Carl‐Philipp Heisenberg

In a large-scale screen, we isolated mutants displaying a specific visible phenotype in embryos or early larvae of the zebrafish, Danio rerio. Males were mutagenized with ethylnitrosourea (ENU) and F2 families of single pair matings between sibling F1 fish, heterozygous for a mutagenized genome, were raised. Egg lays were obtained from several crosses between F2 siblings, resulting in scoring of 3857 mutagenized genomes. F3 progeny were scored at the second, third and sixth day of development, using a stereomicroscope. In a subsequent screen, fixed embryos were analyzed for correct retinotectal projection. A total of 4264 mutants were identified. Two thirds of the mutants displaying rather general abnormalities were eventually discarded. We kept and characterized 1163 mutants. In complementation crosses performed between mutants with similar phenotypes, 894 mutants have been assigned to 372 genes. The average allele frequency is 2.4. We identified genes involved in early development, notochord, brain, spinal cord, somites, muscles, heart, circulation, blood, skin, fin, eye, otic vesicle, jaw and branchial arches, pigment pattern, pigment formation, gut, liver, motility and touch response. Our collection contains alleles of almost all previously described zebrafish mutants. From the allele frequencies and other considerations we estimate that the 372 genes defined by the mutants probably represent more than half of all genes that could have been discovered using the criteria of our screen. Here we give an overview of the spectrum of mutant phenotypes obtained, and discuss the limits and the potentials of a genetic saturation screen in the zebrafish.

Zebrafish embryos and larvae have stage-specific patterns of motility or locomotion. Two embryonic structures accomplish this behavior: the central nervous system (CNS) and skeletal muscles. To identify genes that are functionally involved in mediating and controlling different patterns of embryonic and larval motility, we included a simple touch response test in our zebrafish large-scale genetic screen. In total we identified 166 mutants with specific defects in embryonic motility. These mutants fall into 14 phenotypically distinct groups comprising at least 48 genes. Here we describe the various phenotypic groups including mutants with no or reduced motility, mechanosensory defective mutants, 'spastic' mutants, circling mutants and motor circuit defective mutants. In 63 mutants, defining 18 genes, striation of somitic muscles is reduced. Phenotypic analysis provides evidence that these 18 genes have distinct and consecutive functions during somitic muscle development. The genes sloth (slo) and frozen (fro) already act during myoblast differentiation, while 13 genes appear to function later, in the formation of myofibers and the organization of sarcomeres. Mutations in four other genes result in muscle-specific degeneration. 103 mutations, defining at least 30 genes, cause no obvious defects in muscle formation and may instead affect neuronal development. Analysis of the behavioral defects suggests that these genes participate in the diverse locomotion patterns observed, such as touch response, rhythmic tail movements, equilibrium control, or that they simply confer general motility to the animal. In some of these mutants specific defects in the developing nervous system are detected. Mutations in two genes, nevermind (nev) and macho (mao), affect axonal projection in the optic tectum, whereas axon formation and elongation of motorneurons are disrupted by mutations in the diwanka (diw) and the unplugged (unp) genes.