School of Biological Sciences, University of Sussex, Brighton, BN1 9QG, UK.,
Drosophila melanogaster shows a stereotypical patterning on the notum of micro- and macrochaetae bristles, which are the visible part of the adult peripheral nervous system. The precise patterning of the notum provides an excellent two-dimensional model for studying pattern formation. Mutations involved in this pathway are easily observed as they usually cause the loss or gain of bristles. In order for a cell in the notum to become a macrochaete, a specific program of events occurs. Clusters of cells in the wing disc increase their expression of the proneural genes, achaete (ac) and scute (sc), which gives them the potential to become a sensory mother cell (SMC). These cells then compete, through cell-cell interactions of the neurogenic genes, to become a SMC. Once the SMC has been specified, an inhibitory signal prevents other cells in the cluster from taking on this fate.
The wingless signalling pathway is necessary for a subset of bristles on the notum. shaggy-zeste white 3 (sgg-zw3) a segment polarity gene encoding a ubiquitously expressed serine-threonine kinase is involved in the wingless (wg) signalling pathway. wingless, which is also a segment polarity gene, is a secreted protein and it inactivates sgg-zw3 through the dishevelled (dsh) protein. Inactivation of sgg-zw3 increases the expression of ac and sc providing a bias to form a SMC. dsh is now shown to also directly inactivate the Notch (N)receptor resulting in feed-back on the wg pathway.
Using the Flp/Frt system, sgg- clones marked by the absence of the plac protein were generated in the wing disc. achaete expression, which is essential for the formation of a SMC, was visualised using a mouse monoclonal antibody. In the presumptive wing blade, previous data have shown ectopic ac can be seen throughout the clones. We have shown that notal clones show a different response to the loss of sgg-zw3. As well as causing abnormal folding around the edge of the clone, there is ectopic ac expression. In contrast to clones in the presumptive wing blade, clones in the notum show non-uniform expression of ac. Only cells at the borders of the clone show expression, whilst central cells, unlike wing-pouch clones, show no ac expression.
This data suggests that the wingless signalling pathway is involved in the correct positioning of a subset of bristles on the notum. The secretion of Wg protein from its stripe of expression in the notum could act as a positional cue for these bristles. We are now looking at the expression of ac in relation to wg expression and we are also looking at the epistasis between wg and extramacrochaete (emc), a gene involved in the inhibition of ac and sc expression.