Lead Graduate Student: Kathreen Bitner
Background: A fundamental component of the modern theory of life-history evolution has been the concept of trade-offs. Previous research in our laboratory has shown that populations of Drosophila melanogaster that have adapted to (i) high levels of ammonia in the larval food, (ii) high levels of urea in the larval food, and (iii) high larval densities, all exhibit an alteration of a fundamental behavioral phenotype of the larvae: their feeding rates. under controlled conditions with few stressors, feeding rates are tightly correlated to larval competitive ability, with very fast feeding, fast growing, competitively superior larvae being inefficient at processing food they consume. Therefore, we believe that adaptation to elevated ammonia and urea has resulted in the evolution of an efficiency-maximizing strategy due to the relationship between the energy demands required to survive in these new environments on one hand, and the central roles played by larval feeding rates and digestive efficiency on the other
Research Objectives: Develop 20 large independent experimental populations that have adapted to different nitrogen waste products in their larval food, or levels of crowding and collect samples from these populations every five generations for later DNA sequencing. The experiment shall compare physiological measures related to energy acquisition and use in control and experimental populations. These measures include metabolic rates, food intake rates, larval growth rates, and digestive tract structure and function. Finally RNA expression in control and selected populations in both the control and experimental environments will be measured to determine the metabolic targets of selection.
Methods and Analysis: This study will focus on a collection of 20 experimental populations of Drosophila melanogaster all derived from a common source population. There will be (i) five independent control populations kept at low density and raised in standard food, (ii) five populations selected for resistance to high levels of ammonia in the larval food environment but also raised at low density, (iii) five populations selected for resistance to high levels of urea in the larval food environment but kept at low larval densities, and (iv) five populations raised on standard food and high larval densities. To test our hypotheses, we will measure the following in larvae from the different lines: (1) Energy acquisition, to confirm the costs to our proposed metabolic tradeoffs; (2) digestive and metabolic function, to examine the physiology underlying these trade-offs; and (3) energy budgets, to test how energy is allocated in the different lines.
Intellectual Merits: Although many studies have touched on the ideas of energy trade-offs many lack important components to test this idea thoroughly. Only very few studies have actually attempted to demonstrate the resource basis of trade-offs by quantifying acquisition or allocation of resources. This goal is made more important by the suggestion that some life-history traits may exhibit negative correlations that are not due to limiting energy but are the result of molecular signaling. This possibility suggests that direct measurements must be used to infer energetic trade-offs. Simple plausibility arguments are insufficient. We propose to do this with well-defined experimental populations whose evolutionary history and trajectories we have followed in detail thereby overcoming limitations of other studies.
Broader Impacts: We suggest that adaptation to high levels of ammonia and urea in the larval food environment require adaptations that are energetically expensive (e.g., elevated GDH expression). If the feeding rates of our larva follow our hypothesis, we expect the ammonia- and urea-adapted lines to (i) show higher digestive efficiency, (ii) spend less of their energy budget on growth and maintenance (e.g., digestive tract size and function), even when placed in environments without urea and ammonia, (iii) grow more slowly, and thus, (iv) be inferior competitors in food limited environments lacking of any stressors. With this study and the direct measurements we will make, will be able to infer energetic trade-offs that are occurring.