Figure 9.

Summary. The factors and their interactions present in domains II, IV, PPN, and the MF are schematized. Do main IV expresses the differentiation transcription factors So, Eya, and Dac. Differentiated cells also express Hh, which induces Dpp expression in the MF. Dpp represses Hth and induces Hairy. Hth represses Hairy. The combination of Hth–Tsh–Ey is mutually antagonistic with So–Eya–Dac. Ey also directly activates So expression (Niimi et al. 1999), perhaps in the PPN domain. Also indicated is a positive requirement for Wg signaling for Hth expression in domain II (data not shown). Wg, together with the factors present in domain II, is proposed to promote proliferation in the eye disc. Not indicated here is our observation that Eya and Dac collaborate with Dpp to repress Hth and our inference that an unknown short-range signal in the MF represses Tsh and Ey.