Antennal Transcriptome Analysis of Odorant Reception Genes in the Red Turpentine Beetle (RTB), Dendroctonus valens

Abstract

Background: The red turpentine beetle (RTB), Dendroctonus valens LeConte (Coleoptera: Curculionidae, Scolytinae), is a destructive invasive pest of conifers which has become the second most important forest pest nationwide in China. Dendroctonus valens is known to use host odors and aggregation pheromones, as well as non-host volatiles, in host location and mass-attack modulation, and thus antennal olfaction is of the utmost importance for the beetles' survival and fitness. However, information on the genes underlying olfaction has been lacking in D. valens. Here, we report the antennal transcriptome of D. valens from next-generation sequencing, with the goal of identifying the olfaction gene repertoire that is involved in D. valens odor-processing.

Results: We obtained 51 million reads that were assembled into 61,889 genes, including 39,831 contigs and 22,058 unigenes. In total, we identified 68 novel putative odorant reception genes, including 21 transcripts encoding for putative odorant binding proteins (OBP), six chemosensory proteins (CSP), four sensory neuron membrane proteins (SNMP), 22 odorant receptors (OR), four gustatory receptors (GR), three ionotropic receptors (IR), and eight ionotropic glutamate receptors. We also identified 155 odorant/xenobiotic degradation enzymes from the antennal transcriptome, putatively identified to be involved in olfaction processes including cytochrome P450s, glutathione-S-transferases, and aldehyde dehydrogenase. Predicted protein sequences were compared with counterparts in Tribolium castaneum, Megacyllene caryae, Ips typographus, Dendroctonus ponderosae, and Agrilus planipennis.

Conclusion: The antennal transcriptome described here represents the first study of the repertoire of odor processing genes in D. valens. The genes reported here provide a significant addition to the pool of identified olfactory genes in Coleoptera, which might represent novel targets for insect management. The results from our study also will assist with evolutionary analyses of coleopteran olfaction.

Fig 1. Distribution of unigene size in the D. valens transcriptome assembly.

Fig 2. Percentage of homologous hits of the D. valens transcripts to other insect species.

The D. valens transcripts were searched by BlastX against the non-redundancy protein database with a cutoff E-value of 10⁻⁵. Species which have more than 0.5% matching hits to the D. valens transcripts are shown.

Fig 3. Gene ontology (GO) classification of the D. valens transcripts with Blast2GO program.

Fig 4. The number of chemosensory genes in different insect species, obtained from antenna transcriptome (4a) or genome (4b).

The digits by the histogram bars represent number of chemosensory genes in different subfamilies (OBP:CSP:SNMP:OR:IR). The data were obtained from the current study for D. valens and from references [7,20,24,91] for Drosophila melanogaster, [92] for Sesamia inferens,[,,,–95] for Bomby mori, [7,20,24,91] for Tribolium castaneum,[5] for I. typographus and D. ponderosae, [1] for Manduca sexta, [26] for Agrilus planipennis.

Fig 5. Phylogenetic tree of putative OBPs from Dendroctonus valens (Dval), Ips typographus (Ityp), Dendroctonus ponderosae (Dpon), Tribolium castaneum (Tcas) and Agrilus planipennis (Ap).

The D. valens translated unigenes are shown in blue. Amino acid sequences are given in S1 Fig. The tree was constructed with MEGA5.0, using the neighbor-joining method. Values indicated at the nodes are bootstrap values based on 1000 replicates, and the bootstrap values below 50% are not shown.

Fig 6. Multiple sequences alignment of OBPs of Dendroctonus valens (Dval) with other insects OBPs.

Analyses included OBPs and pheromone binding proteins (PBPs). Amino acid sequences are given in S2 Fig.

Fig 7. Phylogenetic tree of putative CSPs from Dendroctonus valens (Dval), Ips typographus (Ityp), Dendroctonus ponderosae (Dpon), Tribolium castaneum (Tcas) and Drosophila melanogaster (Dmel).

The D. valens translated unigenes are shown in blue. Amino acid sequences are given in S3 Fig. The tree was constructed with MEGA5.0, using the neighbor-joining method. Values indicated at the nodes are bootstrap values based on 1000 replicates, and the bootstrap values below 50% are not shown.

Fig 8. Phylogenetic tree of putative ORs and GRs from Dendroctonus valens (Dval), Ips typographus (Ityp), Dendroctonus ponderosae (Dpon)and Tribolium castaneum (Tcas).

The D. valens translated unigenes are shown in blue. The branch containing GRs was used as outgroup to root the tree. The different subgroups (numbered 1–7 according to [35,53], and 7a-7b) are discussed in the main text. Originally, DponOR15Fix was found within group 2 [29], as indicated here by the numbers in brackets. Amino acid sequences are given in S4 Fig. The tree was constructed with MEGA5.0, using the neighbor-joining method. Values indicated at the nodes are bootstrap values based on 1000 replicates, and the bootstrap values below 50% are not shown.

Fig 9. Phylogenetic tree of putative SNMPs from Dendroctonus valens (Dval), Ips typographus (Ityp), Dendroctonus ponderosae (Dpon), Tribolium castaneum (Tcas) and Drosophila melanogaster (Dmel).

The D. valens translated unigenes are shown in blue. Accession numbers are given in S5 Fig. The tree was constructed with MEGA5.0, using the neighbor-joining method. Values indicated at the nodes are bootstrap values based on 1,000 replicates, and the bootstrap values below 50% are not shown.