Notch signalling by the ligand Delta-like 4 (Dll4) is essential for normal vascular remodelling, yet the precise way in which the pathway influences the behaviour of endothelial cells remains a mystery. Using the embryonic zebrafish, we show that, when Dll4-Notch signalling is defective, endothelial cells continue to migrate and proliferate when they should normally stop these processes. Artificial overactivation of the Notch pathway has opposite consequences. When vascular endothelial growth factor (Vegf) signalling and Dll4-Notch signalling are both blocked, the endothelial cells remain quiescent. Thus, Dll4-Notch signalling acts as an angiogenic `off' switch by making endothelial cells unresponsive to Vegf.

The zebrafish is an important vertebrate model for the mutational analysis of genes effecting developmental processes. Understanding the relationship between zebrafish genes and mutations with those of humans will require understanding the syntenic correspondence between the zebrafish and human genomes. High throughput gene and EST mapping projects in zebrafish are now facilitating this goal. Map positions for 523 zebrafish genes and ESTs with predicted human orthologs reveal extensive contiguous blocks of synteny between the zebrafish and human genomes. Eighty percent of genes and ESTs analyzed belong to conserved synteny groups (two or more genes linked in both zebrafish and human) and 56% of all genes analyzed fall in 118 homology segments (uninterrupted segments containing two or more contiguous genes or ESTs with conserved map order between the zebrafish and human genomes). This work now provides a syntenic relationship to the human genome for the majority of the zebrafish genome.

A fundamental problem in developmental biology concerns how multipotent precursors choose specific fates. Neural crest cells (NCCs) are multipotent, yet the mechanisms driving specific fate choices remain incompletely understood. Sox10 is required for specification of neural cells and melanocytes from NCCs. Like sox10 mutants, zebrafish shady mutants lack iridophores; we have proposed that sox10 and shady are required for iridophore specification from NCCs. We show using diverse approaches that shady encodes zebrafish leukocyte tyrosine kinase (Ltk). Cell transplantation studies show that Ltk acts cell-autonomously within the iridophore lineage. Consistent with this, ltk is expressed in a subset of NCCs, before becoming restricted to the iridophore lineage. Marker analysis reveals a primary defect in iridophore specification in ltk mutants. We saw no evidence for a fate-shift of neural crest cells into other pigment cell fates and some NCCs were subsequently lost by apoptosis. These features are also characteristic of the neural crest cell phenotype in sox10 mutants, leading us to examine iridophores in sox10 mutants. As expected, sox10 mutants largely lacked iridophore markers at late stages. In addition, sox10 mutants unexpectedly showed more ltk-expressing cells than wild-type siblings. These cells remained in a premigratory position and expressed sox10 but not the earliest neural crest markers and may represent multipotent, but partially-restricted, progenitors. In summary, we have discovered a novel signalling pathway in NCC development and demonstrate fate specification of iridophores as the first identified role for Ltk.

SUMMARYThe ventrally expressed secreted polypeptide endothelin1 (Edn1) patterns the skeleton derived from the first two pharyngeal arches into dorsal, intermediate and ventral domains. Edn1 activates expression of many genes, including hand2 and Dlx genes. We wanted to know how hand2/Dlx genes might generate distinct domain identities. Here, we show that differential expression of hand2 and Dlx genes delineates domain boundaries before and during cartilage morphogenesis. Knockdown of the broadly expressed genes dlx1a and dlx2a results in both dorsal and intermediate defects, In zebrafish, dlx3b and dlx5a are redundantly required for patterning specifically within intermediate domain-derived skeleton (Walker et al., 2006). By contrast, zebrafish hand2 nulls exhibit loss of lower jaws, but not upper jaws . hand2 is expressed ventral to nkx3.2, a marker of the jaw joint region . In zebrafish, hand2 mutants, nkx3.2 expands ventrally, indicating that hand2 patterns lower jaw identity in part by repressing jaw joint identity . However, it was unclear whether hand2 represses intermediate domain identity, because hand2 mutants consistently lose jointed-jaw skeleton .Fate-mapping experiments have indicated approximately where skeletal patterning domains arise within early pharyngeal arches Crump et al., 2004;Eberhart et al., 2006). However, these fate maps lacked the precision to directly connect early gene expression patterns to later skeletal shapes. Here, we present expression patterns that allow us to precisely define the dorsal, intermediate and ventral domains within zebrafish pharyngeal arches. We propose that the ventral domain comprises the hand2-expressing pharyngeal arch region, and the skeletal elements that are formed in this region. The ventral domain contains most of Meckel's and ceratohyal cartilages, and the dentary bone. The intermediate domain is the region of pharyngeal arches that expresses all Dlx genes, besides dlx2b (which is not expressed in anterior arches). Expression of the most restricted Dlx gene, dlx4a, reveals the borders of the intermediate domain. The intermediate domain includes the jaw joint region, and the second arch joint region, as well as the opercle and branchiostegal bones. Arch mesenchymal expression of dlx3b and dlx4b is also restricted to the intermediate domain. The dorsal domain is the region of the pharyngeal arch dorsal to dlx4a expression. Because dlx2a is expressed throughout the arch dorsoventral axis, co-labeling of dlx2a and dlx4a reveals the dorsal domain. The dorsal domain contains most of the palatoquadrate cartilage, including the distinctive pterygoid process, the hyomandibular cartilage and the maxillary bone. dlx5a and dlx6a expression does not correspond to a single domain.In addition to defining D-I-V domains, this report examines the functional requirements for D-I-V patterning. We show that along with dlx3b and dlx5a, dlx4b is also redundantly required for intermediate domain skeleton. We report a transgenic revealing the expression pattern of trps1, a gen...

The zebrafish is an excellent genetic system for the study of vertebrate development and disease. In an effort to provide a rapid and robust tool for zebrafish gene mapping, a panel of radiation hybrids (RH) was produced by fusion of irradiated zebrafish AB9 cells with mouse B78 cells. The overall retention of zebrafish sequences in the 93 RH cell lines that constitute the LN54 panel is 22%. Characterization of the LN54 panel with 849 simple sequence length polymorphism markers, 84 cloned genes and 122 expressed sequence tags allowed the production of an RH map whose total size was 11,501 centiRays. From this value, we estimated the average breakpoint frequency of the LN54 RH panel to correspond to 1 centiRay ‫؍‬ 148 kilobase. Placement of a group of 235 unbiased markers on the RH map suggests that the map generated for the LN54 panel, at present, covers 88% of the zebrafish genome. Comparison of marker positions in RH and meiotic maps indicated a 96% concordance. Mapping expressed sequence tags and cloned genes by using the LN54 panel should prove to be a valuable method for the identification of candidate genes for specific mutations in zebrafish.Somatic-cell hybrids and radiation hybrids (RHs) have played a key role in the mapping of human and mouse genes (1-7). Cell hybrids constitute one of the most expedient methods for assigning genes to chromosomes or chromosome segments, because mapping with cell hybrids does not require gene polymorphism. RHs are generated by irradiating cells from a donor species, causing random chromosomal breaks, and fusing these to a cell line from a different species. Donor-cell chromosome fragments are retained to different extents in the ensuing hybrid cells. Typing a panel of RHs with PCR-based sequence-tagged sites creates an RH map in which the frequency of breakpoints between two markers is proportional to the distance between them.The large collection of mutations produced in the zebrafish constitutes a valuable resource for the study of vertebrate developmental mechanisms (8-10). The efficient identification of the genes disrupted by mutation in zebrafish requires dense maps of the genome. Meiotic maps based on rapidamplified polymorphic DNA sequences and microsatellite markers have been produced (11-16). Since localization of cDNAs and expressed sequence tags (ESTs) on meiotic maps requires the identification of polymorphisms, the use of RH mapping is a valuable complementary method suitable for high-throughput cDNA/EST mapping projects to identify candidate genes for available mutants.We have previously shown that stable transfer of zebrafish chromosomes or chromosome segments to a rodent cell line was possible (17). Markers from the simple sequence-length polymorphism (SSLP) meiotic map could be anchored on a panel of zebrafish/mouse somatic-cell hybrids (14). Furthermore, Kwok et al. (18) demonstrated that RH technology could be used for nonmammalian vertebrates. In the present study, we report characterization of LN54, a zebrafish RH panel composed of 93 cell...