Spinosad-resistance mechanisms of Bactrocera dorsalis, one of the most important agricultural pests worldwide, were investigated. Resistance levels to spinosad in a B. dorsalis strain from Taiwan were more than 2000-fold, but showed no cross resistance to imidacloprid or fipronil. Combined biochemical and synergistic data indicated that target-site insensitivity is the major resistance component. The gene encoding the nAChR subunit alpha 6 (Bdα6), the putative molecular target of spinosad, was isolated using PCR and RACE techniques. The full-length cDNA of Bdα6 from spinosad-susceptible strains had an open reading frame of 1467 bp and codes for a typical nAChR subunit. Two isoforms of exon 3 (3a and 3b) and exon 8 (8a and 8b), and four full-length splicing variants were found in the susceptible strain. All transcripts from the spinosad-resistant strain were truncated and coded for apparently non-functional Bdα6. Genetic linkage analysis further associated spinosad-resistance phenotype with the truncated Bdα6 forms. This finding is consistent with a previous study in Plutella xylostella. Small deletions and insertions and consequent premature stop codons in exon 7 were associated with the truncated transcripts at the cDNA level. Analysis of genomic DNA sequences (intron 2 and exons 3-6) failed to detect exon 5 in resistant flies. In addition, a mutation in Bdα6 intron 2, just before the truncated/mis-splicing region and in same location with a mutation previously reported in the Pxylα6 gene, was identified in the resistant flies. RNA editing was investigated but was not found to be associated with resistance. While the demonstration of truncated transcripts causing resistance was outlined, the mechanism responsible for generating truncated transcripts remains unknown.

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