The Drosophila retina is patterned by a morphogenetic wave driven by the Hedgehog signaling protein. Hedgehog, secreted by the first neurons, induces neuronal differentiation and hedgehog expression in nearby uncommitted cells, thereby propagating the wave. Evidence is presented here that the zebrafish Hedgehog homolog, Sonic Hedgehog, is also expressed in the first retinal neurons, and that Sonic Hedgehog drives a wave of neurogenesis across the retina, strikingly similar to the wave in Drosophila. The conservation of this patterning mechanism is unexpected, given the highly divergent structures of vertebrate and invertebrate eyes, and supports a common evolutionary origin of the animal visual system.

During organogenesis, the foregut endoderm gives rise to the many different cell types that comprise the hepatopancreatic system, including hepatic, pancreatic and gallbladder cells, as well as the epithelial cells of the hepatopancreatic ductal system that connects these organs together and with the intestine. However, the mechanisms responsible for demarcating ducts versus organs are poorly understood. Here, we show that Fgf10 signaling from the adjacent mesenchyme is responsible for refining the boundaries between the hepatopancreatic duct and organs. In zebrafish fgf10 mutants, the hepatopancreatic ductal epithelium is severely dysmorphic, and cells of the hepatopancreatic ductal system and adjacent intestine misdifferentiate toward hepatic and pancreatic fates. Furthermore, Fgf10 also functions to prevent the differentiation of the proximal pancreas and liver into hepatic and pancreatic cells, respectively. These data shed light onto how the multipotent cells of the foregut endoderm, and subsequently those of the hepatopancreatic duct, are directed toward different organ fates.

The development of vertebrate limb buds is triggered in the lateral plate mesoderm by a cascade of genes, including members of the Fgf and Wnt families, as well as the transcription factor tbx5. Fgf8, which is expressed in the intermediate mesoderm, is thought to initiate forelimb formation by activating wnt2b, which then induces the expression of tbx5 in the adjacent lateral plate mesoderm. Tbx5, in turn, is required for the activation of fgf10, which relays the limb inducing signal to the overlying ectoderm. We show that the zebrafish fgf24 gene, which belongs to the Fgf8/17/18 subfamily of Fgf ligands, acts downstream of tbx5 to activate fgf10 expression in the lateral plate mesoderm. We also show that fgf24 activity is necessary for the migration of tbx5-expressing cells to the fin bud, and for the activation of shh, but not hand2, expression in the posterior fin bud.

SummarySonic hedgehog, secreted by amacrine cells, acts as a short-range signal to direct differentiation and lamination in the zebrafish retina

No abstract

Recent large-scale mutagenesis screens have made the zebrafish the first vertebrate organism to allow a forward genetic approach to the discovery of developmental control genes. Mutations can be cloned positionally, or placed on a simple sequence length polymorphism (SSLP) map to match them with mapped candidate genes and expressed sequence tags (ESTs). To facilitate the mapping of candidate genes and to increase the density of markers available for positional cloning, we have created a radiation hybrid (RH) map of the zebrafish genome. This technique is based on somatic cell hybrid lines produced by fusion of lethally irradiated cells of the species of interest with a rodent cell line. Random fragments of the donor chromosomes are integrated into recipient chromosomes or retained as separate minichromosomes. The radiation-induced breakpoints can be used for mapping in a manner analogous to genetic mapping, but at higher resolution and without a need for polymorphism. Genome-wide maps exist for the human, based on three RH panels of different resolutions, as well as for the dog, rat and mouse. For our map of the zebrafish genome, we used an existing RH panel and 1,451 sequence tagged site (STS) markers, including SSLPs, cloned candidate genes and ESTs. Of these, 1,275 (87.9%) have significant linkage to at least one other marker. The fraction of ESTs with significant linkage, which can be used as an estimate of map coverage, is 81.9%. We found the average marker retention frequency to be 18.4%. One cR3000 is equivalent to 61 kb, resulting in a potential resolution of approximately 350 kb.

There were mistakes in the Materials and methods 'Treatment with DEAB, SU5402' section on p. 2806 of this paper. The whole section should therefore be replaced with the following.4-Diethylaminobenzaldehyde (DEAB) (Fluka) and the FGF receptor inhibitor SU5402 (Calbiochem) were dissolved in dimethylsulfoxide (DMSO) and used at a concentration of 10 M and 16 M, respectively. DEAB treatment was performed from 30% epiboly onwards.Incubations were carried out in the dark at 28°C. The authors apologise to readers for the mistakes.

Clock output pathways play a pivotal role by relaying timing information from the circadian clock to a diversity of physiological systems. Both cell-autonomous and systemic mechanisms have been implicated as clock outputs; however, the relative importance and interplay between these mechanisms are poorly understood. The cell cycle represents a highly conserved regulatory target of the circadian timing system. Previously, we have demonstrated that in zebrafish, the circadian clock has the capacity to generate daily rhythms of S phase by a cell-autonomous mechanism in vitro. Here, by studying a panel of zebrafish mutants, we reveal that the pituitary–adrenal axis also plays an essential role in establishing these rhythms in the whole animal. Mutants with a reduction or a complete absence of corticotrope pituitary cells show attenuated cell-proliferation rhythms, whereas expression of circadian clock genes is not affected. We show that the corticotrope deficiency is associated with reduced cortisol levels, implicating glucocorticoids as a component of a systemic signaling pathway required for circadian cell cycle rhythmicity. Strikingly, high-amplitude rhythms can be rescued by exposing mutant larvae to a tonic concentration of a glucocorticoid agonist. Our work suggests that cell-autonomous clock mechanisms are not sufficient to establish circadian cell cycle rhythms at the whole-animal level. Instead, they act in concert with a systemic signaling environment of which glucocorticoids are an essential part.