Organ development is directed by selector gene networks. Eye development in the 
fruit fly Drosophila melanogaster is driven by the highly conserved selector 
gene network referred to as the "retinal determination gene network," composed 
of approximately 20 factors, whose core comprises twin of eyeless (toy), eyeless 
(ey), sine oculis (so), dachshund (dac), and eyes absent (eya). These genes 
encode transcriptional regulators that are each necessary for normal eye 
development, and sufficient to direct ectopic eye development when misexpressed. 
While it is well documented that the downstream genes so, eya, and dac are 
necessary not only during early growth and determination stages but also during 
the differentiation phase of retinal development, it remains unknown how the 
retinal determination gene network terminates its functions in determination and 
begins to promote differentiation. Here, we identify a switch in the regulation 
of ey by the downstream retinal determination genes, which is essential for the 
transition from determination to differentiation. We found that central to the 
transition is a switch from positive regulation of ey transcription to negative 
regulation and that both types of regulation require so. Our results suggest a 
model in which the retinal determination gene network is rewired to end the 
growth and determination stage of eye development and trigger terminal 
differentiation. We conclude that changes in the regulatory relationships among 
members of the retinal determination gene network are a driving force for key 
transitions in retinal development.