Selection on the timing of adult emergence results in altered circadian clocks in fruit flies Drosophila melanogaster

To investigate whether circadian clocks in fruit flies Drosophila melanogaster evolve as a consequence of selection on the timing of adult emergence, we raised four replicate populations each of early (early(1..4)) and late (late(1..4)) emerging flies by selecting for adults that emerged during the morning and the evening hours. We estimated the percentage of flies that emerged during the two selection windows to evaluate the direct response to selection, and the circadian phenotypes of adult emergence and locomotor activity rhythms under light/dark (LD) and constant darkness (DD) to assess the correlated response to selection. After 55 generations, the percentage of flies emerging during the morning window increased in the early populations, but decreased in the late populations. The percentage of flies emerging during the evening window increased in the late populations, but decreased in the early populations. The time course and waveform of emergence and locomotor activity rhythms of the selected populations diverged from each other as well as from the controls. Further, the circadian periodicity of the early populations was significantly shorter than the controls, while that of the late populations was significantly longer than the controls. The light-induced phase response curve of the selected populations differed significantly within groups as well as from the controls. Such modifications in the circadian phenotypes of the selected populations due to heritable changes in genetic architecture, in response to imposed selection pressure, suggest that the circadian clocks underlying emergence and locomotor activity rhythms in D. melanogaster evolve as a correlated response to selection on the timing of adult emergence.