Temporally Changing Roles of Morphogen Dorsal in the Drosophila Early Embryo

Morphogen gradients provide positional cues during development, with cell fate specification proceeding in a morphogen concentration-dependent manner during patterning. However, morphogens also are dynamic as their concentrations change not only in space but also in time, but how these dynamics are translated into cell fate specification over time is not well understood. To provide a better understanding of morphogens’ temporal roles, we studied how Drosophila dorsal-ventral body patterning is controlled by the dynamic morphogen Dorsal (Dl). Dl is present in a nuclear-cytoplasmic gradient along the dorsal-ventral (DV) axis, but Dl levels also continuously increase between and within nuclear cell divisions associated with the early syncytial embryo. To experimentally manipulate Dl levels in time in order to determine whether these dynamics are important, we developed a light-activated degradation system. The blue light inducible degron domain, BLID, was fused to the C-terminus of Dl by genomic editing using CRISPR-Cas9. To assay effects on temporally manipulated Dl levels, we combined this light-inducible degradation system with the MS2-MCP.GFP nascent transcript imaging system, and used to monitor transcription changes in vivo at the snail (sna) locus, a gene requiring high Dl levels. We found that while high Dl levels are required for sna activation at early nuclear cycle 14, late expression can be supported even if Dl levels are extinguished. Twist, an early Dl target gene, is later auto-activating and can support the later sna expression without Dl. Surprisingly, we found that peak levels of Dl, present at late nuclear cycle 14, are required only to fine tune, in particular to decrease, sna levels. This differential action of Dl, first functioning as an activator and next as a damper of expression, is manifest by the coordinate action of two enhancers acting at the sna locus. Here, we highlight how morphogen roles change in time, and suggest that this may be a general characteristic of dynamic morphogens that allows them to control developmental patterning.