Quantification of Images.

All images used for quantifications were carefully acquired avoiding overexposure or underexposure. For each genotype, data from flies with RU486 feeding (RU+) and flies without RU486 feeding (RU−) were acquired at the same time and under the same microscope parameters. Four to six consecutive slices of each fly, which contained the dendritic region of mushroom body (calyx) surrounded by Kenyon cells, were selected and calculated using Imaris software. Mean intensity of pMAPK in nuclei was divided by that in calyx to get the mean intensity ratio. Area of nuclei overlapped by strong pMAPK signal (judged by intensity above mean in calyx) was divided by total nuclei area. Specifically, in UAS-YFP-msk/+; elav-GS/+ (RU+) flies, total nuclei area was further selected by intensity of YFP signal. All statistics were calculated from at least six brains. For each genotype, statistics in the induced group (RU+) are shown as fold change compared with the uninduced group (RU−).