Functional Interactions of Genes Mediating Convergent Extension,knypekandtrilobite,during the Partitioning of the Eye Primordium in Zebrafish

AbstractVertebrate eye development in the anterior region of the neural plate involves a series of inductive interactions dependent on the underlying prechordal plate and signals from the midline of the neural plate, including Hedgehog. The mechanisms controlling the spatiotemporal expression pattern ofhedgehoggenes are currently not understood. Cyclopia is observed intrilobite(tri) andknypek(kny) mutants with affected convergent extension of the embryonic axis during gastrulation. Here, we demonstrate thattrimutants show a high frequency of partial or complete cyclopia,knymutants exhibit cyclopia infrequently, whileknym119trim209double-mutant embryos have dramatically reduced convergent extension and are completely cyclopic. We analyzed the relationships between the convergent extension defect, the expression ofhedgehogand prechordal plate genes, and the formation of cyclopia inknym119andtrim209mutants. Our results correlate the cyclopia phenotype with the abnormal location ofhh-expressing cells with respect to the optic primordium. We show that cyclopia in these mutants is not due to an incompetence oftriandknycells to respond to Hedgehog signaling. Rather, it is a consequence of exceeding a critical distance (>40–50 μm) betweenhedgehog-expressing cells and the prospective eye field. We hypothesize that at this distance, midline cells are not in an appropriate position to physically separate the eye field and that HH and other signals do not reach the appropriate target cells. Furthermore,triandknyhave overlapping functions in establishing proper alignment of the anterior neural plate and midline cells expressingshhandtwhhgenes when the partitioning of the eye primordium takes place