Temporal dynamics of pair-rule stripes in living <i>Drosophila</i> embryos.

Abstract

Traditional studies of gene regulation in the <i>Drosophila</i> embryo centered primarily on the analysis of fixed tissues. These methods provided considerable insight into the spatial control of gene activity, such as the borders of <i>eve</i> stripe 2, but yielded only limited information about temporal dynamics. The advent of quantitative live-imaging and genome-editing methods permits the detailed examination of the temporal control of endogenous gene activity. Here, we present evidence that the pair-rule genes <i>fushi tarazu</i> (<i>ftz</i>) and <i>even-skipped</i> (<i>eve</i>) undergo dynamic shifts in gene expression. We observe sequential anterior shifting of the stripes along the anterior to posterior axis, with stripe 1 exhibiting movement before stripe 2 and the more posterior stripes. Conversely, posterior stripes shift over greater distances (two or three nuclei) than anterior stripes (one or two nuclei). Shifting of the <i>ftz</i> and <i>eve</i> stripes are slightly offset, with <i>ftz</i> moving faster than <i>eve</i> This observation is consistent with previous genetic studies, suggesting that <i>eve</i> is epistatic to <i>ftz</i> The precision of pair-rule temporal dynamics might depend on enhancer-enhancer interactions within the <i>eve</i> locus, since removal of the endogenous <i>eve</i> stripe 1 enhancer via CRISPR/Cas9 genome editing led to precocious and expanded expression of <i>eve</i> stripe 2. These observations raise the possibility of an added layer of complexity in the positional information encoded by the segmentation gene regulatory network.