Antennal transcriptome analysis of the chemosensory gene families in the tree killing bark beetles, Ips typographus and Dendroctonus ponderosae (Coleoptera: Curculionidae: Scolytinae)

Abstract

Background: The European spruce bark beetle, Ips typographus, and the North American mountain pine beetle, Dendroctonus ponderosae (Coleoptera: Curculionidae: Scolytinae), are severe pests of coniferous forests. Both bark beetle species utilize aggregation pheromones to coordinate mass-attacks on host trees, while odorants from host and non-host trees modulate the pheromone response. Thus, the bark beetle olfactory sense is of utmost importance for fitness. However, information on the genes underlying olfactory detection has been lacking in bark beetles and is limited in Coleoptera. We assembled antennal transcriptomes from next-generation sequencing of I. typographus and D. ponderosae to identify members of the major chemosensory multi-gene families.

Results: Gene ontology (GO) annotation indicated that the relative abundance of transcripts associated with specific GO terms was highly similar in the two species. Transcripts with terms related to olfactory function were found in both species. Focusing on the chemosensory gene families, we identified 15 putative odorant binding proteins (OBP), 6 chemosensory proteins (CSP), 3 sensory neuron membrane proteins (SNMP), 43 odorant receptors (OR), 6 gustatory receptors (GR), and 7 ionotropic receptors (IR) in I. typographus; and 31 putative OBPs, 11 CSPs, 3 SNMPs, 49 ORs, 2 GRs, and 15 IRs in D. ponderosae. Predicted protein sequences were compared with counterparts in the flour beetle, Tribolium castaneum, the cerambycid beetle, Megacyllene caryae, and the fruit fly, Drosophila melanogaster. The most notable result was found among the ORs, for which large bark beetle-specific expansions were found. However, some clades contained receptors from all four beetle species, indicating a degree of conservation among some coleopteran OR lineages. Putative GRs for carbon dioxide and orthologues for the conserved antennal IRs were included in the identified receptor sets.

Conclusions: The protein families important for chemoreception have now been identified in three coleopteran species (four species for the ORs). Thus, this study allows for improved evolutionary analyses of coleopteran olfaction. Identification of these proteins in two of the most destructive forest pests, sharing many semiochemicals, is especially important as they might represent novel targets for population control.

Figure 1

Gene ontology (GO) results. GO analysis corresponding to 8,713 contig sequences in Ips typographus and 10,713 isotigs in Dendroctonus ponderosae, as predicted for their involvement in A) molecular function (level 3 GO categorization) and B) biological process (level 2). For results presented as detailed bar diagrams, see Additional file 2.

Figure 2

Maximum-likelihood dendrogram based on protein sequences of candidate odorant binding proteins (OBPs). Included are OBPs from Ips typographus (Ityp), Dendroctonus ponderosae (Dpon), Tribolium castaneum (Tcas) and Drosophila melanogaster (Dmel). One major beetle-specific expansion of Minus-C OBPs is evident. Bark beetle proteins in this expansion have lost cysteine residue C2 and C5. Two OBPs in I. typographus and one in D. ponderosae belong to the Plus-C group characterized by additional cysteine residues. Numbers refer to support values, which are only displayed when <; 0.9.

Figure 3

Maximum-likelihood dendrogram based on protein sequences of candidate chemosensory proteins (CSPs). Included are CSPs from Ips typographus (Ityp), Dendroctonus ponderosae (Dpon), Tribolium castaneum (Tcas) and Drosophila melanogaster (Dmel). Numbers refer to support values, which are only displayed when <; 0.9.

Figure 4

Maximum-likelihood dendrogram based on protein sequences of candidate sensory neuron membrane proteins (SNMPs). Included are SNMPs from Ips typographus (Ityp), Dendroctonus ponderosae (Dpon), Tribolium castaneum (Tcas) and Drosophila melanogaster (Dmel). The T. castaneum orthologue of Croqemort, a non-SNMP member of the CD36 family, was used as outgroup to root the tree. Numbers refer to support values, which are only displayed when <; 0.9.

Figure 5

Maximum-likelihood dendrogram based on protein sequences of candidate odorant receptors (ORs) and gustatory receptors (GRs). Included are ORs and GRs from Ips typographus (Ityp), Dendroctonus ponderosae (Dpon), Tribolium castaneum (Tcas) and Megacyllene caryae (Mcar). The branch containing bark beetle GRs was used as outgroup to root the tree. The different subgroups (numbered 1–7 according to [29,53], and 7a-7b) are discussed in the main text. Originally, TcasOr339 and TcasOr340 were found within group 4 and 6 [53], as indicated here by the numbers in brackets. Numbers at nodes refer to support values, which are only displayed when <; 0.9.

Figure 6

Maximum-likelihood dendrogram based on protein sequences of candidate ionotropic receptors (IRs). Included are IRs from Ips typographus (Ityp), Dendroctonus ponderosae (Dpon), Tribolium castaneum (Tcas) and Drosophila melanogaster (Dmel). Numbers refer to support values, which are only displayed when <; 0.9.