An individual-based simulation framework exploring the ecology and mechanistic underpinnings of larval crowding in laboratory populations of Drosophila

Abstract

The study of larval competition in laboratory populations of Drosophila, implemented via the crowding of larval cultures, has contributed greatly to the understanding of the ecology of competition, the evolution of larval competitive ability, and formed the basis of rigorous testing of the theory of density-dependent selection. Earlier studies led to the view that the outcomes of larval competition, and resulting evolutionary consequences of crowding-adaptation, could largely be understood by varying the starting density of individuals in a crowded culture. However, recent studies have shown that the results of adaptation to larval crowding may not be well predicted by the total larval density (i.e., total starting individuals/total volume of food). Cultures raised at the same total density but at different egg number and food volume combinations were shown to have different underlying density-specific fitness functions, and crowding-adaptation in each of these cultures was attained through different evolutionary trajectories as well. A recent study showed that cultures with not just the same density, but the same egg and food volume combination, achieved through food columns of differing diameter and height, could also differ greatly in fitness-related trait outcomes. In that study, the density of larvae in the feeding band (volume of food close to the surface in contact with air, to which larval feeding is largely restricted) was a very important factor in predicting the outcomes of larval competition. Given these recent findings, it is important to understand the overall role of feeding band density, and how it influences density-specific fitness functions in different kinds of crowded cultures. As the older models of larval competition are now insufficient to capture current empirical data, we constructed an individual-based simulation framework informed in part by these more recent findings, in order to better understand the evolutionary ecology and mechanistic underpinnings of larval competition, and predict robust experiments for expanding our understanding of the process of larval competition in Drosophila.