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.

Genetics and mechanisms of insecticide resistance in Ceratitis capitata and its implications for resistance management

“…Bactrocera cucurbitae (Hsu et al, 2012a). Moreover, selection in the laboratory has resulted in several highly resistant insect strains including Drosophila melanogaster (Watson et al, 2010), Bactrocera dorsalis (Hsu et al, 2012) and C. capitata (Ureña et al, 2019).…”

“…In some cases it has been possible to establish an association between dominance and molecular mechanism (Bourguet and Raymond, 1998). For example, spinosad resistance mediated by null or truncated transcripts of the α6 subunit of the nAChR is usually inherited as an autosomal recessive trait (Perry et al, 2007;Baxter et al, 2010;Ureña et al, 2019). The inheritance of pyrethroids resistance as a consequence of VGSC mutations has been found to be incompletely recessive (Farnham, 1977;Scott, 2019).…”

“…However, it was not long until resistance to lambda-cyhalothrin was detected in 2009-2010 in Spanish populations from the Mediterranean area (Arouri et al, 2015). On the contrary, field populations remain susceptible to spinosad (Ureña et al, 2019). The use of the pyrethroid deltamethrin in lure and kill devices has increased in the last years, but there are no records of the susceptibility of Spanish field populations to this insecticide.…”

“…Apart from resistance events found in the field, C. capitata has also demonstrated its capacity to develop resistance under laboratory selection pressure with different insecticides: DDT (15-fold) (Orphanidis et al, 1980), dieldrin (68-fold) (Busch-Petersen and Wood, 1986), permethrin (up to 20-fold) (Busch-Petersen and Wood, 1983), lambda-cyhalothrin (205-fold) (Arouri et al, 2015) and spinosad (about 500-fold) (Ureña et al, 2019). The potential to develop spinosad resistance under selection pressure is especially relevant, since it is at present the main insecticide used for the control of this pest in Spanish citrus crops.…”

“…Spinosad resistance has been associated to a target-site resistance process based on the appearance of truncated transcripts of the α6 subunit of the nAChR (Ureña et al, 2019). Authors investigated the resistance mechanism in the laboratory-selected strain JW-100s, originated from a field population, and found different resistant alleles that had varied during the selection process with spinosad.…”