Segregation of Eye and Antenna Fates: Initiation and Maintenance. Y. Henry Sun^1,2, Cheng-Wei Wang^1,2, Hui-Yu Ku^1,2. 1) Department of Life Sciences and Institute of Genome Sciences, National Yang-Ming University, Taipei, Taiwan, Republic of China; 2) Institute of Molecular Biology, Academia Sinica, Nankang, Taipei, Taiwan, Republic of China.

   A general problem in development is how do adjacent primordia adopt different developmental fates and stably maintain their distinct fates. In Drosophila, the adult eye and antenna originate from the embryonic eye-antenna primordium. We used the G-TRACE lineage tracing method to show that these cells proliferate to form the larval eye-antenna disc. Classical mitotic clonal analysis and disc transplantation experiment suggested that the eye and antenna fates segregated before late second instar (L2). However we found, by twin-spot MARCM, that a significant proportion of clones induced at late L2 can still cross the eye and antenna disc boundary. In L1 eye-antenna disc, the nuclear factors eyeless (ey), twin of eyeless (toy), sine oculis (so), eye gone (eyg), homothorax (hth), and teashirt (tsh) are expressed uniformly. The expression became segregated at mid-L2 with the expression of Cut (Ct) in the antenna disc and the restriction of Ey in the eye disc. We have recently demonstrated that once segregated, the eye and antenna fates are stably maintained by the mutual repression between the eye pathway factors (Ey and So) and antenna pathway factors (Ct and Hth) (Wang and Sun, 2012, Development 139:3413-21). However, these cell-autonomous repressions can only create a salt-and-pepper pattern. A coordinated segregation of eye and antenna cells would require additional patterning mechanism. We found that the initial bias of the two fields comes from induction by an external signal, the Egfr ligand Spitz (Spi). Spi, acting through the Egfr signaling pathway, induced Ct expression in the presumptive antennal field. As Ct and Spi spreads progressively through the antennal field, Ey expression was driven out by Ct. This initial bias thus directed the progressive segregation of the two fields. Our results revealed the molecular mechanisms for the initiation and maintenance of eye and antenna fate segregation.