Evolution of novel structures is often made possible by changes in the timing or 
spatial expression of genes regulating development. Macrochaetes, large sensory 
bristles arranged into species-specific stereotypical patterns, are an 
evolutionary novelty of cyclorraphous flies and are associated with changes in 
both the temporal and spatial expression of the proneural genes achaete (ac) and 
scute (sc). Changes in spatial expression are associated with the evolution of 
cis-regulatory sequences, but it is not known how temporal regulation is 
achieved. One factor required for ac-sc expression, the expression of which 
coincides temporally with that of ac-sc in the notum, is Wingless (Wg; also 
known as Wnt). Wingless downregulates the activity of the serine/threonine 
kinase Shaggy (Sgg; also known as GSK-3). We demonstrate that Scute is 
phosphorylated by Sgg on a serine residue and that mutation of this residue 
results in a form of Sc with heightened proneural activity that can rescue the 
loss of bristles characteristic of wg mutants. We suggest that the 
phosphorylated form of Sc has reduced transcriptional activity such that sc is 
unable to autoregulate, an essential function for the segregation of bristle 
precursors. Sgg also phosphorylates Pannier, a transcriptional activator of 
ac-sc, the activity of which is similarly dampened when in the phosphorylated 
state. Furthermore, we show that Wg signalling does not act directly via a 
cis-regulatory element of the ac-sc genes. We suggest that temporal control of 
ac-sc activity in cyclorraphous flies is likely to be regulated by permissive 
factors and might therefore not be encoded at the level of ac-sc gene sequences.