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High resistance to Bacillus sphaericus binary toxin in culex pipiens (Diptera: Culicidae): the complex situation of
Christina Nielsen-Leroux1, Nicole Pasteur, Jérĵme Prètre
1Institut Pasteur, Bactéries Entomopathogénes, Paris, France. cnielsen@pasteur.fr
Abstract:
This study was aimed at clarifying the nature of the resistance to Bacillus sphaericus Neide (Bs) that Culex pipiens L. has developed in west Mediterranean countries, France, and Tunisia. Two recessive and sex-linked mutants, sp-1R and sp-2R, were previously detected in southern France. Here, the Tunisian resistance was also shown to involve a single recessive and sex-linked gene that was temporarily named sp-T(R). In addition, sp-1R, sp-2R, and sp-T(R) were shown to separately confer a similar high resistance level (> 5,000-fold) in the homozygous state. Knowing that sp-1R resistance does not alter the binding of Bs binary toxin to its specific receptor, we investigated this character in sp-2RR and sp-T(RR) homozygotes. This was performed by in vitro experiments in which larval brush border membrane fractions (BBMF) were exposed to the 125I-Bin2 toxin of B. sphaericus strain 1593. The toxin-receptor binding was found disrupted by sp-2R but not by sp-T(R). Comparing the binding kinetics among nine Culex pipiens strains of diverse origins revealed that the Bs receptors of sp-1RR and Sp-T(RR) homozygous larvae were displaying the highest affinity toward Bs binary toxins. These results are discussed with regard to alternative assumptions on the dynamics of high Bs-resistance and on the emerging possibilities to test them in a near future.
Insights
Resistance in Culex pipiens mosquitoes to Bacillus sphaericus (Bs) involves a single gene. This gene, identified in Tunisia as sp-T(R), confers high resistance without affecting toxin-receptor binding, unlike other resistance genes.
Area of Science:
- Entomology
- Molecular Biology
- Genetics
Background:
- Culex pipiens mosquitoes have developed resistance to Bacillus sphaericus (Bs) in Mediterranean regions.
- Previous studies identified two sex-linked recessive mutants (sp-1R, sp-2R) conferring Bs resistance in France.
Purpose of the Study:
- To elucidate the genetic basis of Bs resistance in Tunisian Culex pipiens.
- To investigate the effect of resistance mutations on the binding of Bs binary toxin to its receptor.
Main Methods:
- Genetic analysis to identify the resistance gene in Tunisian mosquitoes (sp-T(R)).
- In vitro binding assays using 125I-labeled Bs binary toxin and larval brush border membrane fractions.
- Kinetic analysis of toxin-receptor interactions in various Culex pipiens strains.
Main Results:
- The Tunisian resistance (sp-T(R)) is controlled by a single, recessive, sex-linked gene, similar to previously identified genes.
- sp-2R mutation disrupts toxin-receptor binding, while sp-T(R) does not.
- Homozygous larvae with sp-1R and sp-T(R) exhibit high affinity for Bs binary toxins.
Conclusions:
- Bs resistance in Culex pipiens involves distinct genetic mechanisms, with some affecting toxin-receptor interactions and others not.
- The sp-T(R) mutation represents a novel mechanism of high-level Bs resistance, preserving toxin-receptor binding affinity.
