Modularity of transcription factors. (a) Expression of Twi and Mef2 and their co-occurrence at genomic sites at different developmental stages. Compared are stages 5 to 9 (approximately 2-4 h AEL) and stages 9 to 11 (approximately 4-6 h AEL). In the earlier time frame, strong mesodermal Twi expression but only weak Mef2 expression can be observed. Half of the genomic sites that see occupancy by either TF at some stage during development show only Twi occupancy (43%) or Twi/Mef2 (8%) double-occupancy. By 4 to 6 h AEL, the majority of the mesoderm develops into muscle, which is characterized by strong Mef2 expression. Genomic occupancy follows this trend, showing that most sites with previous double-occupancy maintain that state while one-third of the previous Twi-only sites gain a Mef2 partner site. Overall, more than half of the Twi sites then show double-occupancy. The images showing TF expression are reproduced from . ChIP data are from . (b) A map of potential pairwise interactions between transcription factors. For all of the 373 TFs for which BDGP expression data are available, the frequency of co-occurrence in at least one tissue is color-coded (as fraction of sampled time frames). Most TFs are co-expressed in at least one spatio-temporal coordinate. (c) Transcription factor expression exhibits modular behavior. More modules of two (left panel) or three (right panel) transcription factors show precisely the same expression from one to the next developmental stage than is to be expected at random. Interestingly, this is an intrinsic feature of all genes and not a specific property of the transcription factors. Error bars indicate the standard deviation observed in the random experiments.