Variation in organophosphate resistance and esterase activity in Culex quinquefasciatus Say from California

Abstract

Organophosphate resistance in Culex quinquefasciatus Say from California is inherited as a monofactorial character (R) indissociable from a highly active esterase (H allele) and previous experiments suggested that H and R are not unique alleles but rather a series of alleles that differ in their degree of expressivity. In the present study, 4 strains (A, B, C and D) homozygous for different forms of the R and H genes were isolated. These strains displayed different degrees of resistance and different levels of esterase activity. Esterase activity as well as resistance increased from one resistant strain to the other. In addition, in the offspring of crosses between the A or B strain and a susceptible (S) strain, esterase activity and resistance (LC!) were half those of the resistant parent. The relationship existing between the degree of resistance and esterase activity confirmed that both characters are under the genetic control of the same gene. The hypothesis according to which both esterase activity and resistance levels are the result of an amplification (multiple duplication) of a detoxifying gene is discussed.