Optogenetic rescue of a developmental patterning mutant

Animal embryos are partitioned into spatial domains by complex patterns of signaling and gene expression. Yet it is still largely unknown what features of a developmental signal (e.g., the minimum duration, spatial extent, or number of distinct concentrations) are essential to be compatible with life. Part of the challenge arises because it has been impossible to “paint” an arbitrary signaling pattern on an embryo to test its sufficiency. Here we demonstrate exactly this capability by combining optogenetic control of Ras/Erk signaling with the genetic loss of terminal signaling in early embryos. Simple all-or-none light inputs at the embryonic termini were able to completely rescue normal development, generating viable larvae and fertile adults from this otherwise-lethal genetic mutant. Systematically varying illumination parameters further revealed that at least three distinct developmental programs are triggered by different cumulative doses of Erk, and that a wide range of spatial patterns permit normal tissue movements during gastrulation. These results open the door to the targeted design of complex morphogenetic outcomes as well as the ability to correct patterning errors that underlie developmental defects.