Conservation of symmetry breaking at the level of chromatin accessibility between fly species with unrelated anterior determinants
Establishing the anterior-posterior (AP) body axis is a fundamental process during embryogenesis, and the fruit fly, Drosophila melanogaster, provides one of the best-known case studies. But for unknown reasons, different species of flies (Diptera) establish the AP axis through unrelated, structurally distinct anterior determinants (ADs). The AD of Drosophila, Bicoid (Bcd), initiates symmetry-breaking during nuclear cleavage cycles (NCs) when ubiquitous pioneer factors, such as Zelda (Zld), drive zygotic genome activation (ZGA) at the level chromatin accessibility by nucleosome depletion. While Bcd engages in a concentration-dependent competition with nucleosomes at the loci of a small set of transcription factor (TF) genes that are expressed in the anterior embryo, it remains unknown whether unrelated ADs of other fly species function in the same way and target homologous genes. We have examined the symmetry-breaking mechanism of a moth fly, Clogmia albipunctata, in which a maternally expressed transcript isoform of the pair-rule segmentation gene odd-paired serves as AD. We provide a de novo assembly and annotation of the Clogmia genome and describe how Clogmias orthologs of zelda (Cal-zld) and odd-paired (Cal-opa) affect chromatin accessibility and gene expression. Our results suggest direct roles of Cal-zld in opening and closing chromatin during nuclear cleavage cycles (NCs) and show that during the early phase of ZGA maternal Cal-opa activity promotes chromatin accessibility and anterior expression at Clogmias homeobrain and sloppy-paired loci. These genes are not known as key targets of Bcd but may serve a more widely conserved role in the initiation of anterior pattern formation given their early anterior expression and function in head development in insects. We conclude that the ADs of Drosophila and Clogmia differ in their target genes but share the mechanism of concentration-dependent nucleosome depletion.