Spatial and temporal order in the developing Drosophila eye

There are many instances in development where a regular arrangement of cell fates self-organizes through cell-cell interactions, yet the dynamics by which these patterns arise, and the underlying logic, often remain elusive. In the developing Drosophila eye, regular rows of light-receiving units emerge in the wake of a traveling differentiation front to form a crystal-like array. The propagation of this pattern is thought to proceed by templating, with inhibitory signaling from each row providing a negative template for the next, but its dynamics had not been directly observed. Live imaging reveals unanticipated oscillations of the proneural factor Atonal, associated with pulses of Notch signaling activity. Our observations inform a new relay model for eye patterning, in which dynamic signaling from row n triggers differentiation at row n+2, conveying both spatial and temporal information to propagate crystal-like order.