FGF8 IS MUTATED IN ZEBRAFISH ACEREBELLAR (ACE) MUTANTS AND IS REQUIRED FOR MAINTENANCE OF MIDBRAIN-HINDBRAIN BOUNDARY DEVELOPMENT AND SOMITOGENESIS

We describe the isolation of zebrafish Fgf8 and its expression during gastrulation, somitogenesis, fin bud and early brain development. By d emonstrating genetic linkage and by analysing the structure of the Fgf 8 gene, me show that acerebellar is a zebrafish Fgf8 mutation that may inactivate Fgf8 function. Homozygous acerebellar embryos lack a cereb ellum and the midbrain-hindbrain boundary organizer. Fgf8 function is required to maintain, but not initiate, expression of Pax2.1 and other marker genes in this area. We show that Fgf8 and Pax2.1 are activated in adjacent domains that only later become overlapping, and activatio n of Fgf8 occurs normally in ilo isthmus embryos that are mutant for P ax2.1. These findings suggest that multiple signaling pathways are ind ependently activated in the midbrain-hindbrain boundary primordium dur ing gastrulation, and that Fgf8 functions later during somitogenesis t o polarize the midbrain. Fgf8 is also expressed in a dorsoventral grad ient during gastrulation and ectopically expressed Fgf8 can dorsalize embryos, Nevertheless, acerebellar mutants show only mild dorsoventral patterning defects. Also, in spite of the prominent role suggested fo r Fgf8 in limb development, the pectoral fins are largely unaffected i n the mutants. Fgf8 is therefore required in development of several im portant signaling centers in the zebrafish embryo, but may be redundan t or dispensable for others.