Resistance to Bacillus sphaericus Involves Different Mechanisms in Culex pipiens (Diptera: Culicidae) Larvae

Abstract

Field Culex pipiens pipiens (L.) mosquitoes that were collected after a control failure with Spherimos in southern France developed high resistance (> 10,000-fold) to Bacillus sphaericus crystal toxin after < 8 generations of laboratory selection. We show that this resistance is encoded by a single major recessive gene on linkage group I at 22.1 recombination units from the sex locus, and that it is not associated with any loss of binding affinity between brush border membrane fractions and the B. sphaericus radiolabeled toxin. Thus, in Southern France, resistance differs from the high B. sphaericus resistance developed after laboratory selection of Californian C. p. quinquefasciatus. This demonstrates that at least 2 different mechanisms may confer high levels of resistance to B. sphaericus crystal toxin in mosquitoes of the C. pipiens complex. These results have important implications for mosquito control strategies.

Insecticide Resistance and Vector Control

“…The mechanisms of resistance to B. sphaericus are not yet defined (46,47), but more than one mechanism seems to be involved (48). Resistance to B. thuringiensis has resulted from reduced binding of the toxin to the brush border in the lumen of the insect gut (49,50) or by enhanced digestion of toxin by gut proteases (51).…”

Detection of an Allele Conferring Resistance to Bacillus sphaericus Binary Toxin in Culex quinquefasciatus Populations by Molecular Screening

“…Disruption of the interaction between the toxin and the midgut is the major mechanism underlying resistance, and it has already been reported from different laboratory-or fieldselected colonies (25,26,27,33). Unrelated mechanisms may also be involved in resistance, but they have not been characterized so far (25,27).Molecular studies revealed that mutations in the genes cpm1 and cqm1, which prevent the production of functional membrane-bound receptors, are the main reasons behind the lack of binding of the Bin toxin to the midgut epithelium. Four cpm1/cqm1 resistance alleles were found in Culex populations of different origins.…”

“…The action of the Bin toxin on Culex larvae relies on its specific binding to those membrane-bound receptors (24). Disruption of the interaction between the toxin and the midgut is the major mechanism underlying resistance, and it has already been reported from different laboratory-or fieldselected colonies (25,26,27,33). Unrelated mechanisms may also be involved in resistance, but they have not been characterized so far (25,27).…”

Various Levels of Cross-Resistance to Bacillus sphaericus Strains in Culex pipiens (Diptera: Culicidae) Colonies Resistant to B. sphaericus Strain 2362

“…The following three susceptible and four resistant colonies of C. pipiens pipiens and C. pipiens quinquefasciatus were investigated in this study: (i) JRMM-R, a laboratory-selected field colony of C. pipiens quinquefasciatus with fivefold resistance, and its parental susceptible colony, JRMM-S (F-S and F-SEL, respectively) (25,26); (ii) KOCHI, a C. pipiens quinquefasciatus colony with high-level (Ͼ2,000-fold) resistance to B. sphaericus that was field selected from an area of southern India treated with B. sphaericus strain 1593M (22), and Madurai, a susceptible laboratory colony of C. pipiens quinquefasciatus; (iii) SPHAE, a C. pipiens pipiens colony with high-level (Ͼ50,000fold) resistance that was generated by field selection from an area of southern France treated with B. sphaericus strain 2362 (16,33); and (iv) GeoR, a C. pipiens quinquefasciatus colony with high-level (Ͼ50,000-fold) resistance that was produced by G. P. Georghiou, who selected field-collected larvae from California with B. sphaericus 2362 in the laboratory (15,36). The SPHAE and GeoR colonies were established from egg rafts kindly provided by Nicole Pasteur (University of Montpellier II, Montpellier, France) (SPHAE) and by G. P. Georghiou and Margareth Wirth (University of California, Riverside) (GeoR) and were reared at the Pasteur Institute in the Entomopathogenic Bacteria Unit.…”

“…Levels of stable laboratory-selected resistance of between 35-fold and more than 100,000-fold have been reported, suggesting that there may be different resistance mechanisms. Investigations of the mechanisms and genetics of resistance to B. sphaericus have been carried out for some of the resistant populations (15,16,36).…”