CELL-AUTONOMOUS SHIFT FROM AXIAL TO PARAXIAL MESODERMAL DEVELOPMENT IN ZEBRAFISH FLOATING HEAD MUTANTS

Zebrafish floating head mutant embryos lack notochord and develop semi tic muscle in its place. This may result from incorrect specification of the notochord domain at gastrulation, or from respecification of no tochord progenitors to form muscle, In genetic mosaics,floating head a cts cell autonomously, Transplanted wild-type cells differentiate into notochord in mutant hosts; however, cells from, floating head mutant donors produce muscle rather than notochord in wild-type hosts. Consis tent with respecification, markers of axial mesoderm are initially exp ressed in floating head mutant gastrulas, but expression does not pers ist, Axial cells also inappropriately express markers of paraxial meso derm. Thus, single cells in the mutant midline transiently co-express genes that are normally specific to either axial or paraxial mesoderm, Since floating head mutants produce some floor plate in the ventral n eural tube, midline mesoderm may also retain early signaling capabilit ies. Our results suggest that wild-type floating head provides an esse ntial step in maintaining, rather than initiating, development of noto chord-forming axial mesoderm.