Anterior movement of ventral diencephalic precursors separates the primordial eye field in the neural plate and requires cyclops

A currently favored hypothesis postulates that a single field of cells in the neural plate forms bilateral retinas. To learn how retinal precursors segregate, we followed individual labeled neural plate cells in zebrafish. In the late gastrula, a single field of odd-paired-like-expressing cells contributed to both retinas, bordered posteriorly by diencephalic precursors expressing mariposa. Median mariposa-expressing cells moved anteriorly, separating the eyes, and formed ventral anterior diencephalon, the presumptive hypothalamus. In cyclops mutants, corresponding cells failed to move anteriorly, a ventral diencephalon never formed, and the eyes remained fused. Ablation of the region containing these cells induced cyclopia in wild types. Our results indicate that movement of a median subpopulation of diencephalic precursors separates retinal precursors into left and right eyes. Wild-type cyclops gene function is required for these morphogenetic movements.