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

[EN] 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(3aQ68Delta 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(3aQ68Delta 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.