Isl-1 has previously been established as the earliest marker of developing chicken spinal motor neurons where it is regulated by inductive signals from the floorplate and notochord. We now report that, in zebrafish, the expression of Isl-1 is initiated in Rohon-Beard cells, primary motor neurons, interneurons and cranial ganglia, hours before the neural tube itself is formed. The expression is initiated simultaneously in the Rohon-Beard cells and the primary motor neurons, at the axial level of the presumptive first somite. The Isl-1-expressing motor neurons appear on either side of the ventral midline whereas the interneurons and Rohon-Beard cells initiate expression while located at the edge of the germinal shield. Isl-1 expression is initiated in these cells before the formation of a differentiated notochord. Isl-1 is expressed in the various functional classes of primary neurons at 24 hours postfertilization. This selective expression of a homeodomain protein in the primary neurons implies that these neurons share a common program of early development and that they have evolved and been selected for as a coordinated system. One of the functions of the primary neurons is to send long axons which pioneer the major axon tracts in the zebrafish embryo. An evolutionary conserved functional role for Isl-1 in the expression of the pioneering phenotype of the primary neurons is suggested.

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