Localized and transient transcription of Hox genes suggests a link betweenpatterning and the segmentation clock

During development, Hox gene transcription is activated in presomitic mesod erm with a time sequence that follows the order of the genes along the chro mosome. Here, we show that Hoxd1 and other Hox genes display dynamic stripe s of expression within presomitic mesoderm. The underlying transcriptional bursts may reflect the mechanism that coordinates Hox gene activation with somitogenesis. This mechanism appears to depend upon Notch signaling, as mi ce deficient for RBPJk, the effector of the Notch pathway, showed severely reduced Hoxd gene expression in presomitic mesoderm. These results suggest a molecular link between Hox gene activation and the segmentation clock. Su ch a linkage would efficiently keep in phase the production of novel segmen ts with their morphological specification.