Novel Mutations Associated with Resistance to Bacillus sphaericus in a Polymorphic Region of the Culex quinquefasciatus cqm1 Gene

Abstract

ABSTRACTBin toxin fromBacillus sphaericusacts onCulex quinquefasciatuslarvae by binding to Cqm1 midgut-bound receptors, and disruption of thecqm1gene is the major cause of resistance. The goal of this work was to screen for a laboratory-selected resistancecqm1RECallele in field populations in the city of Recife, Brazil, and to describe other resistance-associated polymorphisms in thecqm1gene. Thecqm1RECallele was detected in the four nontreated populations surveyed at frequencies from 0.001 to 0.017, and sequence analysis from these samples revealed a novel resistant allele (cqm1REC-D16) displaying a 16-nucletotide (nt) deletion which is distinct from the 19-nt deletion associated withcqm1REC. Yet a third resistant allele (cqm1REC-D25), displaying a 25-nt deletion, was identified in samples from a treated area exposed toB. sphaericus. A comparison of the three deletion events revealed that all are located within the same 208-nt region amplified during the screening procedure. They also introduce equivalent frameshifts in the sequence and generate the same premature stop codon, leading to putative transcripts encoding truncated proteins which are unable to locate to the midgut epithelium. The populations analyzed in this study contained a variety of alleles with mutations disrupting the function of the corresponding Bin toxin receptor. Their locations reveal a hot spot that can be exploited to assess the resistance risk through DNA screening.