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.Ìý
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First, we use fluorescence correlation spectroscopy to examine protein dynamicsÌýin vivo. We find there are spatial differences inÌýBcdÌýdiffusivity along the AP axis, withÌýBcdÌýdiffusing more rapidly in the posterior. We establish that such spatially varying differences inÌýBcd dynamics are sufficient to explain howÌýBcdÌýcan have a steep exponential gradient in the anterior half of the embryo and yet still have an observable fraction ofÌýBcdÌýnear the posterior pole. In the nucleus, we demonstrate thatÌýBcdÌýdynamics are impacted by binding to DNA. Addition of theÌýBcdÌýhomeodomain toÌýeGFP::NLS qualitatively replicates theÌýBcd concentration profile, suggesting this domain regulatesÌýBcdÌýdynamics. Our results reveal how a long-range gradient can form while retaining a steep profile through much of its range.
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Second,Ìýwe take advantage of theÌýSunTagÌýreporter system to quantitatively examine the spatiotemporal profile ofÌýbcdÌýmRNA translationÌýin vivo. We demonstrate that Processing body (P body) localisation in the oocyte suppresses prematureÌýbcdÌýmRNA translation. Upon egg laying,ÌýbcdÌýmRNA 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,ÌýbcdÌýmRNA relocates to the apical domain of the nuclear environment and translation continues near the embryo surface. InÌýn.c.Ìý14,ÌýbcdÌýmRNA localises to newly formed P bodies and further translation is not detected.Ìý
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We use these observations to build a modified source-diffusion-degradation model ofÌýBcd gradient formation that leads to an exponential gradient byÌýn.c.Ìý12 and is consistent with other experimental measurements ofÌýBcdÌýdynamics. Consequently, we see that the spatiotemporal dynamics of BicoidÌýare highly regulated throughout oogenesis and early embryo development.