Gap junction-mediated signaling coordinates Rhodopsin coupling for <i>Drosophila</i> color vision

The Drosophila compound eye is composed of approximately 800 ommatidia, and every ommatidium contains eight photoreceptor cells, six outer cells (R1-R6) and two inner cells (R7 and R8), and accessory cells (cone and pigment cells). The expression of rhodopsin genes in R7 and R8 is highly coordinated through an instructive signal from R7 to R8. The activity of the homeodomain protein Defective proventriculus in R1 is also required to transmit this instructive signal, suggesting that cell-cell communication between R7, R1, and R8 is important to generate the pattern of Rh expression in R7/R8 (Rhodopsin coupling). As cell junctions play crucial roles in maintaining the structural and functional integrity of tissues, we tested whether cell junction proteins are involved in the interactions between photoreceptor cells. Here, we demonstrate that gap junction proteins innexin 2 and innexin 7 in accessory cells are necessary for transmitting signals from R7 to R8. In addition, Notch-mediated accessory cell development and Rhodopsin coupling in R7/R8 are highly correlated. Our results provide evidence that functional coupling of two different neurons, R7 and R8, is established through gap junction-mediated signaling from adjacent accessory cells.