Foxi2 and Sox3 are master regulators controlling ectoderm germ layer specification.

Unlabelled: In vertebrates, germ layer specification represents a critical transition where pluripotent cells acquire lineage-specific identities. We identify the maternal transcription factors Foxi2 and Sox3 to be pivotal master regulators of ectodermal germ layer specification in . Ectopic co-expression of Foxi2 and Sox3 in prospective endodermal tissue induces the expression of ectodermal markers while suppressing mesendodermal markers. Transcriptomics analyses reveal that Foxi2 and Sox3 jointly and independently regulate hundreds of ectodermal target genes. During early cleavage stages, Foxi2 and Sox3 pre-bind to key cis-regulatory modules (CRMs), marking sites that later recruit Ep300 and facilitate H3K27ac deposition, thereby shaping the epigenetic landscape of the ectodermal genome. These CRMs are highly enriched within ectoderm-specific super-enhancers (SEs). Our findings highlight the pivotal role of ectodermal SE-associated CRMs in precise and robust ectodermal gene activation, establishing Foxi2 and Sox3 as central architects of ectodermal lineage specification.

Highlights: Foxi2 and Sox3 are master regulators for the ectodermal germ layer and sub-lineagesFive ectodermal cell states of early gastrulae are regulated by Foxi2 and Sox3Foxi2 and Sox3 binding sites in the genome are enriched in ectoderm super-enhancersSE-associated genes show high expression levels, low noise, stabilizing ectodermal regulation.