Hh and Wg: inducers or stabilisers of cell fate?

As Jeff Axelrod and co-workers state in their paper (see p. 2767), `a fundamental concept in development is that secreted molecules such as Wg and Hh generate pattern by inducing cell fate'. They now cast doubt on this concept by reporting that, in Drosophila embryos, Wg and Hh generate pattern by inhibiting specific switches in cell identity rather than by specifying cell identity. They reached this conclusion by studying the specification and patterning of the segmental grooves that develop immediately posterior to the Hh-secreting, en-expressing cell stripes in the fly embryonic epidermis. By identifying Odd as a groove cell (GC) marker, the authors traced GC lineage and found that Wg, by maintaining En in these stripes, inhibits the development of Odd-expressing GC precursors. Thus, Wg signalling stops cells from switching to a GC identity, while cells beyond its reach can make this switch. Similarly, Hh, in a subsequent step, refines the GC pattern by blocking another transition towards a more posterior fate. How general this inhibition of cell identity progression by patterning signals is,awaits further investigation.