THE FUNCTION OF SILBERBLICK IN THE POSITIONING OF THE EYE ANLAGE IN THE ZEBRAFISH EMBRYO

In zebrafish, as in other vertebrates, an initially singular eye held within the neural plate has to split during morphogenesis to allow the development of two separated eyes. It has been suggested that anterio r progression of midline tissue within the neural plate is involved in the bilateralization of the eye held. Mutations in the recently ident ified silberblick (slb) gene cause an incomplete separation of the eye s. During gastrulation and early somitogenesis, the ventral midline of the central nervous system (CNS) together with the underlying axial m esendoderm is shortened and broadened in slb embryos. While in wild-ty pe embryos the ventral CNS midline extends to the anterior limit of th e neural plate at the end of gastrulation, there is a gap between the anterior tip of the ventral CNS midline and the anterior edge of the n eural plate in slb. To investigate the cause for the shortening of the ventral CNS midline in slb we determined the fate of labeled ventral CNS midline cells in wild-type and slb embryos at different stages of development. In slb, anterior migration of ventral CNS midline cells i s impaired, which indicates that migration of these cells is needed fo r elongation of the ventral CNS midline. The anterior shortening of th e ventral CNS midline in slb leads to medial instead of bilateral indu ction of optic stalks followed by a partial fusion of the eyes at late r developmental stages. The analysis of the sIb phenotype indicates th at anterior migration of midline cells within the neural plate is requ ired for proper induction and subsequent bilateralization of an initia lly singular eye field. These findings may therefore provide a startin g point in elucidating the role of neural plate morphogenesis in posit ioning of the eyes. (C) 1997 Academic Press.