THE IROQUOIS COMPLEX CONTROLS THE SOMATOTOPY OF DROSOPHILA NOTUM MECHANOSENSORY PROJECTIONS

Sensory neurons can establish topologically ordered projections in the central nervous system, thereby building an internal representation o f the external world. We analyze how this ordering is genetically cont rolled in Drosophila, using as a model system the neurons that innerva te the mechanosensory bristles on the back of the by (the notum), Sens ory neurons innervating the medially located bristles send an axonal b ranch that crosses the central nervous system midline, defining a 'med ial' identity, while the ones that innervate the lateral bristles send no such branch, defining a 'lateral' identity. We analyze the role of the proneural genes achaete and scute, which are involved in the form ation of the medial and lateral bristles, and we show that they have n o effect on the 'medial' and 'lateral' identities of the neurons. We a lso analyze the role of the prepattern genes araucan and caupolican, t wo members of the iroquois gene complex which are required for the exp ression of achaete and scute in the lateral region of the notum, and w e show that their expression is responsible for the 'lateral' identity of the projection.