Conservation of symmetry breaking at the level of chromatin accessibility between fly species with unrelated anterior determinants.

Establishing the anterior-posterior body axis is a fundamental process during embryogenesis, and the fruit fly, , provides one of the best-known case studies of this process. In Drosophila, localized mRNA of serves as anterior determinant (AD). Bicoid engages in a concentration-dependent competition with nucleosomes and initiates symmetry-breaking along the AP axis by promoting chromatin accessibility at the loci of transcription factor (TF) genes that are expressed in the anterior of the embryo. However, ADs of other fly species are unrelated and structurally distinct, and little is known about how they function. We addressed this question in the moth fly, , in which a maternally expressed transcript isoform of the pair-rule segmentation gene is localized in the anterior egg and has been co-opted as AD. We provide a assembly and annotation of the Clogmia genome and describe how knockdown of and maternal transcript affect chromatin accessibility and expression of TF-encoding loci. The results of these experiments suggest direct roles of Cal-Zld in opening and closing chromatin during nuclear cleavage cycles and show that Clogmia's maternal activity promotes chromatin accessibility and anterior expression during the early phase of zygotic genome activation at Clogmia's and loci. We conclude that unrelated dipteran ADs initiate anterior-posterior axis-specification at the level of enhancer accessibility and that and homologs 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 other insects.