INHERITANCE OF LARVAL RESISTANCE TO PERMETHRIN IN AEDES-AEGYPTI AND ASSOCIATION WITH SEX-RATIO DISTORTION AND LIFE-HISTORY VARIATION

The genetic mechanisms that confer larval permethrin resistance were i nvestigated in two strains of Aedes aegypti, vectors of yellow fever a nd dengue hemorrhagic fever. Larval resistance to permethrin in an Ae. aegypti held-collected resistant Couva (R) strain was associated with the sex-determining locus by analysis of backcrosses to the susceptib le Rockefeller (S) strain. The median lethal concentrations (LC(50)s) of these strains were 23.1 (95% confidence interval = 22.0-24.3) and 2 .2 (2.0-2.3) parts/billion of permethrin, respectively. The estimated resistance ratio (RR) for the R strain was 10.8 (10.3-11.4) compared w ith the S strain. Resistance was inherited as partly recessive (domina nce [D] = -0.3) with an estimated RR of 2.3 (2.1-2.4) in the F-1 hybri ds when the R parent was male, There were also significantly male-bias ed sex ratios for this cross. In contrast, inheritance was slightly do minant (D = 0.19) with an estimated RR of 4.1 (3.8-4.4) when the R par ent was female, and no significant sex ratio bias of progeny was obser ved. Analysis revealed a strong paternal-strain effect in bioassay mor tality, sex ratio, egg hatch, and fecundity. A maternal-strain effect was also evident for bioassay mortality. Similarly, a strong maternal by paternal strain interaction was also evident for sex ratio. Progeny of single-family backcrosses of F-1 hybrids to R were statistically h omogeneous for sex ratio, duration of oviposition, fecundity, and hatc h rate. A significant increase in male bias was found for only one bac kcross to R, after treatment with permethrin. In contrast, complex pat terns of inheritance of life histories were observed among backcrosses to S. Backcrosses to S had greater mean fecundities, shorter mean tim es to the start of oviposition, anti shorter mean oviposition periods than did backcrosses to R. Hatch rates were statistically homogeneous among backcrosses, but all strikingly reduced relative to the parental generation. Times of start and duration of oviposition were highly ne gatively correlated with fecundity (first gonotropic cycle only) and r ate of egg hatch, Females with lower fecundities had lower hatch rates , and there was a threshold of approximately 80 eggs per female, below which no eggs hatched. Generally aii backcrosses had higher LC(50)s t han expected from single-locus inheritance. Association between sex bi as and inheritance of resistance was apparent, but no single genetic l inkage model based on current understanding of sex chromosome genetics was consistent with these observations. These results may have epidem iologic importance considering that permethrin-soaked bed nets are bei ng used in many countries to control the biting activity of disease ve ctors.