To achieve the "constancy of the wild-type," the developing organism must be buffered against stochastic fluctuations and environmental perturbations [1]. This phenotypic buffering has been theorized to arise from a variety of genetic mechanisms [2] and is widely thought to be adaptive and essential for viability. In the Drosophila blastoderm embryo, staining with antibodies against the active, phosphorylated form of the bone morphogenetic protein (BMP) signal transducer Mad, pMad [3, 4], or visualization of the spatial pattern of BMP-receptor interactions [5] reveals a spatially bistable pattern of BMP signaling centered on the dorsal midline. This signaling event is essential for the specification of dorsal cell fates, including the extraembryonic amnioserosa [5-7]. BMP signaling is initiated by facilitated extracellular diffusion [4, 8] that localizes BMP ligands dorsally. BMP signaling then activates an intracellular positive feedback circuit that promotes future BMP-receptor interactions [5, 6]. Here, we identify a genetic network comprising three genes that canalizes this BMP signaling event. The BMP target eiger(egr) acts in the positive feedback circuit to promote signaling, while the BMP binding protein encoded by crossveinless-2 (cv-2) antagonizes signaling. Expression of both genes requires the early activity of the homeobox gene zerknullt (zen). Two Drosophila species lacking early zen expression have high variability in BMP signaling. These data both detail a new mechanism that generates developmental canalization and identify an example of a species with noncanalized axial patterning.
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