Segmentation is one of the fundamental mechanisms by which animals develop. In vertebrates, the somites—mesoderm in the vertebrate embryo, occurring in pairs along the notochord that give rise to muscles and vertebrae—develop through successive waves of gene expression known as the segmentation clock. The Notch gene is essential for this process in vertebrates, but in the fruit fly Notch is not involved in generating a segmented embryo, suggesting that vertebrates and invertebrates have evolved different mechanisms for generating segments. In the June 19 Nature, Angelika Stollewerk and colleagues at the University of Cologne demonstrate that this is not the case for the spider, Cupiennius salei (Nature, 423:863-865, June 19, 2003).

Stollewerk et al. observed that, unlike Drosophila, Notch and its ligand Delta are expressed in stripes in the spider embryo. This fact alone strongly suggests that Notch signaling is involved in segmentation in this species....

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