Fig. 8. The genetic hierarchy that governs antenna development. (A) Dll and hth, probably in conjunction with exd, are required for the antennal expression of ss, dac, ato and sal, which in turn are required for various aspects of antennal differentiation. Dll is required independently for the activation of arista-less (al), Bar and bab (Campbell and Tomlinson, 1998; Chu et al., 2002; Gorfinkiel et al., 1997; Kojima et al., 2000), while hth is required independently for the activation of cut. (B) In addition to activating genes required for specific aspects of antennal differentiation, the homeotic functions of Dll, hth, exd and ss also include repression of distinct aspects of leg differentiation. For instance, while Dll and hth are required for activation of the low level expression of dac in a3, they also prevent the high level dac expression necessary for medial leg differentiation (Dong et al., 2001). hth can also repress dac independent of Dll since ectopic dac expression is observed in hth null clones in a1 and the surrounding head capsule (Dong et al., 2001). Since hth is required for proximal leg as well as antenna differentiation, hth null clones in the proximal antenna are transformed to medial rather than proximal leg structures (P. D. S. D., J. C. and G. P., unpublished observations). ss is downstream of Dll and hth and is required for the repression of distal leg fates (Duncan et al., 1998) (Ian Duncan, personal communication) (this work). (C) Summary of the antennal expression patterns and relationships among Dll, hth, ss, sal, ato, dac and cut.
