Regulation of ocellar specification and size by twin of eyeless and homothorax.

The retinal determination gene network (RDGN) constitutes a paradigm of a gene network controlling organ specification and growth. In this study, we probed the RDGN in the Drosophila ocelli, a set of simple eyes located on the fly's dorsal head, by studying the expression, regulation, and function of toy, hth, eya, and so, members of the Pax6, Meis, Eya, and Six gene families. Our results highlight the role of the pax6 gene toy, together with the hh signaling pathway, in the initiation of eya and so expression; the engagement of eya and so in a feedback loop necessary for their full expression; and the interplay between hh signaling and hth as a mechanism of organ size control, as general regulatory steps in the specification of visual organs.