THE RESPONSE TO SELECTION FOR FAST LARVAL DEVELOPMENT IN DROSOPHILA-MELANOGASTER AND ITS EFFECT ON ADULT WEIGHT - AN EXAMPLE OF A FITNESS TRADE-OFF

A selection experiment using Drosophila melanogaster revealed a strong trade-off between adult weight and larval development time (LDT), sup porting the view that antagonistic pleiotropy for these two fitness tr aits determines mean adult size. Two experimental lines of flies were selected for a shorter LDT (measured from egg laying to pupation). Aft er 15 generations LDT was reduced by an average of 7.9%. The response appeared to be controlled primarily by autosomal loci. A correlated re sponse to the selection was a reduction in adult dry weight: individua ls from the selected populations were on average 15.1% lighter than th e controls. The lighter females of the selected lines showed a 35% dro p in fecundity, bur no change in longevity. Thus, there is no direct r elationship between LDT and adult longevity. The genetic correlation b etween weight and LDT, as measured from their joint response to select ion, was 0.86. Although there was weak evidence for dominance in LDT, there was none for weight, making it unlikely that selection acting on this antagonistic pleiotropy could lead to a stable polymorphism. In all lines, sex differences in weight violated expectations based on in trasex genetic correlations: Females, being larger than males, ought t o require a longer LDT, whereas there was a slight trend in the opposi te direction. Because the sexual dimorphism in size was nor significan tly altered by selection, it appears that the controlling loci are eit her invariant or have very limited pleiotropic effect on developmental time. Iris suggested that they probably control some intrinsic, energ y-intensive developmental process in males.