Date of Award

12-2022

Document Type

Thesis

Degree Name

Master of Science in Biology

Department

Biology

First Reader/Committee Chair

Smith, David

Abstract

One of the current major issues in the control of vector-borne disease is resistance to insecticides. Resistance to one or more insecticides has appeared in over 580 different arthropod species. Widespread resistance has led to the field of Insecticide Resistance Management (IRM). The goal of IRM is to develop insecticide-use strategies that prevent or slow down the evolution of resistance. Computer models have shown that more effective IRM strategies rely on the assumption that resistance carries a fitness cost in the absence of insecticide use.

In the mosquito Culex quinquefasciatus resistance to certain organophosphate insecticides is caused by an increased production of esterase enzymes due to the amplification of certain esterase genes. Individuals in field populations can vary in esterase activity over a continuous range. The potential effects that different levels of esterase activity have on phenotypic traits with potential effects on fitness was investigated in a lab study of a California strain derived from a field collection. Development times (egg hatch to adult) were measured on samples of 500 individuals reared under four combinations of two temperatures: 25 and 30 degrees C; and two diets: a High food diet providing ample food to developing larvae, and a Low food diet that used about 25% of the amount of food provided in the High food diet. Individuals were frozen as adults after emergence for further analysis. The temperature and diet combinations were designed to create different levels of stress on the developing larvae. For each experiment esterase activity and protein content were measured on samples of males and females across the observed range of development times. The effects of temperature and diet on development time, protein content and esterase activity were determined using ANOVA. The relationships among these variables were determined using regression and correlation analysis. Temperature and diet had consistent significant effects on development time and protein content, but not on esterase activity. Coefficients of variation for development time and protein content were substantially higher in experiments using the Low food diet, suggesting increased stress on developing larvae.

Consistent evidence for a fitness trade-off between esterase activity and protein content (i.e. body size) was found in female mosquitoes. Individuals with higher esterase activity tended to have smaller body size in all four experiments. This trade-off was most apparent in the experiment with the most stressful rearing conditions (Low food diet – 30°C; regression p < 0.001), however regression p values for two of the other experiments were < 0.1, and < 0.2 in the final experiment. Surprisingly, in the experiment with the least stressful conditions (High food diet - 25°C), there were highly significant negative relationships between esterase activity and development time in both males and females (regression p values < 0.001). This study represents the first attempt to relate quantitative variation in esterase activity among individuals within a strain with variation in phenotypic traits that have the potential to affect fitness. The implications of these results for IRM models of the evolution of resistance is discussed.

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