Prepatterning the Drosophila notum: the three genes of the iroquois complex play intrinsically distinct roles

The Drosophila thorax exhibits 11 pairs of large sensory organs (macrochaetes) identified by their unique position. Remarkably precise, this pattern provides an excellent model system to study the genetic basis of pattern formation. In imaginal wing discs, the achaete-scute proneural genes are expressed in clusters of cells that prefigure the positions of each macrochaete. The activities of prepatterning genes provide positional cues controlling this expression pattern. The three homeobox genes clustered in the iroquois complex (araucan, caupolican and mirror) are such prepattern genes. mirror is generally characterized as performing functions predominantly different from the other iroquois genes. Conversely, araucan and caupolican are described in previous studies as performing redundant functions in most if not all processes in which they are involved. We have addressed the question of the specific role of each iroquois gene in the prepattern of the notum and we clearly demonstrate that they are intrinsically different in their contribution to this process: caupolican and mirror, but not araucan, are required for the neural patterning of the lateral notum. However, when caupolican and/or mirror expression is reduced, araucan loss of function has an effect on thoracic bristles development. Moreover, the overexpression of araucan is able to rescue caupolican loss of function. We conclude that, although retaining some common functionalities, the Drosophila iroquois genes are in the process of diversification. In addition, caupolican and mirror are required for stripe expression and, therefore, to specify the muscular attachment sites prepattern. Thus, caupolican and mirror may act as common prepattern genes for all structures in the lateral notum.