Dynamics of bicoid mRNA localisation and translation dictate morphogen gradient formation

The transcription factor Bicoid (Bcd) protein guides early Drosophila patterning and is the best-characterised morphogen. The source of the morphogen, bcd mRNA, is maternally deposited during oogenesis and localised to the anterior pole. When and where Bcd protein is produced and how this protein gradient is dynamically shaped remains contentious. Here, we use the SunTag reporter system to quantitatively examine the spatiotemporal profile of bcd mRNA translation in vivo. We show that association with Processing bodies (P bodies) in mature oocytes prevent premature bcd mRNA translation. Following egg activation, bcd mRNA dissociates from P bodies and translation is observed at the anterior pole. Translation remains largely restricted to the anterior domain throughout early development. At cellularisation, translation ceases and the remaining bcd mRNA associates with reformed P bodies, which appear to block any further translation. We use these observations to create a new modified synthesis-diffusion-degradation model of Bcd gradient, which is consistent with experimental measurements. Overall, our study reveals that bcd mRNA translation is tightly controlled in space and time during oogenesis and early embryogenesis.