Sex-specific transcript diversity is regulated by a maternal pioneer factor in early Drosophila embryos

Maternally deposited RNAs and proteins play a crucial role in initiating zygotic transcription during early embryonic development. However, the mechanisms by which maternal factors regulate zygotic transcript diversity early in development remain poorly understood. Furthermore, how early in development sex-specific transcript diversity occurs is not known genome-wide in any organism. Here, we determine that sex-specific transcript diversity occurs much earlier in development than previously thought in Drosophila, concurrent with Zygotic genome activation (ZGA). We use genetic, biochemical, and genomic approaches to demonstrate that the essential maternally-deposited pioneer factor CLAMP (Chromatin linked adapter for MSL proteins) is a key regulator of sex-specific transcript diversity in the early embryo via the following mechanisms: 1) In both sexes, CLAMP directly binds to the gene bodies of female and male sex-specifically spliced genes. 2) In females, CLAMP modulates chromatin accessibility of an alternatively-spliced exon within Sex-lethal, the master regulator of sex determination, to promote protein production. 3) In males, CLAMP regulates Maleless (MLE) distribution, a spliceosome component to prevent aberrant sex-specific splicing. Thus, we demonstrate for the first time how a maternal factor regulates early zygotic transcriptome diversity sex-specifically. We also developed a new tool to measure how splicing changes over time called time2splice.