CASH4, A NOVEL ACHAETE-SCUTE HOMOLOG INDUCED BY HENSENS NODE DURING GENERATION OF THE POSTERIOR NERVOUS-SYSTEM

In vertebrate embryos, the precursor cells of the central nervous syst em (CNS) are induced by signaling from the organizer region. Here we r eport the isolation of a novel vertebrate achaete-scute homolog, cash4 , which is expressed in the presumptive posterior nervous system in re sponse to such signaling. cash4 is first expressed in epiblast cells f lanking the late-phase organizer (Hensen's node), which retains its ab ility to induce cash4 during regression to the caudal end of the embry o. We show that these node-derived signals can be mimicked in vivo by the activity of fibroblast growth factor (FGF). We demonstrate that ca sh4 can substitute for the achaete/scute genes in the fly and that it also has proneural activity in vertebrate embryos. Together these resu lts suggest that cash4 functions as a proneural gene downstream of nod e-derived signals (including EGF) to promote the formation of the neur al precursors that will give rise to the posterior CNS in the chick em bryo.