Antennal transcriptome analysis of olfactory genes and characterizations of odorant binding proteins in two woodwasps, Sirex noctilio and Sirex nitobei (Hymenoptera: Siricidae)

- Most of the olfactory genes identified in two species were homologous.
- In total, 14 OBPs were expressed primarily in the antennae..
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- Different expression in the antennae, external genitals or heads, exhibiting an obvious sex bias, suggested their different role in recognizing sex pheromones or plant volatiles.
- The morphology of the two species, S.
- noctilio adults started to emerge in the field from late June to early September, subse- quently, emerge peak of S.
- Insects use their olfactory systems to sense odors and changes in the environment and thus, to adjust behav- iors such as locating hosts for food, mating, and ovipos- ition [20].
- It has been thought that odorant binding proteins (OBPs) and chemosensory proteins (CSPs) in the lymph can recognize, bind, and transport odor molecules.
- The OBP/CSP-odor molecule complexes then interact with chemosensory receptors, which are located in the den- dritic membrane of OSNs [23, 24].
- These signals are inte- grated in the insect brain to produce behavioral instruc- tions for insects to respond accordingly [27].
- Taken together, our findings identified and compared olfactory genes in the two woodwasps based on antennal tran- scriptome analysis, and established a start point for fur- ther research on molecular mechanisms of olfactory system in symphyta woodwasps..
- nitobei were deposited in the NCBI SRA database (the accession number of S.
- GO annotation in- dicated that the distributions of GO terms in the uni- genes were highly similar between the two species (Figs.
- The similar result was found in the GO annotation between Helicoverpa armigera and H.
- In the molecular function category,.
- We identified 16 and 15 OBPs in the S.
- We found 13 Sno- cOBPs and 14 SnitOBPs with expression values greater than 1 FPKM, while 6 SnocOBPs and 7 Sni- tOBPs exhibited expression values greater than 100 FPKM, indicating high expression of these OBPs in the antennae..
- However, the FPKM value of SnocOBP13 was less than 0.001 in the transcriptome dataset, so it was hardly expressed in antennae.
- OBP4, OBP7, and OBP10 of both woodwasps were clustered in the GOBP lineages with and 1.00 bootstrap support values, respectively..
- We identified 7 SnocCSPs and 6 SnitCSPs in the anten- nal transcriptomes of the two woodwasp species (Add- itional file 2, Table S2).
- The expression values (FPKM) of 4 SnocCSPs and 5 SnitCSPs were greater than 1, while 1 SnocCSP and 3 SnitCSPs displayed expression values greater than 100, indicating that these genes were highly expressed in the woodwasp antennae..
- In the phylogenetic tree (Fig.
- We identified 41 and 43 ORs in the S.
- Most SnocORs and SnitORs were clustered together in the phylogenetic tree (Fig.
- Two special lineages were identified in the tree.
- And the two ORcos were clustered with the honey bee ORco AmelOR2 [32] and other Hymenoptera ORco, suggesting they could function as a complex with other ORs in the woodwasps as the ORcos in other insects.
- greater than 100, indicating that Sirex SNMPs are highly expressed in the antennae..
- In our phylogenetic tree, SnocSNMP1 and SnitSNMP1 clustered in the SNMP1 lineage with a bootstrap support value of 1.00 (Fig.
- We identified 8 and 10 GRs in the S.
- No Sirex GRs clustered in the bitter taste lineages..
- Most Sirex GRs exhibited homology to sugar taste receptors, and one SnocGR and 3 SnitGRs clustered in the sugar taste lineages.
- We identified 13 and 16 IRs in the S.
- Of these, SnocIR6 and SnitIR6 had the greatest expression in the antennal transcriptome with FPKM values of 16.359 and 21.583.
- 8), SnocIR6 and SnitIR6 clustered in the co- receptor IR8a lineages with a bootstrap support value of 1.00, while SnocIR4 and SnitIR4 clustered in the co- receptor IR25a lineages with a bootstrap support value of 1.00.
- mediator [36], while only one IR25a homolog was found in the wasp species C.
- In our phylogenetic tree, most of the S.
- Additionally, we found that most of the S.
- In the same way, some of the.
- To verify OBPs expression in the antennae and characterize the expression profiles of OBPs in 4 chemo- sensory tissues (antennae, legs, heads, and externalia), 10 SnocOBPs and 10 SnitOBPs with high FPKM values were selected for fluorescent quantitative real-time PCR (Figs.
- Primers for OBPs and for an internal reference gene (β-tubulin) were listed in the Additional file 6..
- Most OBPs were expressed mainly in the antennae of the two wood- wasps.
- The observed high expression in the antennae suggested that the OBPs may play a role in binding and transporting odor signals in antennae.
- Firstly, significant species-specific expression was ob- served for many OBP genes, especially those not greatly expressed in the antennae, including OBP3 and OBP10..
- SnocOBP3 was primarily expressed in the genitalia of fe- male S.
- In addition, we also found that some homolo- gous genes mainly expressed in the antennae differ in their expression profiles between the two species.
- For both SnocOBP9 and SnitOBP9 that were identified as PBP homologues, we observed high expression in an- tennae and no detectible expression in the other organs..
- Additionally, SnocOBP11 and SnitOBP11 were more highly expressed in the female external genitalia and male antennae when compared to the male external genitalia and female antennae (Fig.
- Homologous genes of the two species cluster together.
- From our transcriptome analysis, most of the olfactory genes identified in two species were homologous between the two species, supporting their close relationship..
- Number of OBPs varies greatly among different species Compared with OBPs of model species, the numbers of OBPs encoded in the genomes of siricids (16 in S.
- This suggests differences in the modalities of ol- factory discrimination for different insect species..
- This may also be a reason why the number of OBPs in the sirisids is small.
- We did not uncover any Plus-C OBPs in the transcrip- tomes of S.
- For example, Plus-C OBPs were not found in the genomes of Hymenoptera such as A.
- The expression values of the two Minus-C OBPs in the Sirex transcriptomes were extremely high, indicat- ing that these Minus-C OBPs may be specialized into spe- cial OBPs that play a key role in the wasp’s olfactory systems.
- OBPs in the antenna play critical roles in the adaptation of insects to a wide variety of environments and life styles.
- In the honey bee only nine OBPs are antenna-specific, and the remaining genes are expressed either ubiquitously or are tightly regulated in specialized tissues or during development.
- In the mos- quito Aedes aegypti, OBP22 is expressed in antennae and in male reproductive organs, and is transferred to females during mating [52, 53].
- In a previous report, four OBPs and five CSPs were found in the venom gland proteome of S.
- Through sequence align- ment, it was found that the OBPs identified in the venom gland were SnocOBP2, SnocOBP6, SnocOBP9, and SnocOBP11, and the CSP identified was SnocCSP2–.
- An important task in the subsequent re- search is to understand the other nonolfactory functions of these olfactory proteins..
- Among the olfactory genes identified in the transcrip- tomes, we observed that some of the same types of olfactory genes were located on the close loci, such as SnocOBP10 and SnocOBP16, and SnitCSP3 and SnitCSP4.
- In our phylogenetic analysis, we found two genes, SnocORco and SnitORco, that were clustered with the honey bee ORco AmelOR2 [32] and other Hymenoptera ORco, suggesting they could func- tion as a complex with other ORs in the woodwasps as the ORcos in other insects..
- mediator [36], while only one IR25a homolog was found in the woodwasp species C.
- nitobei), which is similar to the number identified in the honey bee but significantly fewer than those in C..
- Furthermore, none of the investigated woodwasp genes was clustered with AmelOR11, which is the receptor for 9-oxo-2-decenoic acid (9-ODA), the main component of the queen retinue pheromone (QRP) in the honey bee.
- Expression of trehalose receptors in siricids indicated 2 and 7 sweet receptors in the transcriptomes of S.
- Trehalose is also present in the body fluid of insects and can be used as a energy source for flying.
- Most of the olfactory genes identified in two species were homologous be- tween the two species.
- The high expression of OBPs in an- tennae supports the function of OBPs in the semio- chemical recognition process.
- Significant tissue-biased or sex-biased expression was observed for many OBP genes, especially those not greatly expressed in the an- tennae, which suggest OBPs are involved in activities of daily living, for example, recognizing sex pheromones or plant volatile components, and guiding normal behaviors such as feeding, mating, or oviposition.
- After performed amino acid sequence align- ment using the Muscle method, phylogenetic trees of ol- factory unigenes were constructed using the neighbor- joining (NJ) method with the P-distances model and pairwise deletion of gaps in the MEGA v6.0 software package.
- The primers of candidate genes were designed in the same way as chemosen- sory genes, and the candidate genes were evaluated by GeNorm and Normfinder.
- Primers for the internal reference gene (β- tubulin) were listed in the Additional file 6..
- Normfinder result of the six reference genes in S.
- HE participated in the design of the study, the transcriptome analysis and performed all experiments in revision edition.
- ZJ participated in the design of the study and revised the draft of the manuscript.
- They are not included in the “ List of Endangered and Protected Animals in China.
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