Co‐selection and replacement of resistance alleles to Lysinibacillus sphaericus in a Culex quinquefasciatus colony

The Cqm1 alpha-glucosidase, expressed within the midgut of Culex quinquefasciatus mosquito larvae, is the receptor for the Binary toxin (Bin) from the entomopathogen Lysinibacillus sphaericus. Mutations of the Cqm1 alpha-glucosidase gene cause high resistance levels to this bacterium in both field and laboratory populations, and a previously described allele, cqm1(REC), was found to be associated with a laboratory-resistant colony (R2362). This study described the identification of a novel resistance allele, cqm1(REC-2), that was co-selected with cqm1(REC) within the R2362 colony. The two alleles display distinct mutations but both generate premature stop codons that prevent the expression of midgut-bound Cqm1 proteins. Using a PCR-based assay to monitor the frequency of each allele during long-term maintenance of the resistant colony, cqm1(REC) was found to predominate early on but later was replaced by cqm1(REC-2) as the most abundant resistance allele. Homozygous larvae for each allele were then generated that displayed similar high-resistance phenotypes with equivalent low levels of transcript and lack of protein expression for both cqm1(REC) and cqm1(REC-2). In progeny from a cross of homozygous individuals for each allele at a 1 : 1 ratio, analyzed for ten subsequent generations, cqm1(REC) showed a higher frequency than cqm1(REC-2). The replacement of cqm1(REC) by cqm1(REC-2) observed in the R2362 colony, kept for 210 generations, indicates changes in fitness related to traits that are unknown but linked to these two alleles, and constitutes a unique example of evolution of resistance within a controlled laboratory environment.