Diversification of oral and aboral mesodermal regulatory states in pregastrular sea urchin embryos

Specification of the non-skeletogenic mesoderm (NSM) in sea urchin embryos depends on Delta signaling. Signal reception leads to expression of regulatory genes that later contribute to the aboral NSM regulatory state. In oral NSM, this is replaced by a distinct oral regulatory state in consequence of Nodal signaling. Through regulome wide analysis we identify the homeobox gene not as an immediate Nodal target. not expression in NSM causes extinction of the aboral regulatory state in the oral NSM, and expression of a new suite of regulatory genes. All NSM specific regulatory genes are henceforth expressed exclusively, in oral or aboral domains, presaging the mesodermal cell types that will emerge. We have analyzed the regulatory linkages within the aboral NSM gene regulatory network. A linchpin of this network is gataE which as we show is a direct Gcm target and part of a feedback loop locking down the aboral regulatory state.

► Early mesoderm segments display distinct and non-overlapping regulatory states.
► Regulome wide analysis identified the pan-oral not gene as a target of Nodal signaling.
► not disrupts maintenance of the aboral regulatory state and institutes oral mesoderm.
► A three-way feedback loop in the aboral mesoderm locks down the aboral regulatory state.