LOCALIZED HEDGEHOG ACTIVITY CONTROLS SPATIAL LIMITS OF WINGLESS TRANSCRIPTION IN THE DROSOPHILA EMBRYO

CELL patterning in the body segments of the Drosophila embryo requires activity of the segment polarity genes, a molecularly heterogeneous g roup defined by a generic mutant phenotype1. Two of these genes, wingl ess (wg) and hedgehog (hh), encode proteins that enter the secretory p athway2-4, implicating them as signals that instruct the fates of neig hbouring cells5. Genetic analysis has identified wg transcription as o ne of the targets of hh activity6,7 and it has been suggested that the spatial control of wg expression depends on the limited range of the hh signal and the differential competence of responding cells8. I have tested this model by driving ubiquitous expression of the hh gene usi ng the Hsp70 promoter. Here I report that, as predicted, this causes t he ectopic activation of wg in only a subset of the cells of each para segment. Using another target of hh activity as a probe, I demonstrate that the competence of cells to express wg is independent of their ab ility to receive the hh signal. Finally, I show that wg activation req uires the function of the segment polarity gene fused, suggesting that the putative hh signal is transduced by the serine/threonine kinase t hat fused encodes.