The roles of the homeobox genes aristaless and Distal-less in patterning the legs and wings of Drosophila

In the leg and wing imaginal discs of Drosophila, the expression domains of the homeobox genes aristaless (al) and Distal-less (Dll) are defined by th e secreted signaling molecules Wingless (Wg) and Decapentaplegic (Dpp), Her e, the roles played by al and Dll in patterning the legs and wings have bee n investigated through loss of function studies. In the developing leg, al is expressed at the presumptive tip and a molecularly defined null allele o f al reveals that its only function in patterning the leg appears to be to direct the growth and differentiation of the structures at the tip. In cont rast, Dll has previously been shown to be required for the development of a ll of the leg more distal than the coxa. Dll protein can be detected in a c entral domain in leg discs throughout most of larval development, and in ma ture discs this domain corresponds to the distal-most region of the leg, th e tarsus and the distal tibia, Clonal analysis reveals that late in develop ment these are the only regions in which Dll function is required. However, earlier in development Dll is required in more proximal regions of the leg suggesting it is expressed at high levels in these cells early in developm ent but not later. This reveals a correlation between a temporal requiremen t for Dll and position along the proximodistal axis; how this may relate to the generation of the P/D axis is discussed. Dll is required in the distal regions of the leg for the expression of tarsal-specific genes including a l and bric-a-brac, Dll mutant cells in the leg sort out from wild-type cell s suggesting one function of Dll here is to control adhesive properties of cells. Dll is also required for the normal development of the wing, primari ly for the differentiation of the wing margin.