Dynamic Hh signaling can generate temporal information during tissue patterning

The differentiation of tissues and organs requires that cells exchange information in space and time. Spatial information is often conveyed by morphogens, molecules that disperse across receiving cells generating signaling gradients. Cells translate such concentration gradients into space-dependent patterns of gene expression and cellular behavior [1, 2]. But could morphogen gradients also convey developmental time? Here, investigating the developmental role of Hh on a component of the Drosophila visual system, the ocellar retina, we discovered that ocellar cells use the non-linear gradient of Hh as a temporal cue, collectively performing the biological equivalent of a mathematical logarithmic transformation. In this way, a morphogen diffusing from a non-moving source is decoded as a wave of differentiating photoreceptors that travels at constant speed throughout the retinal epithelium.