The Effects of Starvation Selection on Drosophila Melanogaster Life History and Development

In nature, animals may endure periods of famine to complete their life cycles. Starvation stress will increase in populations as climates around the world change. To predict how populations may respond to such a stress, laboratory experimentation becomes essential. The evolutionary process of adaptation, its innovations and their trade-offs, can be studied in populations experiencing starvation stress. For this purpose outbred populations ofDrosophila melanogasterwere selected for starvation resistance in the laboratory. After 60+ generations of starvation selection the starvation-selected flies have gone from surviving 2-3 days without food to 12-14 days without food. How this amazing feat of resistance is achieved in these flies is the subject of this dissertation.Drosophilahave three mechanisms for increasing their starvation resistance. 1) Increase energy reserves, 2) decrease rate of energy use, and 3) require less energy to maintain life. Here I examined each of these strategies in the starvation-selected flies. Starvation-selected flies store nearly 3 times the amount of lipids considered normal and use those lipids at a slower rate by having lowered their metabolic rate. These findings support the use of mechanisms 1 and 2 to survive starvation stress; however no evidence supporting mechanism 3 was discovered. The lipids, so important for surviving starvation, were found to be accumulated during larval development. The storage of such large amounts of lipids may also be causing a trade-off between storage of different energetic nutrients. Acquiring starvation resistance has affected other life history traits negatively. Fecundity is low in starvation-selected flies, and egg-to-puparium development is extended by at least 24 hours, decreasing the overall fitness of the starvation-selected fly populations. This extension in development is of particular interest, because the lipid stores used to resist starvation are accumulated during larval development; an extension in development may contribute to extra lipid stores. The delay in larval development is most likely due to a delay in the hormonal cascade responsible for regulating development. Larval development time was shortened significantly in flies fed 20-hydroxyecdysone (20E) early, but lipid content was only reduced by a small amount in the starvation-selected flies. Development time therefore contributes to lipid stores to some extent, but lipid metabolism during development must also play a significant role. The delay in the hormonal cascade responsible for regulating development, and no change in the rate of mass accumulation, in combination are consistent with a model developed inManduca sextathat selection for starvation resistance is positively selecting for longer development time and larger body size. This evolutionary model may have promise as a model for studying and predicting evolutionary mechanisms indrosophilaas well. Overall the starvation-selected flies provide an excellent model for investigating starvation resistant mechanisms and how they evolved under selection. Energy storage predominates these mechanisms at the expense of changes in development. The findings brought together here contribute significantly to our understanding of the physiological mechanisms behind starvation resistance and contribute to developing models to predict the evolutionary outcome of starvation stress