ZEBRAFISH PRIMARY NEURONS INITIATE EXPRESSION OF THE LIM HOMEODOMAIN PROTEIN ISL-1 AT THE END OF GASTRULATION

Isl-1 has previously been established as the earliest marker of develo ping chicken spinal motor neurons where it is regulated by inductive s ignals from the floor-plate and notochord. We now report that, in zebr afish, the expression of Isl-1 is initiated in Rohon-Beard cells, prim ary motor neurons, interneurons and cranial ganglia, hours before the neural tube itself is formed. The expression is initiated simultaneous ly in the Rohon-Beard cells and the primary motor neurons, at the axia l level of the presumptive first somite. The Isl-1-expressing motor ne urons appear on either side of the ventral midline whereas the interne urons and Rohon-Beard cells initiate expression while located at the e dge of the germinal shield. Isl-1 expression is initiated in these cel ls before the formation of a differentiated notochord. Isl-1 is expres sed in the various functional classes of primary neurons at 24 hours p ostfertilization. This selective expression of a homeodomain protein i n the primary neurons implies that these neurons share a common progra m of early development and that they have evolved and been selected fo r 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 zebr afish embryo. An evolutionary conserved functional role for Isl-1 in t he expression of the pioneering phenotype of the primary neurons is su ggested.