her9 promotes floor plate development in zebrafish

Notochord, floor plate, and in anamniotes hypochord, are vertebrate embryonic midline structures that are the sources of molecules that pattern the nervous system, somites, and dorsal aorta. Midline precursor cells arise from the dorsal organizer during gastrulation, and Notch signaling is an important regulator of midline cell fate specification. To understand fully how Notch signaling regulates midline development, we investigated the role of potential Notch target genes. We show here that midline precursors express her9, a member of the hairy/Enhancer of split gene family. Although her9 inhibits notochord development and promotes floor plate specification, her9 expression in floor plate cells appears not to require Notch signaling. We show that, instead, her9 is a downstream effector of Nodal signaling for floor plate specification.