Formation of the bicoid morphogen gradient: an mRNA gradient dictates the protein gradient

The Bicoid (Bcd) protein gradient is generally believed to be established in pre-blastoderm Drosophila embryos by the diffusion of Bcd protein after translation of maternal mRNA, which serves as a strictly localized source of Bcd at the anterior pole. However, we previously published evidence that the Bcd gradient is preceded by a bcd mRNA gradient. Here, we have revisited and extended this observation by showing that the bcd mRNA and Bcd protein gradient profiles are virtually identical at all times. This confirms our previous conclusion that the Bcd gradient is produced by a bcd mRNA gradient rather than by diffusion. Based on our observation that bcd mRNA colocalizes with Staufen (Stau), we propose that the bcd mRNA gradient forms by a novel mechanism involving quasi-random active transport of a Stau-bcd mRNA complex through a nonpolar microtubular network, which confines the bcd mRNA to the cortex of the embryo.