Dynamic interpretation of maternal inputs by the Drosophila segmentation gene network

Patterning of body parts in multicellular organisms relies on the interpretation of transcription factor (TF) concentrations by genetic networks. To determine the extent by which absolute TF concentration dictates gene expression and morphogenesis programs that ultimately lead to patterns in Drosophila embryos, we manipulate maternally supplied patterning determinants and measure readout concentration at the position of various developmental markers. When we increase the overall amount of the maternal TF Bicoid 5-fold, Bicoid concentrations in cells at positions of the cephalic furrow, an early morphological marker, differ by a factor of two. This finding apparently contradicts the traditional threshold-dependent readout model, which predicts that the Bicoid concentrations at these positions should be identical. In contrast, Bicoid concentration at target gene expression boundaries is nearly unchanged during early developmental times, but adjusts dynamically toward the same 2-fold chang...