Nicotinic acetylcholine receptor subunit α6 associated with spinosad resistance in Rhyzopertha dominica (Coleoptera: Bostrichidae).

The lesser grain borer, Rhyzopertha dominica, which is a primary pest of stored products, breaks up whole grains and makes them susceptible to secondary infestation by other pests. Insecticide application is the main control measure against this borer. A resistant strain of R. dominica against the insecticide, spinosad, was selected in the laboratory. The full-length cDNA of the target site of spinosad, nicotinic acetylcholine receptor subunit α6, from R. dominica (Rdα6) was cloned and analyzed using reverse transcription PCR and rapid amplification of cDNA ends. The complete 2133-bp cDNA contains the open reading frame of 1497 bp encoding a 498-amino-acid protein. There are four predicted transmembrane (TM) regions, and six extracellular ligand-binding sites at the N-terminus, upstream from the first TM in Rdα6. Three mutations have been found in the resistant strain compared with the susceptible one: (1) a 181-bp fragment truncated at the N-terminus, resulting in the appearance of a premature stop codon, (2) one missing bp at the position 997, causing a frame-shift mutation, and (3) an 87-bp fragment truncated in the TM2 region. In addition, real-time quantitative PCR was applied to detect the transcriptional expression of Rdα6 in both the susceptible and resistant strains. The results indicated that the expression of Rdα6 was significantly lower in then resistant strain than in susceptible one. In conclusion, mutation of Rdα6 may cause R. dominica resistant to spinosad due to target site insensitivity.