Enhancement of larval immune system traits as a correlated response to selection for rapid development in Drosophila melanogaster

We have shown earlier that the evolution of rapid development is accompanied by a correlated decrease in larval feeding rate and competitive ability in laboratory populations of Drosophila melanogaster (Prasad et al. 2001; Shakarad et al. 2005). Here, we show that our faster developing populations have evolved higher hemocyte density and phenol oxidase activity in the larval hemolymph. The increased hemocyte density could be responsible for the evolution of decreased feeding rate as hemocytes and the cephalopharyngeal musculature share common embryonic precursor cells (Kraajiveld et al. 2001). We also show that the bacterial load in larval food vials of the faster developing populations is substantially higher than in controls. Our results suggest that the evolution of reduced competitive ability in the faster developing populations is probably due to larval feeding rate trading off with enhanced larval immune system function. Enhanced larval immune function, in turn, is most likely selected for due to the role of hemocytes (Lanot et al. 2001, Wood and Jacinto 2007) and phenol oxidase (Pentz et al. 1986) in development, and perhaps also due to inadvertent selection on immune performance resulting from the higher bacterial load faced by larvae in the faster developing populations.