Evolution of increased larval competitive ability inDrosophila melanogasterwithout increased larval feeding rate
Authors: Sarangi Manaswini , Nagarajan Archana , Dey Snigdhadip , Bose Joy , Joshi Amitabh 
Published: October 2015

Multiple experimental evolution studies onD. melanogasterin the 1980s and 1990s indicated that enhanced competitive ability evolved primarily through increased larval tolerance to nitrogenous wastes and increased larval feeding and foraging rate, at the cost of efficiency of food conversion to biomass, and this became the widely accepted view of how adaptation to larval crowding evolves in fruitflies. We recently showed that populations ofD. ananassaeandD. n. nasutasubjected to extreme larval crowding evolved greater competitive ability without evolving higher feeding rates, primarily through a combination of reduced larval duration, faster attainment of minimum critical size for pupation, greater efficiency of food conversion to biomass, increased pupation height and, perhaps, greater urea/ammonia tolerance. This was a very different suite of traits than that seen to evolve under similar selection inD.melanogasterand was closer to the expectations from the theory ofK-selection. At that time, we suggested two possible reasons for the differences in the phenotypic correlates of greater competitive ability seen in the studies withD. melanogasterand the other two species. First, thatD. ananassaeandD. n. nasutahad a very different genetic architecture of traits affecting competitive ability compared to the long-term, laboratory populations ofD. melanogasterused in the earlier studies, either because the populations of the former two species were relatively recently wild-caught, or by virtue of being different species. Second, that the different evolutionary trajectories inD. ananassaeandD. n. nasutaversusD. melanogasterwere a reflection of differences in the manner in which larval crowding was imposed in the two sets of selection experiments. TheD. melanogasterstudies used a higher absolute density of eggs per unit volume of food, and a substantially larger total volume of food, than the studies onD. ananassaeandD. n. nasuta. Here, we show that long-term laboratory populations ofD. melanogaster, descended from some of the populations used in the earlier studies, evolve essentially the same set of traits as theD. ananassaeandD. n. nasutacrowding-adapted populations when subjected to a similar larval density at low absolute volumes of food. As in the case ofD. ananassaeandD. n. nasuta, and in stark contrast to earlier studies withD. melanogaster, these crowding-adapted populations ofD. melanogasterdid not evolve greater larval feeding rates as a correlate of increased competitive ability. The present results clearly suggest that the suite of phenotypes through which the evolution of greater competitive ability is achieved in fruitflies depends critically not just on larval density per unit volume of food, but also on the total amount of food available in the culture vials. We discuss these results in the context of an hypothesis about how larval density and the height of the food column in culture vials might interact to alter the fitness costs and benefits of increased larval feeding rates, thus resulting in different routes to the evolution of greater competitive ability, depending on the details of exactly how the larval crowding was implemented.