Multiple mutations in the nicotinic acetylcholine receptor Ccα6 gene associated with resistance to spinosad in medfly

Abstract

Spinosad is an insecticide widely used for the control of insect pest species, including Mediterranean fruit fly, Ceratitis capitata. Its target site is the α6 subunit of the nicotinic acetylcholine receptors, and different mutations in this subunit confer resistance to spinosad in diverse insect species. The insect α6 gene contains 12 exons, with mutually exclusive versions of exons 3 (3a, 3b) and 8 (8a, 8b, 8c). We report here the selection of a medfly strain highly resistant to spinosad, JW-100 s, and we identify three recessive Ccα6 mutant alleles in the JW-100 s population: (i) Ccα6^3aQ68* containing a point mutation that generates a premature stop codon on exon 3a (3aQ68*); (ii) Ccα6^3aAG>AT containing a point mutation in the 5' splicing site of exon 3a (3aAG > AT); and (iii) Ccα6^3aQ68*-K352* that contains the mutation 3aQ68* and another point mutation on exon 10 (K352*). Though our analysis of the susceptibility to spinosad in field populations indicates that resistance has not yet evolved, a better understanding of the mechanism of action of spinosad is essential to implement sustainable management practices to avoid the development of resistance in field populations.

Figure 1

Schematic representation of the potential isoforms coded by the Ccα6 alleles. Numbered boxes represent the exons. White figures refer to full-length wild-type isoforms, while grey figures indicate truncated isoforms. (A) Isoforms produced by the Ccα6 wild-type, with the alternative exons 3a/3b and 8a/8b/8c; TM, transmembrane domains; A-F, ligand-binding loops; inverted triangle, signature cysteines for nAChR α subunits. (B) Truncated isoforms produced by Ccα6^3aQ68*-K352*. (C) Isoforms coded by the Ccα6^3aQ68*. (D) Isoforms produced by Ccα6^3aAG>AT.