MUTATIONS AFFECTING THE DEVELOPMENT OF THE EMBRYONIC ZEBRAFISH BRAIN

In a large scale mutagenesis screen for embryonic mutants in zebrafish , we have identified 63 mutations in 24 loci affecting the morphogenes is of the zebrafish brain, The expression of marker genes and the inte grity of the axonal scaffold have been studied to investigate abnormal ities in regionalization, neurogenesis and axonogenesis in the brain, Mutants can be broadly classified into two groups, one affecting regio nalization along the anterior-posterior or dorsal-ventral axis, and th e other affecting general features of brain morphology, The first grou p includes one locus that is required to generate the anlage of the mi dbrain-hindbrain boundary region at the beginning of somitogenesis. Fo ur loci were identified that affect dorsal-ventral patterning of the b rain, including the previously described cyclops locus, Mutant embryos of this class show a reduction of ventral neuroectodermal structures and variable fusion of the eyes, The second group includes a large cla ss of mutations affecting the formation of brain ventricles. Analysis of this class reveals the requirement of a functional cardiovascular s ystem for ventricle enlargement during embryogenesis. Mutations in one locus lead to the formation of supernumerary primary neurons, a remin iscent of neurogenic mutants in Other mutant phenotypes described here range from abnormalities in the fasciculation and outgrowth of axons to defects in the diameter of the neural tube. The identified loci est ablish the genetic foundation for a further analysis of the developmen t of the zebrafish embryonic brain, phenotype Drosophila.