How enhancers regulate wavelike gene expression patterns: Novel enhancer prediction and live reporter systems identify an enhancer associated with the arrest of pair-rule waves in the short-germ beetleTribolium

AbstractA key problem in development is to understand how genes turn on or off at the right place and right time during embryogenesis. Such decisions are made by non-coding sequences called ‘enhancers’. Much of our models of how enhancers work rely on the assumption that genes are activatedde novoas stable domains across embryonic tissues. Such view has been strengthened by the intensive landmark studies of the early patterning of the anterior-posterior (AP) axis of theDrosophilaembryo, where indeed gene expression domains seem to arise more or less stably. However, careful analysis of gene expressions in other model systems (including the AP patterning in vertebrates and short-germ insects like the beetleTribolium castaneum) painted a different, very dynamic view of gene regulation, where genes are oftentimes expressed in a wavelike fashion. How such gene expression waves are mediated at the enhancer level is so far unclear. Here we establish the AP patterning of the short-germ beetleTriboliumas a model system to study dynamic and temporal pattern formation at the enhancer level. To that end, we established an enhancer prediction system inTriboliumbased on time- and tissue-specific ATAC-seq and an enhancer live reporter system based on MS2 tagging. Using this experimental framework, we discovered severalTriboliumenhancers, and assessed the spatiotemporal activities of some of them in live embryos. We found our data consistent with a model in which the timing of gene expression during embryonic pattern formation is mediated by a balancing act between enhancers that induce rapid changes in gene expressions (that we call ‘dynamic enhancers’) and enhancers that stabilizes gene expressions (that we call ‘static enhancers’).