The concentration gradient formed by Bicoid (Bcd), a transcription factor that patterns anterior Drosophila development, declines exponentially with distance from the anterior pole of the syncytial embryo, and is a standard paradigm for how a morphogen provides positional information. Now,Markus Noll, Stefan Baumgartner and colleagues challenge a widely accepted model - the SDD model - for the establishment of this gradient (see ). The SDD model proposes that the gradient arises when Bcd is synthesised from a bcdmRNA source localised at the anterior pole; Bcd then diffuses away and is uniformly degraded. However, the authors convincingly show that, instead of bcd mRNA being localised anteriorly, a bcd mRNA gradient is formed and maintained during the syncytial stages, in agreement with publications predating the SDD model. Because Bicoid mRNA and protein patterns are similar to those of Staufen protein, which functions in mRNA localisation in oogenesis, the authors propose a gradient-formation model based on active mRNA transport, which awaits further experimental testing.
