The α6 nicotinic acetylcholine receptor subunit of Frankliniella occidentalis is not involved in resistance to spinosad.

Insects evolve resistance which constrains the sustainable use of insecticides. Spinosyns, a class of environmentally-friendly macrolide insecticides, is not an exception. The mode of inheritance and the mechanisms of resistance to spinosad (the most common spinosyn insecticide) in Frankliniella occidentalis (Western flower thrips, WFT) were investigated in this study. Resistance (170,000-fold) was autosomal and completely recessive. Recent studies showed that deletion of the nicotinic acetylcholine receptor α6 subunit gene resulted in strains of Drosophila melanogaster, Plutella xylostella and Bactrocera dorsalis that are resistant to spinosad, indicating that nAChRα6 subunit maybe important for the toxic action of this insecticide. Conversely, a G275E mutation of this subunit in F. occidentalis was recently proposed as the mechanism of resistance to spinosad. We cloned and characterized nAChRα6 from three susceptible and two spinosad resistant strains from China and the USA. The Foα6 cDNA is 1873bp and the open reading frame is 1458bp which encodes 485 amino acid residues with a predicted molecular weight of 53.5-kDa, the 5' and 3' UTRs are 121 and 294bp, respectively. There was no difference in the cDNA sequence between the resistant and susceptible thrips, suggesting the G275E mutation does not confer resistance in these populations. Ten isoforms of Foα6, arising from alternative splicing, were isolated and did not differ between the spinosad-susceptible and resistant strains. Quantitative real time PCR analysis showed Foα6 was highly expressed in the first instar larva, pupa and adult, and the expression levels were 3.67, 2.47, 1.38 times that of the second instar larva. The expression level was not significantly different between the susceptible and resistant strains. These results indicate that Foα6 is not involved in resistance to spinosad in F. occidentalis from China and the USA.