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First published online 9 July 2008
doi: 10.1242/dev.017814
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1 Department of Pathology, Stanford University School of Medicine, 300 Pasteur
Drive, Stanford, CA 94305, USA.
2 Department of Genetics, Howard Hughes Medical Institute, Harvard Medical
School, 77 Avenue Louis Pasteur, Boston, MA 02115, USA.
* Author for correspondence (e-mail: jaxelrod{at}stanford.edu)
Accepted 18 June 2008
A fundamental concept in development is that secreted molecules such as Wingless (Wg) and Hedgehog (Hh) generate pattern by inducing cell fate. By following markers of cellular identity posterior to the Wg- and Hh-expressing cells in the Drosophila dorsal embryonic epidermis, we provide evidence that neither Wg nor Hh specifies the identity of the cell types they pattern. Rather, they maintain pre-existing cellular identities that are otherwise unstable and progress stepwise towards a default fate. Wg and Hh therefore generate pattern by inhibiting specific switches in cell identity, showing that the specification and the patterning of a given cell are uncoupled. Sequential binary decisions without induction of cell identity give rise to both the groove cells and their posterior neighbors. The combination of independent progression of cell identity and arrest of progression by signals facilitates accurate patterning of an extremely plastic developing epidermis.
Key words: Drosophila, Hedgehog, Wingless, Embryo, Patterning
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