Fig. 1

Comparison of Bcd gradient formation, as explained by the ARTS and the SDD models. Pictures represent midsagittal confocal planes or schematic drawings of embryos oriented with their dorsal side up and anterior to the left. Relative intensities of the crude confocal pictures were converted to a color scale with values of 0–255 (8-bit), as shown in inserts of (a) and (f), respectively. Nomenclature of nuclear cycles follows that of [28]. (a) freshly fertilized embryo stained for bcd mRNA. (b, c) in the ARTS (active RNA transport, synthesis) model, the bcd mRNA (red, arrows) is actively transported along microtubules (b, green) to form the mRNA gradient. The mRNA gradient then serves as template for translation of the Bcd protein to form the protein gradient (c, blue). (d, e) in the SDD (synthesis, diffusion, degradation) model, the mRNA (d, red) is proposed to stay at the tip at all times. The mRNA is translated to produce the Bcd protein (d, blue) which diffuses throughout the whole embryo (e). After 3 h, the nuclei at the 14th nuclear cycle are filled with Bcd protein which forms a precise morphogen gradient (f). Please note that for both models, the start and end points are identical (a, f), but they differ considerably in their mechanisms