on the image to enlarge.)

Gene ontology (GO) annotation of the unigenes was obtained using Blast2GO pipeline according to the blastx search against nr. From the 66,560 final unigenes set, a total of 10,940 unigenes were assigned various GO terms. In the molecular function category, the genes expressed in the antennae were mostly enriched to molecular binding activity (e.g., nucleotide, ion and odorant binding) and catalytic activity (e.g., hydrolase and oxidoreductase). In the biological process terms, cellular and metabolic processes were the most represented. In the cellular component terms, cell, cell part and organelle were the most abundant (Figure 1B).

Identification of Candidate Chemosensory Receptors

The unigenes related to candidate chemosensory receptors were identified by keyword search of the blastx annotation. The predicted protein sequences of the unigenes were further searched by PSI-blastp with known Lepidopteran chemosensory receptors [4] to indentify more candidate ORs. We identified 47 distinct unigenes that were putative OR genes. Of these, 23 sequences were full-length OR genes because they have intact open reading frames with a general length of 1,200bp and 5-7 transmembrane domains, which are characteristic of typical insect ORs.

The C. suppressalis Orco co-receptor orthologue was easily detected as it has a high degree of identity with the conserved insect co-receptor: this gene was named CsupOrco. Six unigenes were considered to be putative pheromone receptors because they shared considerable similarity with previously characterized Lepidopteran pheromone receptors and were clustered together into one subgroup in the phylogenetic tree (Figure 2). These 6 candidate ORs were named as “CsupPRx” (x=1 through 6), to more clearly indicate function, as previously reported Lepidopteran pheromone receptors were not clearly separated from the general odorant receptors and followed no orderly numbering system. The naming convention followed in this report is also consistent with general naming of odorant binding proteins, where the pheromone binding proteins (PBPs) are distinguished from other odorant binding proteins (OBPs). Other candidate ORs were highly divergent and shared low similarity with other insect ORs, which is common for insect olfactory receptor genes. These genes were named as “CsupOR”, followed by a numeral, in descending order of their coding region lengths.

Phylogenetic analysis was performed with ORs from B. mori, H. armigera, H. virescens, and PR sequences from P. xylostella and some Crambidae insects. For the relatively conserved PR genes, the CsupPR4 and CsupPR5 were clustered together with the Crambidae pheromone receptor 1 and 3. CsupPR1, 2, 3 and 6 were not closely grouped with the Crambidae PRs but clustered with the P. xylostella PR clade with high bootstrap support. Almost all CsupOR candidates clustered with at least one Lepidopteran orthologous gene in the phylogenetic tree. No C. suppressalis specific OR family expansion was discovered in our phylogenetic tree.

Information including unigene reference, length, and best blastx hit of all 47 odorant receptors are listed in Table 2. The sequences are listed in Additional File 1: Supplementary Material S1.

Identification of Candidate Ionotropic Receptors

The putative IR genes in the C. suppressalis antennal transcriptome were represented according to their similarity to known insect IRs. Bioinformatic analysis led to the identification of 20 candidates IRs, in which 13 sequences contain a full-length ORF, the remaining 7 sequences are marked as incomplete due to lacking a complete 5' or 3' terminus. The insect IRs contained three transmembrane domains. TMHMM2.0 predicted 10 IR candidates with three transmembrane domains (Table 3).

 Figure 2 

Phylogenetic tree of candidate CsupORs with known lepidopteran OR sequences. Harm: H. armigera; Hvir: H. virescens; Bmor: B. mori; Pxyl: P. xylostella; Osca: Ostrinia scapulalis; Onub: Ostrinia nubilalis; Ozea: Ostrinia zealis; Ofur: Ostrinia furnacalis; Opal: Ostrinia palustralis; Ozag: Ostrinia zaguliaevi: Oova: Ostrinia ovalipennis; Olat: Ostrinia latipennis. The clade in blue indicates the pheromone receptor gene clade.

Int J Biol Sci Image (Click on the image to enlarge.)
 Table 2 

Unigenes of candidate olfactory receptors

Gene nameLength (nt)ORF (aa)Unigene referenceStatusTMD (No.)EvalueBLASTx best hit
ORco
CsupOrco3864474CL5509.Contig1Complete ORF70.0E+00gb|AFQ94048.1|olfactory receptor 2 [Chilo suppressalis]
Pheromone receptor
CsupPR11492424Unigene14957Complete ORF61.0E-94gb|ADB89183.1|odorant receptor 6 [Ostrinia nubilalis]
CsupPR22220367Unigene18611Complete ORF42.0E-95gb|ADB89183.1|odorant receptor 6 [Ostrinia nubilalis]
CsupPR32044264CL1103.Contig5Complete ORF61.0E-56gb|AFK30402.1|E-race odorant receptor 6 [Ostrinia nubilalis]
CsupPR4483160CL812.Contig25',3' lost22.0E-28dbj|BAG71417.1|olfactory receptor-1 [Diaphania indica]
CsupPR529799CL4759.Contig15',3' lost12.0E-21dbj|BAH57981.1|olfactory receptor [Ostrinia latipennis]
CsupPR625986Unigene437135',3' lost26.0E-10gb|AFK30403.1|odorant receptor 6 [Ostrinia furnacalis]
Other odorant receptor
CsupOR11537457Unigene28449Complete ORF75.0E-152ref|NP_001116817.1|olfactory receptor-like [Bombyx mori]
CsupOR21371446Unigene15165Complete ORF62.0E-179ref|NP_001155301.1|olfactory receptor 60 [Bombyx mori]
CsupOR31612439Unigene29790Complete ORF55.0E-56dbj|BAH66328.1|olfactory receptor [Bombyx mori]
CsupOR41443432CL3655.Contig2Complete ORF61.0E-140gb|AFL70813.1|odorant receptor 50, partial [Manduca sexta]
CsupOR51445429Unigene22904Complete ORF74.0E-141ref|NP_001166607.1|olfactory receptor 44 [Bombyx mori]
CsupOR61381423CL5260.Contig2Complete ORF61.0E-131gb|AFL70813.1|odorant receptor 50, partial [Manduca sexta]
CsupOR72645423Unigene26044Complete ORF42.0E-27gb|EHJ75140.1|olfactory receptor [Danaus plexippus]
CsupOR81423422Unigene990Complete ORF53.0E-114ref|NP_001166893.1|olfactory receptor 27 [Bombyx mori]
CsupOR91429422CL3918.Contig45' lost64.0E-112gb|AFC91736.1|putative odorant receptor OR28 [Cydia pomonella]
CsupOR101600414Unigene247415' lost74.0E-116ref|NP_001091818.1|olfactory receptor 42 [Bombyx mori]
CsupOR111270406CL5145.Contig2Complete ORF61.0E-10sp|P81922|Odorant receptor 47b [Drosophila melanogaster]
CsupOR121501402Unigene11744Complete ORF54.0E-116gb|ACC63240.1|olfactory receptor 20, partial [Helicoverpa armigera]
CsupOR131318402CL2287.Contig1Complete ORF77.0E-109tpg|DAA05986.1|TPA: TPA_exp: odorant receptor 30 [Bombyx mori]
CsupOR141209400Unigene45765' lost61.0E-119ref|NP_001166616.1|olfactory receptor 54 [Bombyx mori]
CsupOR151275400Unigene35932Complete ORF51.0E-150ref|NP_001166617.1|olfactory receptor 56 [Bombyx mori]
CsupOR161344397Unigene335203' lost63.0E-135ref|NP_001166613.1|olfactory receptor 22 [Bombyx mori]
CsupOR171365397CL458.Contig1Complete ORF66.0E-162gb|AFC91721.1|putative odorant receptor OR12 [Cydia pomonella]
CsupOR181442397CL3235.Contig23' lost67.0E-134ref|NP_001166894.1|olfactory receptor 29 [Bombyx mori]
CsupOR191632395CL545.Contig1Complete ORF58.0E-54gb|EHJ63141.1|olfactory receptor [Danaus plexippus]
CsupOR201382390CL1707.Contig3Complete ORF56.0E-124emb|CAG38118.1|putative chemosensory receptor 17 [Heliothis virescens]
CsupOR211220386CL727.Contig1Complete ORF52.0E-94ref|NP_001166620.1|olfactory receptor 63 [Bombyx mori]
CsupOR221388381Unigene18694Complete ORF64.0E-118gb|AFC91732.1|putative odorant receptor OR24 [Cydia pomonella]
CsupOR231193379Unigene186265' lost69.0E-60ref|NP_001166606.1|olfactory receptor 23 [Bombyx mori]
CsupOR241270378CL380.Contig1Complete ORF43.0E-78gb|AFC91719.1|putative odorant receptor OR10 [Cydia pomonella]
CsupOR25863377Unigene175545' lost23.0E-86ref|NP_001166895.1|olfactory receptor 18 [Bombyx mori]
CsupOR261214375Unigene223795' lost42.0E-54ref|NP_001091790.1|candidate olfactory receptor [Bombyx mori]
CsupOR27545370Unigene245765' lost41.0E-26tpg|DAA05974.1|TPA: TPA_exp: odorant receptor 15 [Bombyx mori]
CsupOR281061353Unigene229275',3' lost63.0E-135ref|NP_001104832.2|olfactory receptor 16 [Bombyx mori]
CsupOR291060348Unigene336765' lost64.0E-118gb|AEF32141.1|odorant receptor [Spodoptera exigua]
CsupOR301170346CL1602.Contig35' lost52.0E-67gb|AFC91739.1|putative odorant receptor OR31 [Cydia pomonella]
CsupOR311022340Unigene286615',3' lost61.0E-26gb|EHJ65088.1|olfactory receptor 44 [Danaus plexippus]
CsupOR322686332CL4999.Contig1Complete ORF63.0E-43ref|NP_001091791.1|candidate olfactory receptor [Bombyx mori]
CsupOR33971323Unigene302185',3' lost48.0E-87ref|NP_001091790.1|candidate olfactory receptor [Bombyx mori]
CsupOR341000295Unigene358815' lost48.0E-82ref|NP_001166892.1|olfactory receptor 36 [Bombyx mori]
CsupOR35911272CL5748.Contig25' lost26.0E-76gb|AFC91725.1|putative odorant receptor OR17 [Cydia pomonella]
CsupOR36793264Unigene268345',3' lost45.0E-65ref|NP_001091818.1|olfactory receptor 42 [Bombyx mori]
CsupOR37834245CL296.Contig25' lost21.0E-66dbj|BAH66322.1|olfactory receptor [Bombyx mori]
CsupOR38657219Unigene353705',3' lost41.0E-72ref|NP_001166892.1|olfactory receptor 36 [Bombyx mori]
CsupOR39604163CL4235.Contig23' lost33.0E-16dbj|BAH66322.1|olfactory receptor [Bombyx mori]
CsupOR40406120CL4235.Contig13' lost21.0E-11ref|NP_001104828.1|olfactory receptor 25 [Bombyx mori]
 Table 3 

Unigenes of candidate ionotropic receptors

Gene nameLength (nt)ORF (aa)Unigene
reference
StatusTMD (No.)EvalueBLASTx best hit
CsupIR11934574Unigene354215' lost30.0E+00gb|EHJ72198.1|putative ionotropic glutamate receptor-invertebrate [Danaus plexippus]
CsupIR1.12121656CL4511.Contig3Complete ORF31.0E-113gb|EHJ72198.1|putative ionotropic glutamate receptor-invertebrate [Danaus plexippus]
CsupIR22147320CL2718.Contig3Complete ORF44.0E-163gb|AAB62572.1|GABA-gated chloride channel isoform a3 [Heliothis virescens]
CsupIR21a2972844Unigene11518Complete ORF30.0E+00gb|ADR64678.1|putative chemosensory ionotropic receptor IR21a [Spodoptera littoralis]
CsupIR25a3304927Unigene17452Complete ORF32.0E-75sp|P39087|Glutamate receptor, ionotropic kainate 2 [Mus musculus]
CsupIR32100474Unigene29712Complete ORF40.0E+00gb|EHJ68597.1|putative glycine receptor beta precursor [Danaus plexippus]
CsupIR4931277Unigene9045',3' lost25.0E-12sp|Q68Y21|Glutamate receptor delta-2 subunit [Danio rerio]
CsupIR40a2918707CL4571.Contig2Complete ORF31.0E-15sp|Q9ULK0|Glutamate receptor delta-1 subunit [Homo sapiens]
CsupIR41a1989598CL876.Contig25' lost30.0E+00gb|AFC91758.1|putative ionotropic receptor IR41a [Cydia pomonella]
CsupIR64a1258380Unigene228855' lost48.0E-17sp|Q68Y21|Glutamate receptor delta-2 subunit [Danio rerio]
CsupIR68a2044674Unigene14878Complete ORF60.0E+00gb|ADR64682.1|putative chemosensory ionotropic receptor IR68a [Spodoptera littoralis]
CsupIR75d1145294CL349.Contig25' lost11.0E-11sp|P34299|Glutamate receptor 1 [Caenorhabditis elegans]
CsupIR75p2200615CL46.Contig4Complete ORF30.0E+00gb|ADR64684.1|putative chemosensory ionotropic receptor IR75p [Spodoptera littoralis]
CsupIR75p.11429441CL2655.Contig25' lost40.0E+00gb|EHJ72019.1|putative ionotropic glutamate receptor-invertebrate [Danaus plexippus]
CsupIR75q1588196Unigene98385',3' lost21.0E-56gb|AFC91752.1|putative ionotropic receptor IR75q2 [Cydia pomonella]
CsupIR75q22011635CL1806.Contig1Complete ORF40.0E+00gb|AFC91752.1|putative ionotropic receptor IR75q2 [Cydia pomonella]
CsupIR76b2070547Unigene33212Complete ORF30.0E+00gb|AFC91765.1|putative ionotropic receptor IR76b [Cydia pomonella]
CsupIR87a2067647Unigene8213Complete ORF64.0E-07sp|O43424|Glutamate receptor delta-2 subunit [Homo sapiens]
CsupIR8a3058912Unigene17458Complete ORF30.0E+00gb|AFC91764.1|putative ionotropic receptor IR8a, partial [Cydia pomonella]
CsupIR93a2878877CL2805.Contig1Complete ORF31.0E-33sp|Q63226|Glutamate receptor delta-2 subunit [Rattus norvegicus]

To distinguish putative IRs from iGluRs, putative C. suppressalis IRs were aligned with IR orthologoues from D. melanogaster, B. mori, S. littoralis and some D. melanogaster iGluRs for a phylogenetic analysis. The results revealed a clear segregation between DmeliGluRs and insect IRs (Figure 3). In the phylogenetic tree of IRs, most C. suppressalis IR candidates clustered with their ionotropic receptor orthologues into a separate clade. According to their positions in phylogenetic tree and strong bootstrap support, 15 of 20 candidate C. suppressalis IRs were given names consistent with the number and suffix of the Dmel/Bmor/Slit IR orthologues in the same clade.

Two of the remaining 5 IR sequences, CL4511.Contig3 and CL2655.Contig2, were clustered into the SlitIR1 and Slit/Bmor IR75p clades, respectively, with reliable bootstrap support, forming small expansions with the CsupIR1 and CsupIR75p genes. Considering that these two sequences contain typical IR characteristics, these two sequences may likely be C. suppressalis specific genes, or their orthologues haven't been detected in other insects. These two sequences were named as “CsupIR1.1” and “CsupIR75p.1”, respectively. The other 3 sequences, CL2718.Contig3, Unigene29712 and Unigene904, had low bootstrap values unable to clearly demonstrate their phylogenetic positions, were named as “CsupIR2”, “CsupIR3” and “CsupIR4”, respectively. The information including unigene reference, length, and best blastx hit of all the 20 IRs are listed in Table 3. The sequences of all 20 IRs were listed in SAdditional File 1: upplementary Material S1.

Identification of Putative Odorant-binding Proteins and Chemosensory Proteins

In addition to keyword searching and PSI-Blast, we also used motif scanning to detect the conserved 6 cysteine residues pattern (C1-X5-39-C2-X3-C3-X21-44-C4-X7-12-C5-X8-C6) [22] of the putative odorant-binding proteins. In our transcript set, we identified 26 different sequences encoding odorant binding proteins, including 4 PBPs and 2 GOBPs. In these 26 sequences, 23 had intact ORFs detected; 3 unigenes failed in the signal peptide test which is performed by SignalP. Sequence alignment showed that almost all the putative OBPs shared the classic six-cysteine motif, except 4 sequences (CsupOBP4, 10, 11 and 13), which grouped into the “minus-C” subgroup with their second cysteine residues missing [23]. It was also notable that all 4 “minus-C” OBPs had a lysine residue in place of the C2 cysteine (Figure 4). In the phylogenetic tree, the PBP and GOBP sequences were clustered respectively into the PBP and GOBP clades as expected (Figure 5). All candidate OBP sequences were clustered with at least one lepidopteran orthologue. Comparing our putative OBPs with NCBI records of C. suppressalis, we identified 10 as “discovered genes”, which are GOBP1, 2, PBP1, 2, 3, OBP2, 8, 13, 14 and 17. All of these “discovered genes” have identities over 96% in amino acid to their most similar NCBI records. Therefore, we named these candidate GOBPs and PBPs following the existing NCBI records. We named the candidate OBPs as “CsupOBP” followed by a numeral in descending order of their coding region lengths, as the numbering of existing C. suppressalis OBP records is confusing (Table 4).

Bioinformatic analysis led to the identification of 21 different sequences encoding candidate CSPs. Among them, 18 sequences have full-length ORFs and signal peptides; Due to incomplete N-termini, the remaining 3 failed in the SignalP test. The conserved cysteine pattern of C1-X6-8-C2-X18-C3-X2-C4 [24] and the six-helix secondary-structure were retained in all 21 candidate CSPs (Figure 6). Neighbor-joining tree analysis showed that all of the 21 sequences clustered with Lepidopteran orthologous genes (Figure 7). These candidate CSPs were named as “CsupCSP” followed by a numeral in descending order of their coding region lengths. The information on the CSPs is listed in Table 5. The sequences are listed in Additional File 1: Supplement Material S1.

Identification of Candidate Sensory Neuron Membrane Proteins

SNMPs are thought to be involved in the recognition of Lepidopteran pheromone, since they were first identified in Lepidopteran pheromone-sensitive neurons [25, 26]. SNMPs of two families, SNMP1 and 2, were discovered in our C. suppressalis antennal transcriptome. Unigene35775 showed a 99% identity to the CsupSNMP1 published in Genebank. And the CL173.contig15 covered the whole sequence of the CsupSNMP2 (GI: 406668637). Our SNMP unigene sequences are available in Additional File 1: Supplementary Material S1.

 Figure 3 

Phylogenetic tree of candidate CsupIRs with known lepidopteran IRs and iGluRs. Dmel: D. melanogaster,Bmor: B. mori, Slit: S. littoralis. The clade in blue indicates the iGluR gene clade; the clade in purple indicates the IR8a/IR25a clade.

Int J Biol Sci Image (Click on the image to enlarge.)
 Figure 4 

Sequences alignment of putative CsupOBPs. The conserved cysteine residues were marked with “*”. Because of the overly long sequence of CsupOBP1, the CsupOBP1 is not included in the multisequence alignment.

Int J Biol Sci Image (Click on the image to enlarge.)
 Figure 5 

Phylogenetic tree of candidate CsupOBPs with known lepidopteran OBPs. Bmor: B. mori, Hv: H. virescens. The clade in blue indicates the PBP gene clade; the clade in purple indicates the GOBP clade.

Int J Biol Sci Image (Click on the image to enlarge.)
 Figure 6 

Sequences alignment of putative CsupCSPs. The conserved cysteine residues were marked with “*”.

Int J Biol Sci Image (Click on the image to enlarge.)
 Figure 7 

Phylogenetic tree of candidate CsupCSPs with known lepidopteran CSPs. Bmor: B. mori, Hv: H. virescens.

Int J Biol Sci Image (Click on the image to enlarge.)
 Table 4 

Unigenes of candidate odorant binding proteins

Gene nameLength (nt)ORF (aa)Unigene
reference
StatusSignal PeptideEvalueBLASTx best hit
CsupGOBP1907173CL3430.Contig1Complete ORFY3.0E-97gb|ACJ07126.1|general odorant binding protein 1 [Chilo suppressalis]
CsupGOBP21068162Unigene17531Complete ORFY1.0E-86gb|ACJ07120.1|general odorant binding protein 2 [Chilo suppressalis]
CsupPBP1988184Unigene28597Complete ORFN1.0E-91gb|ADK66921.1|pheromone binding protein 1 [Chilo suppressalis]
CsupPBP24508165CL470.Contig5Complete ORFY1.0E-89gb|ACJ07123.1|pheromone binding protein 2 [Chilo suppressalis]
CsupPBP3731170Unigene28180Complete ORFY1.0E-91gb|ADL09140.1|pheromone binding protein 3 [Chilo suppressalis]
CsupPBP4689165Unigene31774Complete ORFY1.0E-38gb|ADT78499.1|pheromone binding protein 5 [Ostrinia nubilalis]
CsupOBP11224351CL5570.Contig1Complete ORFY6.0E-21dbj|BAI82446.1|odorant binding protein 6 [Delia antiqua]
CsupOBP21212251Unigene24952Complete ORFY1.0E-142gb|ADD71058.1|odorant-binding protein [Chilo suppressalis]
CsupOBP3917242Unigene28587Complete ORFY5.0E-09gb|EFA09155.1|odorant binding protein 22 [Tribolium castaneum]
CsupOBP41038194CL2795.Contig2Complete ORFY1.0E-44gb|EHJ77172.1|odorant binding protein [Danaus plexippus]
CsupOBP5753184Unigene31885Complete ORFY6.0E-35emb|CAX63249.1|odorant-binding protein SaveOBP4 precursor, partial [Sitobion avenae]
CsupOBP61201184Unigene22461Complete ORFY1.0E-40ref|NP_001159621.1|odorant binding protein LOC100307012 precursor [Bombyx mori]
CsupOBP7599156Unigene29832Complete ORFY2.0E-14gb|AFI57166.1|odorant-binding protein 17 [Helicoverpa armigera]
CsupOBP8608153Unigene31806Complete ORFY2.0E-79gb|AER27567.1|odorant binding protein [Chilo suppressalis]
CsupOBP9697150Unigene13466Complete ORFY7.0E-27gb|AAR28762.1|odorant-binding protein [Spodoptera frugiperda]
CsupOBP10620147Unigene17320Complete ORFY2.0E-37dbj|BAI44701.1|odorant binding protein [Bombyx mori]
CsupOBP111046147Unigene542Complete ORFY5.0E-60gb|AFG72998.1|odorant-binding protein 1 [Cnaphalocrocis medinalis]
CsupOBP121847145Unigene4662Complete ORFN4.0E-45gb|AFG72998.1|odorant-binding protein 1 [Cnaphalocrocis medinalis]
CsupOBP131151145Unigene152065' lostY8.0E-60gb|AFI57166.1|odorant-binding protein 17 [Helicoverpa armigera]
CsupOBP14534140Unigene22895Complete ORFY4.0E-15gb|ACX53795.1|odorant binding protein [Heliothis virescens]
CsupOBP15978139Unigene3748Complete ORFY2.0E-63gb|AFG73000.1|odorant-binding protein 2 [Cnaphalocrocis medinalis]
CsupOBP16636138Unigene2333Complete ORFY1.0E-38gb|AFI57167.1|odorant-binding protein 18 [Helicoverpa armigera]
CsupOBP171503133CL2095.Contig1Complete ORFY2.0E-24gb|EFA04687.1|odorant binding protein 08 [Tribolium castaneum]
CsupOBP18563129CL5651.Contig2Complete ORFN2.0E-29gb|AFG72998.1|odorant-binding protein 1 [Cnaphalocrocis medinalis]
CsupOBP192038121CL5839.Contig13' lostY5.0E-44gb|EHJ65653.1|odorant-binding protein 1 [Danaus plexippus]
CsupOBP20387113Unigene364763' lostY2.0E-41gb|AFD34173.1|odorant binding protein 5 [Argyresthia conjugella]
 Table 5 

Unigenes of candidate chemosensory proteins

Gene nameLength (nt)ORF (aa)Unigene
reference
StatusSignal PeptidesEvalueBLASTx best hit
CsupCSP1727189Unigene35502_AllComplete ORFY2.00E-37gb|ACX53806.1|chemosensory protein [Heliothis virescens]
CsupCSP23757151CL5573.Contig2_AllComplete ORFY6.00E-38gb|EHJ76401.1|chemosensory protein CSP1 [Danaus plexippus]
CsupCSP31279131Unigene21279_AllComplete ORFY1.00E-52gb|ACX53804.1|chemosensory protein [Heliothis virescens]
CsupCSP41782129Unigene13693_AllComplete ORFY1.00E-17dbj|BAF91712.1|chemosensory protein [Papilio xuthus]
CsupCSP5510128CL2398.Contig1_AllComplete ORFY1.00E-68gb|AFR92093.1|chemosensory protein 9 [Helicoverpa armigera]
CsupCSP6710126Unigene2439_AllComplete ORFY6.00E-47dbj|BAF91714.1|chemosensory protein [Papilio xuthus]
CsupCSP7625126Unigene31984_AllComplete ORFY3.00E-43dbj|BAG71914.1|chemosensory protein 4a [Papilio xuthus]
CsupCSP8503126Unigene39861_AllComplete ORFY5.00E-65gb|AAM77040.1|chemosensory protein 2 [Heliothis virescens]
CsupCSP9481124Unigene39944_AllComplete ORFY2.00E-64gb|ACX53727.1|chemosensory protein [Heliothis virescens]
CsupCSP10577123CL4176.Contig1_AllComplete ORFY1.00E-53dbj|BAG71921.1|chemosensory protein 13 [Papilio xuthus]
CsupCSP11726123Unigene13345_AllComplete ORFY5.00E-46dbj|BAF91716.1|chemosensory protein [Papilio xuthus]
CsupCSP12456123Unigene25770_AllComplete ORFY9.00E-35gb|EHJ78408.1|chemosensory protein [Danaus plexippus]
CsupCSP13769121CL1604.Contig1_AllComplete ORFY5.00E-21gb|EHJ73331.1|chemosensory protein 12 [Danaus plexippus]
CsupCSP141272121CL5801.Contig1_AllComplete ORFY3.00E-56gb|ACX53719.1|chemosensory protein [Heliothis virescens]
CsupCSP15503121Unigene24547_AllComplete ORFY9.00E-29ref|NP_001037068.1|chemosensory protein 7 precursor [Bombyx mori]
CsupCSP16536121Unigene28390_AllComplete ORFY3.00E-41dbj|BAF91717.1|chemosensory protein [Papilio xuthus]
CsupCSP172686120CL4999.Contig1_AllComplete ORFY3.00E-27dbj|BAG71919.1|chemosensory protein 11b [Papilio xuthus]
CsupCSP181153109Unigene15508_AllComplete ORFY1.00E-48dbj|BAF91720.1|chemosensory protein [Papilio xuthus]
CsupCSP1936797CL2398.Contig2_All5' lostN1.00E-51gb|AFR92095.1|chemosensory protein 11 [Helicoverpa armigera]
CsupCSP2030695Unigene38877_All5' lostN1.00E-50gb|AAK53762.1|chemosensory protein [Helicoverpa armigera]
CsupCSP2126889Unigene39576_All5',3' lostN1.00E-39gb|ACX53806.1|chemosensory protein [Heliothis virescens]

Tissue- and Sex- specific Expression of Candidate C. suppressalis OR and IR genes

The expression patterns of the candidate 7 ORs and 20 IRs in male antennae, female antennae and legs were analyzed by semi-quantitative reverse transcription PCR. Results for all of these genes are listed in Figure 8.

 Figure 8 

Tissue- and sex- specific expressions of candidates CsupORs and CsupIRs. M: male antennae, F: female antennae, L: legs.

Int J Biol Sci Image (Click on the image to enlarge.)

All 47 ORs were successfully detected as expressed in our semi-quantitative RT-PCR analysis. These results indicated that these candidate ORs are expressed in the olfactory organ antennae, but not in non-olfactory organs such as legs. Of the six candidate PRs, only CsupPR3 was found to be exclusive to the male antennae. CsupPR1, 2 and 4 have expression detected in both male and female antennae, but the amounts in male antennae were significantly higher than in female antennae. Of the candidate ORs, two sequences (CsupOR39 and CsupOR40) showed a male-specific expression. The remaining ORs were expressed in both sexes, with some differential expression in male or female antennae. Compared with ORs, the candidate IRs showed no big differences between male and female. Additionally, CsupIR3 and CsupIR64a have considerable expression in leg, at similar levels, based upon luminance of the bands in the agarose gels. CsupIR2 and CsupIR75p.1 are also weakly expressed in legs.

Discussion

Our antennal transcriptome sequencing provides a dataset of 47 ORs, 20 IRs, 26 OBPs and 21 CSPs. All of the previously annotated C. suppressalis chemosensory genes available in NCBI were identified in our dataset. Compared to the antennal transcriptomes of M.sexta with 47 ORs [27], C. pomonella with 43 ORs [28] and Helicoverpa armigera with 47 ORs [4], our OR dataset of 47 sequences is of similar quantity. The neuroanatomical study of C. suppressalis' close relative, the European corn borer (ECB) (Ostrinia nubilalis) suggested that there are 64 glomeruli in the antenna lobe of male and female ECB [29]. If the logic holds true that one olfactory receptor type is expressed in OSN type and axonal projects of different OSNs expressing the same olfactory receptor converge on the same antennal lobe glomeruli, our OR dataset of 47 sequences is quite reasonable, for some glomeruli should also be innervated by OSNs expressing other classes of chemoreceptors such as ionotropic receptors [18].

We identified 6 candidate PR genes by their similarities to PRs in other Lepidopterans and physiologic analysis. But the expression profiles of these six sequences showed that not all of them are exclusive in male antenna. Although PR expression in Bombyx mori and some Lepidoptera have been shown to be restricted to male antennae [30, 31], some recent studies gave examples of PR genes expressed in both sexes. In the antennal transcriptome of Helicoverpa armigera, 6 candidate PR genes were discovered; two of them, HarmOR6 and HarmOR13, display expression in male and female antenna [4]. Two candidate PRs identified in S. littoralis were also found to be expressed in antennae of both sexes [32]. Obviously, this phenomenon cannot be simply ascribed to differences between species. A physiology and morphology study suggested that S. littoralis females detect their own pheromone. The rationale behind female pheromone perception has been proposed to be optimization of pheromone production and spatial dispersion of females over host plants [33, 34].

Ionotropic receptors represent a new member of the chemosensory receptor family, and were first discovered in D. melanogaster [18] through genome analyses. The Ionotropic Receptor family is a variant iGluR subfamily. Animal iGluRs have been best characterized for their essential roles in synaptic transmission as receptors for the excitatory neurotransmitter glutamate [35, 36]. IRs share a considerable degree of commonality with the typical iGluRs: first, they are all located to specialized distal membrane domains of neuronal dendrites (OSN cilia and post-synaptic membranes, respectively); secondly, they responds to binding of extracellular ligands (volatile odorants and neurotransmitter); thirdly, they are comprised of multimeric functional complexes (IR8a/25a co-express with other cell-type specific IRs and iGluRs are formed of heteromeric subunits) [20]. It is easy to conjecture that the IR arose from an iGluR with a change in expression localization from an interneuron to a sensilla neuron [20]. In our study, we found 20 IR candidates in C. suppressalis antennae including two co-receptors, IR8a and IR25a. Compared to ORs, the IR family is relatively conserved both in sequence and expression pattern. Among the 20 CsupIRs we discovered, 15 sequences have orthologs found in Dmel/Bmor/Slit IRs; the expression levels have no significant difference between male and female antenna, which is similar to results in S. littoralis IRs [37] and H. armigera IRs [4]. Considering the relatively high sequence conservation and similarities in expression, the functions of CsupIRs are probably conserved as IRs in other Lepidoptera.

Conclusion

Our goal for this study was to identify genes potentially involved in olfactory signal detection in C. suppressalis, and this is well met by a repertoire of 47 ORs, 20 IRs, 26 OBPs and 21 CSPs. Our approach has been proved to be successful in identifying low-expressing chemosensory receptor genes, especially in a non-model pest species without an available genome sequence. Our findings make it possible for future research of the olfactory system of C. suppressalis at the molecular level, and provide information for comparative and functional genomic analyses of related species.

Materials and Methods

Insects

C. suppressalis were obtained from a laboratory colony maintained at the Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing, China. Larvae were reared on an artificial diet at 28±1°C, 70±5% relative humidity, and under a photoperiod of 14:10 (light: dark). After pupation, male and female pupae were separated for adult eclosion in cages kept at 30±1°C, 80±5% RH and 16: 8 light/dark cycles, and were fed with 10% sugar solution. Antennae of unmated male or female individuals were collected 1-3 days after eclosion and immediately frozen in liquid nitrogen, and stored at -70°C until extraction. Antennae were pulled off with tweezers grasped at the very root of the antennae, in order to reserve the intact structure of antennae.

RNA preparing

Frozen antennae were crushed in a liquid nitrogen cooled vitreous homogenizer and total RNA was extracted using TRIzol reagent (Invitrogen, Carlsbad, CA, USA) following the manufacturer's instructions. Residual DNA in total RNA was removed by DNase I (Promega, Madison, WI, USA). Total RNA was dissolved in RNase-free water and RNA integrity was verified by gel electrophoresis. RNA quantity was determined on a Nanodrop ND-2000 spectrophotometer (NanoDrop products, Wilmington, DE, USA).

cDNA library construction and sequencing

Ten micrograms of total RNA extracted from approximately 500 antennae of 1-3 day old adult male or female moths was used to isolate poly-A RNA using oligo(dT) magnetic beads. Poly-A RNA of each sample was digested into short fragments by fragmentation buffer. Random hexamers were used for first-strand cDNA, followed by second-strand cDNA synthesis using RNase H and DNA polymerase I. These dual-strand DNA samples were treated with T4 DNA Polymerase and T4 Polynucleotide Kinase for end-repairing and dA-tailing, followed by adaptor ligation to the dsDNA's dA tail using T4 DNA ligase. Then bands of insert length around 200bp was collected by 2% agarose gel electrophoresis and purified with QIAquick Gel Extraction Kit (Qiagen, Hilden, Germany), and used as templates for PCR amplification to create the cDNA library.

The library was pair-end sequenced using PE90 strategy (paired-end reads of 90 base pairs per read) on Illumina HiSeq™ 2000 (Illumina, San Diego, CA, USA) at the Beijing Genome Institute (Shenzhen, China). Different libraries were sequenced in one lane and raw-reads were sorted by barcodes in the sequencing adaptor.

Assembly

The raw-reads were treated to generate clean-read datasets by the following procedure. First, reads with adaptors or containing unknown nucleotides (Ns) more than 5% were removed directly. Secondly, low-quality reads containing more than 20% suspect-nucleotides of Phred Quality Score less than 10 were filtered out. Finally, both ends of reads were evaluated to trim unreliable ends containing more than 3 successive suspect-nucleotides. Each clean-read dataset of male and female antenna was feed to Trinity r2012-06-08 [38] separately for De novo assembly using paired reads mode and default parameters. Then the Trinity outputs were clustered by TGICL [39]. The consensus cluster sequences and singletons make up the unigenes dataset.

Functional annotation

The annotation of unigenes were performed by NCBI blastx against a pooled database of non-redundant (nr) and SwissProt protein sequences with e-value < 1e-5. The blast results were then imported into Blast2GO [40] pipeline for GO Annotation. Protein coding region prediction was performed by OrfPredictor [41] according to the blast result. The signal peptide of the protein sequences were predicted using SignalP 4.0 [41] server version (http://www.cbs.dtu.dk/services/SignalP/) with default parameters. The transmembrane-domains of annotated genes were predicted using TMHMM [43] server version2.0 (http://www.cbs.dtu.dk/services/TMHMM) with the new model.

Phylogenetic analyses

The phylogenetic reconstruction of C. suppressalis chemosensory genes was performed according to our previous research [4]. Amino acid sequences were aligned using Clustal Omega [44]. Phylogenetic trees were constructed by the neighbor-joining method, with Jones-Taylor-Thornton (JTT) amino acid substitution model, as implemented in MEGA5.2 software. Node support was assessed using a bootstrap procedure of 1000 replicates.

Expression analysis by semi-quantitative reverse transcription PCR

Semi-quantitative reverse transcription PCR was performed to verify the expression of candidate chemosensory genes. Tissue samples were collected from C. suppressalis adult 1 day after eclosion for 3 biological replicates and total RNA were extracted as mentioned above. The cDNA was synthesized from total RNA using RevertAid First Strand cDNA Synthesis Kit (Thermo Scientific, Waltham, MA, USA) with the gDNA removal procedure performed. Gene-specific primers were designed by Primer3 tool (http://frodo.wi.mit.edu/primer3/) and sequences are available in Additional File 2: supplementary material S2. Taq MasterMix (CWBIO, Beijing, China) was used for PCR reactions under general 3-step amplification of 94 °C for 30s, 55-60 °C for 30s, 72 °C for 30s. The PCR cycle-numbers were adjusted respectively for each gene. For most chemosensory genes, cycle-numbers were range from 30 to 34, but for some high-express-level genes like actin and Orco, cycle-numbers were reduced to 25 to 29. PCR products were run on a 2% agarose gel and verified by DNA sequencing.

Abbreviations

iGluR: ionotropic glutamate receptor; OR: odorant receptor; IR: ionotropic receptor; PR: pheromone receptor; PBP: pheromone binding protein; GOBP: general odorant binding protein; OBP: odorant binding protein; CSP: chemosensory protein; SNMP: sensory neuron membrane protein; GO: gene ontology; FPKM : fragments per kb per million fragments; FDR: false discovery rate; JTT: Jones-Taylor-Thornton amino acid substitution model.

Supplementary Material

Attachment Additional File 1 

Supplementary Material S1 Amino acid sequences of C. suppressalis olfactory genes.

Attachment Additional File 2 

Supplementary Material S2 Primers used in the semi-quantitative RT-PCR analysis.

Acknowledgements

We thank Dr. Yunhe Li (Institute of Plant Protection, Chinese Academy of Agricultural Sciences) for providing the insects. This work was supported by National Basic Research Program of China (973 Program, 2012CB114104), National Natural Science Foundation of China (31230062) and International Science & Technology Cooperation Program of China (2013DFG32230).

Data Deposition

The clean reads of the C. suppressalis antennal transcriptome were stored in the NCBI SRA database, under the accession number of SRX497236 and SRX497239.

Competing Interests

The authors have declared that no competing interest exists.

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Author contact

Corresponding address Corresponding author: grwangcn (G.R. Wang)


Received 2014-4-4
Accepted 2014-5-25
Published 2014-7-26