Identification and characterization of chemosensory gene families in the bark beetle, Tomicus yunnanensis.

The bark beetle, Tomicus yunnanensis (Coleoptera: Scolytinae), is a seriously destructive pest of Yunnan pine (Pinus yunnanensis) and is distributed solely in Southwestern China. It has been a challenge to control this pest owing to its resistance to chemical pesticides, which have been used as the main control strategy of this species in recent years. Since this approach will continue until an alternative mitigation strategy is implemented, it is essential to develop novel or improved biocontrol approaches. In the current study, we aimed to identify most, if not all, of the bark beetle's chemosensory genes, and to address their respective phylogenetic relationships and expression characteristics. Digital gene expression (DGE) profiling and a comparison of the profiles at three developmental stages yielded 40,287,265 clean reads and a large number of differentially expressed genes (DEGs), with 21 up- and 20 down-regulated DEGs involved in chemoreception. Transcriptome of the three mixed stages revealed a total of 80 transcripts encoding chemosensory-related proteins comprising 45 odorant-binding proteins (OBPs), 12 chemosensory proteins (CSPs), 20 receptor proteins [9 odorant receptors (ORs), 8 gustatory receptors (GRs) and 3 ionotropic receptors (IRs)] and 3 sensory neuron membrane proteins (SNMPs). As many as 38 full-length sequences were acquired with a combination of transcriptomic analysis and rapid amplification of cDNA ends (RACE) strategy. Phylogenetic analysis showed that T. yunnanensis OBPs were clustered into four sub-groups: 27 Minus-C OBPs, 5 antennal binding proteins (ABPIIs), 10 Classic OBPs and one Plus-C OBP; meanwhile, the ORs were grouped into four clades (1, 2, 7b and Orco). Expression profiles revealed that 66 of 80 genes were detected in the three DGE libraries, and 15 soluble olfactory proteins were antennae-predominant, possibly guiding olfactory-associated behaviors of this beetle. Taken together, our study has provided valuable data for further functional studies of this beetle and will facilitate the identification of potential molecular targets associated with chemosensory reception for use in biocontrol strategies.