Identification and tissue distribution of odorant binding protein genes in the lucerne plant bug Adelphocoris lineolatus (Goeze)

Insect odorant binding proteins (OBPs) are required for insect olfaction perception and play a key role in transporting hydrophobic semiochemicals across the sensillum lymph to the olfactory receptors (Ors). We constructed two high-quality cDNA libraries from the male and female antennae of the lucerne plant bug, Adelphocoris lineolatus (Goeze) (Hemiptera: Miridae), a hemipteran species in a large and economically important phylogenetic group of phytophagous insects. A total of 1538 male ESTs and 1576 female ESTs were sequenced and analyzed, which produced 2915 high-quality ESTs for further analysis. The 2915 ESTs were assembled to 1423 unigenes. Of the 1423 unigenes, 895 (63%) showed no significant similarity with any known GenBank entry. The most prevalent transcripts in the cDNA libraries are OBPs, chemosensory proteins (CSPs), protein takeout precursors, antennae-specific proteins. We identified 14 genes encoding 12 “classical OBPs” with only six conserved cysteines and 2 “Plus-C OBPs” with two additional conserved cysteines and a conserved proline immediately after the sixth cysteine. AlinOBP4 has a very high amino acid identity of 89% to LAP, a well studied OBP of the tarnished plant bug Lygus lineolaris. The expression profiles of the 14 OBPs in different tissues (antennae, heads, thoraxes, abdomens, legs and wings) were measured by real-time qPCR. The results revealed some OBP genes are highly and differentially expressed in male and female antennae, and four OBP genes have an extremely high transcript level in the legs. Of two Plus-C OBP genes, one (AlinOBP14) is highly expressed only in the heads and another (AlinOBP7) is expressed in all olfactory tissues with much lower transcript levels comparing to other OBPs. The possible physiological functions of these OBPs are discussed.

Research highlights
► First report of odorant-binding proteins (OBPs) in the lucerne plant bug, Adelphocoris lineolatus.
► The OBPs are not just expressed in the antennae and have no sexual dimorphism.
► First head specific Plus-C OBP gene AlinOBP14.
► A total of 1538 male antennal ESTs and 1576 female antennal ESTs are sequenced and deposited in the Genbank.