Morphogen gradients provide essential positional information to gene networks through their spatially heterogeneous distribution, yet how they form is still hotly contested, with multiple models proposed for different systems. Here, we focus on the transcription factor Bicoid (Bcd), a morphogen that forms an exponential gradient across the anterior-posterior (AP) axis of the early Drosophila embryo.
First, we use fluorescence correlation spectroscopy to examine protein dynamics in vivo. We find there are spatial differences in Bcd diffusivity along the AP axis, with Bcd diffusing more rapidly in the posterior. We establish that such spatially varying differences in Bcd dynamics are sufficient to explain how Bcd can have a steep exponential gradient in the anterior half of the embryo and yet still have an observable fraction of Bcd near the posterior pole. In the nucleus, we demonstrate that Bcd dynamics are impacted by binding to DNA. Addition of the Bcd homeodomain to eGFP::NLS qualitatively replicates the Bcd concentration profile, suggesting this domain regulates Bcd dynamics. Our results reveal how a long-range gradient can form while retaining a steep profile through much of its range.
Second, we take advantage of the SunTag reporter system to quantitatively examine the spatiotemporal profile of bcd mRNA translation in vivo. We demonstrate that Processing body (P body) localisation in the oocyte suppresses premature bcd mRNA translation. Upon egg laying, bcd mRNA disassociates from P bodies and translation is observed exclusively in the anterior pole of the embryo. Accompanying nuclear migration to the embryo cortex at nuclear cycle (n.c.) 9, bcd mRNA relocates to the apical domain of the nuclear environment and translation continues near the embryo surface. In n.c. 14, bcd mRNA localises to newly formed P bodies and further translation is not detected.
We use these observations to build a modified source-diffusion-degradation model of Bcd gradient formation that leads to an exponential gradient by n.c. 12 and is consistent with other experimental measurements of Bcd dynamics. Consequently, we see that the spatiotemporal dynamics of Bicoid are highly regulated throughout oogenesis and early embryo development.