Mutations affecting neurogenesis and brain morphology in the zebrafish, Danio rerio

In a screen for embryonic mutants in the zebrafish a large number of mutants were isolated with abnormal brain morphology. We describe here 26 mutants in 13 complementation groups that show abnormal development of large regions of the brain. Early neurogenesis is affected in white tail (wit). During segmentation stages, homozygous wit embryos display an irregularly formed neural keel, particularly in the hindbrain. Using a variety of molecular markers, a severe increase in the number of various early differentiating neurons can be demonstrated. In contrast, late differentiating neurons, radial glial cells and some nonneural cell types, such as the neural crest-derived melanoblasts, are much reduced. Somitogenesis appears delayed. In addition, very reduced numbers of melanophores are present posterior to the mid-trunk. The wit phenotype is reminiscent of neurogenic mutants in Drosophila, such as Notch or Delta. In mutant parachute (pac) embryos the general organization of the hindbrain is disturbed and many rounded cells accumulate loosely in the hindbrain and midbrain ventricles. Mutants in a group of 6 genes, snakehead(snk), natter (nat), otter (ott), fullbrain (ful), viper (vip) and white snake (wis) develop collapsed brain ventricles, before showing signs of general degeneration. atlantis (atl), big head (bid), wicked brain (win), scabland (sbd) and eisspalte (ele) mutants have different malformation of the brain folds. Some of them have transient phenotypes, and mutant individuals may grow up to adults.

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 morphogenesis of the zebrafish brain. The expression of marker genes and the integrity of the axonal scaffold have been studied to investigate abnormalities in regionalization, neurogenesis and axonogenesis in the brain. Mutants can be broadly classified into two groups, one affecting regionalization along the anterior-posterior or dorsal-ventral axis, and the other affecting general features of brain morphology. The first group includes one locus that is required to generate the anlage of the midbrain-hindbrain boundary region at the beginning of somitogenesis. Four loci were identified that affect dorsal-ventral patterning of the brain, 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 class of mutations affecting the formation of brain ventricles. Analysis of this class reveals the requirement of a functional cardiovascular system for ventricle enlargement during embryogenesis. Mutations in one locus lead to the formation of supernumerary primary neurons, a phenotype reminiscent of neurogenic mutants in Drosophila. 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 establish the genetic foundation for a further analysis of the development of the zebrafish embryonic brain.

The role of the Enhancer of split complex during cell fate determination in Drosophila

Molecular and genetic data predict that the Enhancer of split locus functions at the end of a pathway dictating appropriate cell fate determination in a number of developmental contexts. We have sought to dissect the role individual member genes of the complex play through a molecular analysis. Of the two principle class of genes, the first, members of the basic helix-loop-helix (bHLH) class of proteins are expressed in specific regions of the embryo in subtle, overlapping patterns in cells that will differentiate as epidermis. The second, groucho, a member of the WD40 class of proteins, is expressed more generally. Immunoprecipitation experiments do not implicate groucho in G protein mediated signal transduction, a known function of many WD40 type proteins. Instead, the nuclear localisation of the protein suggests a relationship to the bHLH members of the complex. Differences in expression of the bHLH genes between neurogenic mutants implies two pathways to their activation during epidermal determination.