Zebrafish have become a popular organism for the study of vertebrate gene function1,2. The virtually transparent embryos of this species, and the ability to accelerate genetic studies by gene knockdown or overexpression, have led to the widespread use of zebrafish in the detailed investigation of vertebrate gene function and increasingly, the study of human genetic disease3–5. However, for effective modelling of human genetic disease it is important to understand the extent to which zebrafish genes and gene structures are related to orthologous human genes. To examine this, we generated a high-quality sequence assembly of the zebrafish genome, made up of an overlapping set of completely sequenced large-insert clones that were ordered and oriented using a high-resolution high-density meiotic map. Detailed automatic and manual annotation provides evidence of more than 26,000 protein-coding genes6, the largest gene set of any vertebrate so far sequenced. Comparison to the human reference genome shows that approximately 70% of human genes have at least one obvious zebrafish orthologue. In addition, the high quality of this genome assembly provides a clearer understanding of key genomic features such as a unique repeat content, a scarcity of pseudogenes, an enrichment of zebrafish-specific genes on chromosome 4 and chromosomal regions that influence sex determination.

Vertebrate gastrulation involves the specification and coordinated movement of large populations of cells that give rise to the ectodermal, mesodermal and endodermal germ layers. Although many of the genes involved in the specification of cell identity during this process have been identified, little is known of the genes that coordinate cell movement. Here we show that the zebrafish silberblick (slb) locus encodes Wnt11 and that Slb/Wnt11 activity is required for cells to undergo correct convergent extension movements during gastrulation. In the absence of Slb/Wnt11 function, abnormal extension of axial tissue results in cyclopia and other midline defects in the head. The requirement for Slb/Wnt11 is cell non-autonomous, and our results indicate that the correct extension of axial tissue is at least partly dependent on medio-lateral cell intercalation in paraxial tissue. We also show that the slb phenotype is rescued by a truncated form of Dishevelled that does not signal through the canonical Wnt pathway, suggesting that, as in flies, Wnt signalling might mediate morphogenetic events through a divergent signal transduction cascade. Our results provide genetic and experimental evidence that Wnt activity in lateral tissues has a crucial role in driving the convergent extension movements underlying vertebrate gastrulation.

Germ cells preserve an individual's genetic information and transmit it to the next generation. Early in development germ cells are set aside and undergo a specialized developmental programme, a hallmark of which is the migration from their site of origin to the future gonad. In Drosophila, several factors have been identified that control germ-cell migration to their target tissues; however, the germ-cell chemoattractant or its receptor have remained unknown. Here we apply genetics and in vivo imaging to show that odysseus, a zebrafish homologue of the G-protein-coupled chemokine receptor Cxcr4, is required specifically in germ cells for their chemotaxis. odysseus mutant germ cells are able to activate the migratory programme, but fail to undergo directed migration towards their target tissue, resulting in randomly dispersed germ cells. SDF-1, the presumptive cognate ligand for Cxcr4, shows a similar loss-of-function phenotype and can recruit germ cells to ectopic sites in the embryo, thus identifying a vertebrate ligand-receptor pair guiding migratory germ cells at all stages of migration towards their target.

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