Comparative analyses of odorant binding protein orthologues in three sympatric Tomicus bark beetles provide insights into functional differentiation of OBPs to ecologically relevant odorants and insecticides

Three Tomicus bark beetles (T. yunnanensis, T. brevipilosus and T. minor) coexist in the trunks of Pinus yunnanensis but possess relatively independent spatio-temporal ecological niches. This makes them become a good case for addressing functional differentiation of chemosensory-related proteins among closely related species in odorant recognition and insecticide sequestering. In this study, we used odorant binding protein (OBP) orthologues highly expressed in antennae to illustrate this question. We first identified 33, 35 and 34 OBPs from T. yunnanensis, T. brevipilosus and T. minor, respectively, with 33 groups of OBP orthologues. Sequence and phylogenetic analyses revealed the conservation and differences of OBP orthologues across three Tomicus beetles. Expression profiles revealed that the majority of Tomicus OBPs were abundantly expressed in antennae where 11 groups of OBP orthologues shared a significantly antenna-enriched distribution. Four groups of antennal OBP orthologues (OBP2, OBP9, OBP10 and OBP16 totaling 12 genes) presented diverse ligand-binding properties, responding differently to five classes of ecologically relevant odorants as well as various insecticides. In particular, OBP orthologues were tuned differentially to host and non-host odorants, and chlorpyrifos was identified as the best ligand for 12 Tomicus OBPs. Our study thus sheds light on functional conservation and divergence of OBP orthologues among three sympatric Tomicus bark beetles, possibly as an implication for the early or late of host colonization, ecological niche differences and the strong habitat adaptation.