Figure 1.

Dorsal-ventral and anterior-posterior patterning genes regulate vnd/NK-2 gene expression (5). The vnd/NK-2 gene is activated initially by dorsal in the ventral half of the embryo (stage 4). Both dorsal and twist are required to activate vnd/NK-2 gene in the hindgut and posterior midgut primordia. The vnd/NK-2 gene is not expressed in the mesodermal anlage because of repression by snail, not in the mesectodermal anlage because of repression by sim, or in part of the lateral neuroectodermal and dorsal epidermal anlagen because of repression mediated by dpp. The stripe of neuroectodermal cells that express the vnd/NK-2 gene is converted into one cluster of vnd/NK-2-positive cells per hemisegment by periodic repression (indicated by white arrowheads) of the vnd/NK-2 gene (stage 8). Another kind of repressor (indicated by green arrowheads) during stages 9–11 converts each cluster into two clusters of neural cells per hemisegment. Arrows represent gene activation, and terminal bars represent gene repression. dpp indirectly mediates repression of the vnd/NK-2 gene in dorsal neuroectoderm via an unidentified repressor. A, anterior; P, posterior; D, dorsal; V, ventral.