Abstract
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.