Lineage and fate in Drosophila: role of the gene tramtrack in sense organ development.

The tactile bristles of the fly comprise four cells that originate from a single precursor cell through a fixed lineage. The gene tramtrack (ttk) plays a crucial role in defining the fates of these cells. Here we analyse the normal pattern of expression of ttk, as well as the effect of ttk overexpression at different steps of the lineage. We show that ttk is never expressed in cells having a neural potential, and that in cells where ttk is expressed, there is a delay between division and the onset of expression. The ectopic expression of ttk before some stage of the cell cycle can block further cell division. Furthermore, this expression transforms neural into non-neural cells, suggesting that ttk acts as a repressor of neural fate at each step of the lineage. Our results suggest that ttk is probably not involved in setting up the mechanism that creates an asymmetry between sister cells, but rather in the implementation of that choice.