Fig. 9. Both conserved and unique interactions regulate bab along the PD axis of the antenna and leg. bab is activated by Dll in both antenna and leg primordia. bab expression is ss dependent in the first through third tarsal segments (t1-3), but not in the fourth tarsal segment (t4) of the leg. The requirement of ss for bab expression in the antenna could not be assessed, because ss null antennae are transformed toward leg. The proximal boundary of bab expression is set by dac in both antenna and leg, but hth and sal also are required for bab repression in the antenna. dac also modulates bab expression levels between rings in both antenna and leg. The dependence of ss expression on Dll has been documented previously (Duncan et al., 1998), as have the dependence of sal (Dong et al., 2000) and dac (Dong et al., 2001) expression on Dll and hth and the mutual antagonism between Dll and dac in the leg (Dong et al., 2001). Solid lines indicate cell-autonomous effects that could be direct. The dotted line indicates that there is a non cell-autonomous component to inter-ring modulation of bab by dac in the antenna. See text for details.