Hindsight modulates Delta expression during Drosophila cone cell induction

The induction of cone cells in the Drosophila larval eye disc by the determined R1/R6 photoreceptor precursor cells requires integration of the Delta-Notch and EGF receptor signaling pathways with the activity of the Lozenge transcription factor. Here, we demonstrate that the zinc-finger transcription factor Hindsight (HNT) is required for normal cone-cell induction. R-cells in which hindsight levels are knocked down using RNAi show normal subtype specification, but these cells have lower levels of the Notch ligand Delta. We show that HNT functions in the determined R1/R6 precursor cells to allow Delta transcription to reach high enough levels at the right time to induce the cone-cell determinants Prospero and D-Pax2 in neighboring cells. The Delta signal emanating from the R1/R6 precursor cells is also required to specify the R7 precursor cell by repressing seven-up. As hindsight mutants have normal R7 cell-fate determination, we infer that there is a lower threshold of Delta required for R7 specification than for cone-cell induction.