Molecular and biological features of Culex quinquefasciatus homozygous larvae for two cqm1 alleles that confer resistance to Lysinibacillus sphaericus larvicides

Abstract

BACKGROUND: Culex quinquefasciatus resistance to the binary toxin from Lysinibacillus sphaericus larvicides can occur because of mutations in the cqm1 gene that prevents the expression of the toxin receptor, Cqm1 ⊍-glucosidase. In a resistant laboratoryselected colony maintained for more than 250 generations, cqm1 REC and cqm1 REC-2 resistance alleles were identified. The major allele initially found, cqm1 REC , became minor and was replaced by cqm1 REC-2 . This study aimed to investigate the features associated with homozygous larvae for each allele to understand the reasons for the allele replacement and to generate knowledge on resistance to microbial larvicides. RESULTS: Homozygous larvae for each allele were compared. Both larvae displayed the same level of resistance to the binary toxin (3500-fold); therefore, a change in phenotype was not the reason for the replacement observed. The lack of Cqm1 expression did not reduce the total specific ⊍-glucosidase activity for homozygous cqm1 REC and cqm1 REC-2 larvae, which were statistically similar to the susceptible strain, using artificial or natural substrates. The expression of eight Cqm1 paralog ⊍-glucosidases was demonstrated in resistant and susceptible larvae. Bioassays in which cqm1 REC or cqm1 REC-2 homozygous larvae were reared under stressful conditions showed that most adults produced were cqm1 REC-2 homozygous (69%). Comparatively, in the offspring of a heterozygous sub-colony reared under optimal conditions for 20 generations, the cqm1 REC allele assumed a higher frequency (0.72). CONCLUSION: Homozygous larvae for each allele exhibited a similar resistant phenotype. However, they presented specific advantages that might favor their selection and can be used in designing resistance management practices.

Bacterial Toxins Active against Mosquitoes: Mode of Action and Resistance

“…The cqm1 REC-2 allele has a nonsense mutation that generates a premature translation termination codon, and the transcripts also code for a truncated soluble protein [362]. Two colonies formed by homozygous individuals for each allele (REC and REC-2) were established [362,365]. Field screenings revealed two other alleles, named cqm1 REC-D16 and cqm1 REC-D25 , which showed deletions of 16-and 25-nt, respectively, resulting in truncated transcripts [361].…”

“…Indeed, several Cx. pipiens colonies resistant to the Bin toxin from L. sphaericus were stably maintained for several years under laboratory conditions [143,[357][358][359]362,365].…”

“…pipiens’ resistance to L. sphaericus Bin toxin, the resistance is often associated with the lack of expression of the toxin receptor Cqm1 α-glucosidase (see Section 4 ). In two highly resistant colonies, it was observed that the lack of Cqm1 did not impact the total α-glucosidase activity in the larvae midgut [ 365 ]. This study suggested that the expression of another α-glucosidases, paralogs of the Cqm1 protein in larvae, could compensate for the lack of Cqm1.…”

Culex quinquefasciatus Resistant to the Binary Toxin from Lysinibacillus sphaericus Displays a Consistent Downregulation of Pantetheinase Transcripts

“…The S colony was established with eggs collected in the city of Recife, Brazil, and has been maintained for more than eleven years, as a reference colony susceptible to insecticides [21]. The REC and REC-2 colonies display resistance ratios (RR) to the Bin toxin of ~4000-fold, attributed to the homozygous genotype for the cqm1 REC and cqm1 REC-2 resistance alleles, respectively [29]. Both Bin-resistant colonies have been continuously maintained and have been exposed to L. sphaericus 2362 at every five generations.…”

A Culex quinquefasciatus strain resistant to the binary toxin from Lysinibacillus sphaericus displays altered enzyme activities and energy reserves

“…Most investigations on L. sphaericus resistance have concentrated efforts to characterize the mechanisms and genes that confer the Bin-resistance itself [ 3 ], while much less is known about other features that could be related to this phenotype. This is particularly important for Bin-resistant strains, such as RIAB59, that have been stably maintained for several generations without a critical impact on their biological traits [ 22 , 23 , 27 , 28 ].…”