Figure 2.

Schematic view of the fly Drosophila melanogaster’s development (in (A–C)), also shown in comparison to that of vertebrates such as the frog Xenopus laevis and humans (in (D)). A fertilized Drosophila egg (A) starts to divide and forms an embryo where the cytoplasm is shared between all nuclei (syncytium, (B)). At this stage of development, a dorsoventral gradient initiates the nuclear localization of Dorsal (Dl, blue), which is the fly homolog of vertebrate NF-κB. Dl is nuclear in the ventral region, as can be seen in the cross-section of the fly embryo depicted in (C). Nuclei without Dl are depicted in red, and a graded amount of nuclear Dorsal is depicted in shades of blue, with the maximal Dl concentration in dark blue. Extracellular regulators such as Decapentaplegic (Dpp) inhibit nuclear orsal, whereas Dpp is inhibited by its antagonist Sog (short gastrulation protein). Vertebrates do not have a ventral nerve cord but have a spinal cord; thus, it would not be entirely surprising if molecular regulators were also turned in their orientation from ventral to dorsal. BMP-4, a homolog of Dpp, is expressed in the ventral domain (D), whereas Chordin, a homolog of Sog, is expressed in the dorsal domain.