Evi1 is necessary for late activation of delta-notch signaling in sea urchin embryos

Embryonic mesoderm specification and differentiation rely on intricate gene regulatory networks. In sea urchin embryos, Ecotropic Viral Integration Site 1 (Evi1) is identified as a key transcriptional regulator that integrates Nodal and Delta-Notch signaling pathways to drive mesodermal development. Through in situ hybridization, hybridization chain reactions, and morpholino-mediated knockdowns, Evi1 is required for activating Notch135 expression and for the formation of mesodermal derivatives, including muscle progenitors and coelomic pouches. Evi1 expression is positively regulated by Nodal signaling: it is upregulated when the Nodal antagonist Lefty is knocked down and lost with Nodal depletion or BMP overexpression. Functionally, Evi1 operates upstream of Notch135, restoring its expression following early Delta-Notch activity and enabling a second signaling phase necessary for mesodermal differentiation. Knockdown of Evi1 results in significant loss of Astacin4 and Myo88, markers for mesoderm and muscle progenitors respectively, and disrupts Pax6 expression, impairing coelomic pouch formation, a precursor to the larval rudiment and adult body plan. These results establish Evi1 as a crucial mediator between Nodal and Delta-Notch pathways in mesodermal patterning. Given its conserved role across deuterostomes, further investigation into the direct transcriptional targets of Evi1 and its later-stage developmental functions could shed light on its broader contributions to organogenesis.