Developmental constraints on the evolution of wing-body allometry in Manduca sexta.

Artificial selection on body size in Manduca sexta produced genetic strains with large and small body sizes. The wing-body allometries of these strains differed significantly from the wild type. Selection on small body size led to a change in the scaling of wing and body size without changing the allometry: the wings were smaller relative to the body, but to the same degree at all body sizes. Selection for large body size led to a change in allometry with a decrease in the allometric coefficient, wing size becoming progressively smaller relative to body as body size increased. When larvae were deprived of food so as to produce adults of a range of small body sizes, all strains retained the same allometric coefficient but showed an increase in the scaling factor. Thus individuals starved as larvae had a smaller adult body size but had proportionally larger wings than fed individuals. We analyzed the developmental processes that could give rise to this pattern of allometries. Differences in the relative growth of body and wing disks can account for the differences in the allometric coefficients among the three body size strains. The change in wing-body allometry at large body sizes was primarily due to an insufficient time period for growth. The available time period for growth of the wing imaginal disks poses a significant constraint on the proportional growth of wings, and thus on the evolution of large body size.