Functional characterization and fitness cost of spinosad-resistant alleles in Ceratitis capitata

The sustainability of control programs for the Mediterranean fruit fly, Ceratitis capitata, for citrus crops in Spain has been threatened by the development of resistance to malathion and lambda-cyhalothrin in recent years. Spinosad is widely used without apparent loss of efficacy. However, a highly resistant strain, JW-100s, has been obtained after laboratory selection. Spinosad resistance in JW-100s has been associated with different mutant alleles of the alpha 6 subunit of the nicotinic acetylcholine receptor (Cc alpha 6) including an isoform-specific truncation allele, Cc alpha 6(3aQ68*). Using the GAL4 > UAS system in Drosophila melanogaster to demonstrate expression of this truncated alpha 6 subunit, in a d alpha 6 loss-of-function genetic background, does not rescue susceptibility to spinosad, while the expression of Cc alpha 6 wild-type isoforms does. We have also generated C. capitata isolines from JW-100s homozygous for: (1) the Cc alpha 6(3aQ68*Delta 3b-4) allele, which contains the mutation 3aQ68*, and (2) the Cc alpha 6(3aQ68*-K352*) allele, which contains the mutations 3aQ68* and K352*. Neither of these produce complete Cc alpha 6 transcripts. The frequency of resistant alleles declined when in competition with individuals carrying the wild-type allele. Through extensive testing of both biological and behavioral fitness traits, we identified a reduced ability of Cc alpha 6(3aQ68*Delta 3b-4) males to detect the parapheromone and to mate with females carrying the Cc alpha 6(3aQ68*-K352*) allele in competition experiments. Thus, not only the potential for spontaneous resistant mutations to arise in Cc alpha 6 but also their fitness costs must be considered when planning resistance management strategies for C. capitata.