Density-dependent selection in Drosophila: evolution of egg size and hatching time

Many different laboratory studies of adaptation to larval crowding in Drosophila spp. have all yielded the evolution of pre-adult competitive ability, even though the ecological context in which crowding was experienced varied across studies. However, the evolution of competitive ability was achieved through different suites of traits in studies wherein crowding was imposed in slightly different ways. Earlier studies showed the evolution of increased competitive ability via increased larval feeding rate and tolerance to nitrogenous waste, at the cost of food to biomass conversion efficiency. However, more recent studies, with crowding imposed at relatively low food levels, showed the evolution of competitive ability via decreased larval development time and body size, and an increase in the time efficiency of conversion of food to biomass, with no change in larval feeding rate or waste tolerance. Taken together, these studies have led to a more nuanced understanding of how the specific details of larval numbers, food amounts etc. can affect which traits evolve to confer increased competitive ability. Here, we report results from a study in which egg size and hatching time were assayed on three sets of populations adapted to larval crowding experienced in slightly different ways, as well as their low density ancestral control populations. Egg size and hatching time are traits that may provide larvae with initial advantages under crowding through increased starting larval size and a temporal head-start, respectively. In each set of populations adapted to some form of larval crowding, the evolution of longer and wider eggs was seen, compared to controls, thus making egg size the first consistent correlate of the evolution of increased larval competitive ability across Drosophila populations experiencing crowding in slightly different ways. Among the crowding-adapted populations, those crowded at the lowest overall eggs/food density, but the highest density of larvae in the feeding band, showed the largest eggs, on an average. All three sets of crowding-adapted populations showed shorter average egg hatching time than controls, but the difference was significant only in the case of populations experiencing the highest feeding band density. Our results underscore the importance of considering factors other than just eggs/food density when studying the evolution of competitive ability, as also the advantages of having multiple selection regimes within one experimental set up, allowing for a more nuanced understanding of the subtlety with which adaptive evolutionary trajectories can vary across even fairly similar selection regimes.