Epiboly, the spreading of the blastoderm over the large yolk cell, is the first morphogenetic movement of the teleost embryo. Examining this movement as a paradigm of vertebrate morphogenesis, we have focused on the epiboly arrest mutant half baked (hab), which segregates as a recessive lethal, including alleles expressing zygotic-maternal dominant (ZMD) effects. Here we show that hab is a mutation in the zebrafish homolog of the adhesion protein E-cadherin. Whereas exclusively recessive alleles of hab produce truncated proteins, dominant alleles all contain transversions in highly conserved amino acids of the extracellular domains, suggesting these alleles produce dominant-negative effects. Antisense oligonucleotides that create specific splicing defects in the hab mRNA phenocopy the recessive phenotypes and, surprisingly, some of the ZMD phenotypes as well. In situ analyses show that during late epiboly hab is expressed in a radial gradient in the non axial epiblast, from high concentrations in the exterior layer of the epiblast to low concentrations in the interior layer of the epiblast. During epiboly, using an asymmetric variant of radial intercalation, epiblast cells from the interior layer sequentially move into the exterior layer and become restricted to that layer; there they participate in subtle cell shape changes that further expand the blastoderm. In hab mutants, when cells intercalate into the exterior layer, they tend to neither change cell shape nor become restricted, and many of these cells 'de-intercalate' and move back into the interior layer. Cell transplantation showed all these defects to be cell-autonomous. Hence, as for the expansion of the mammalian trophoblast at a similar developmental stage, hab/E-cadherin is necessary for the cell rearrangements that spread the teleost blastoderm over the yolk.

In zebrafish, as in Xenopus, the well-orchestrated cell movements of gastrulation can be dissected into several components, including epiboly, involution, convergence and extension. Embryos homozygous for the recessive lethal mutation spt-1(b104) or 'spadetail' have a complex set of defects in the trunk of the embryo that may arise secondarily after loss of one of these movements, convergence, from those precursors that would normally have given rise to trunk somitic mesoderm. We have now tested this hypothesis by transplanting cells between wild-type and mutant embryos, to identify the cells that spt-1 affects directly. Our results show that the mutation autonomously affects only those mesodermal precursors located along the lateral margin of the early gastrula blastoderm. Other mesodermal cells and all ectodermal precursors seem not to require function of the wild-type gene. Our findings reveal an unexpectedly delicate genetic control of vertebrate gastrulation.

No abstract

Mutations causing a visible phenotype in the adult serve as valuable visible genetic markers in multicellular genetic model organisms such as Drosophila melanogaster, Caenorhabditis elegans and Arabidopsis thaliana. In a large scale screen for mutations affecting early development of the zebrafish, we identified a number of mutations that are homozygous viable or semiviable. Here we describe viable mutations which produce visible phenotypes in the adult fish. These predominantly affect the fins and pigmentation, but also the eyes and body length of the adult. A number of dominant mutations caused visible phenotypes in the adult fish. Mutations in three genes, long fin, another long fin and wanda affected fin formation in the adult. Four mutations were found to cause a dominant reduction of the overall body length in the adult. The adult pigment pattern was found to be changed by dominant mutations in wanda, asterix, obelix, leopard, salz and pfeffer. Among the recessive mutations producing visible phenotypes in the homozygous adult, a group of mutations that failed to produce melanin was assayed for tyrosinase activity. Mutations in sandy produced embryos that failed to express tyrosinase activity. These are potentially useful for using tyrosinase as a marker for the generation of transgenic lines of zebrafish.

The study of developmental patterning has been facilitated by the availability of mutations that produce changes in cell fate, in animals such as Caenorhabditis elegans and Drosophila melanogaster. We now describe a zygotic lethal mutation in the zebrafish, Brachydanio rerio, that also changes how particular embryonic cells develop. Severe pattern deficiencies are observed that are restricted to a single body region, the trunk. The mutation may directly affect mesoderm, as somites do not form in the trunk. Head and tail structures, including tail somites, are relatively undisturbed. The earliest detected expression of the mutation is during gastrulation, when movements of mesodermal cells occur incorrectly. We injected prospective trunk mesodermal cells with lineage tracer dye and observed that in mutants these cells may enter a new body region, the tail, and there may express a new fate appropriate for the changed position.

Three of the class I mutants show a change in the pattern of gene expression in the anlage of a brain structure prior to the onset of degeneration. These results suggest that focal cell death may be a useful clue for the detection of early patterning defects of the vertebrate nervous system in regions devoid of visible landmarks.

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.

Vertebrate hematopoiesis first produces primitive (embryonic) lineages and ultimately generates the definitive (adult) blood. While definitive hematopoiesis may produce many diverse blood types via a common multipotent progenitor, primitive hematopoiesis has been thought to produce only erythrocytes or macrophages via progenitors that are unipotent for single blood lineages. Using a variety of in vivo celltracing techniques, we show that primitive blood in zebrafish derives from two different progenitor types. On the dorsal gastrula, blood progenitors are unipotential cells that divide infrequently, populate the rostral blood islands (RBI), and differentiate into macrophages. In contrast, on the ventral gastrula, blood progenitors are multipotential cells with rapid cell cycles, populate the intermediate cell mass (ICM), and differentiate into erythrocytes, neutrophils, and thrombocytes. Our results demonstrate the existence of primitive hematopoietic progenitors that are segregated very early in development and that are specified to produce either a unipotent or a multipotent blood cell lineage.