Two temporal functions of Glass: Ommatidium patterning and photoreceptor differentiation.

Much progress has been made in elucidating the molecular networks required for specifying retinal cells, including photoreceptors, but the downstream mechanisms that maintain identity and regulate differentiation remain poorly understood. Here, we report that the transcription factor Glass has a dual role in establishing a functional Drosophila eye. Utilizing conditional rescue approaches, we confirm that persistent defects in ommatidium patterning combined with cell death correlate with the overall disruption of eye morphology in glass mutants. In addition, we reveal that Glass exhibits a separable role in regulating photoreceptor differentiation. In particular, we demonstrate the apparent loss of glass mutant photoreceptors is not only due to cell death but also a failure of the surviving photoreceptors to complete differentiation. Moreover, the late reintroduction of Glass in these developmentally stalled photoreceptors is capable of restoring differentiation in the absence of correct ommatidium patterning. Mechanistically, transcription profiling at the time of differentiation reveals that Glass is necessary for the expression of many genes implicated in differentiation, i.e. rhabdomere morphogenesis, phototransduction, and synaptogenesis. Specifically, we show Glass directly regulates the expression of Pph13, which encodes a transcription factor necessary for opsin expression and rhabdomere morphogenesis. Finally, we demonstrate the ability of Glass to choreograph photoreceptor differentiation is conserved between Drosophila and Tribolium, two holometabolous insects. Altogether, our work identifies a fundamental regulatory mechanism to generate the full complement of cells required for a functional rhabdomeric visual system and provides a critical framework to investigate the basis of differentiation and maintenance of photoreceptor identity.