Zebrafish pea3 and erm are general targets of FGF8 signaling.

Phenotypic analysis of both zebrafish and mouse has shown that fibroblast growth factor 8 (FGF8) is required for many developmental decisions. To further our understanding of the FGF8 signaling process, we sought to identify new transcriptional targets of the pathway. Here, we propose that two zebrafish ETS genes, pea3 and erm, are general targets of FGF8 signaling, based upon the following observations: both genes are expressed around all early FGF8 signaling sources, both genes are downregulated in fgf8 mutant embryos in all tissues known to require fgf8 function, a pharmacological inhibitor of the FGF pathway completely abolishes expression of both genes, and ectopic expression of fgf8 is sufficient to induce both genes. The finding that pea3 and erm are common transcriptional targets of FGF8 signaling suggests that they are general mediators of FGF8 signaling during development. In addition, we observed that pea3 is often expressed close to an FGF8 source, and erm is expressed in a broader domain. To test whether this differential expression is established by FGF8, we have induced FGF8 ectopically and show that it is sufficient to recapitulate the endogenous nested expression pattern of pea3 and erm.

The endoderm plays an important role in patterning the segmented pharyngeal region in zebrafish (Danio rerio).

The development of the vertebrate head is a highly complex process involving tissues derived from all three germ layers. The endoderm forms pharyngeal pouches, the paraxial mesoderm gives rise to endothelia and muscles, and the neural crest cells, which originate from the embryonic midbrain and hindbrain, migrate ventrally to form cartilage, connective tissue, sensory neurons, and pigment cells. All three tissues form segmental structures: the hindbrain compartmentalizes into rhombomeres, the mesoderm into somitomeres, and the endoderm into serial gill slits. It is not known whether the different segmented tissues in the head develop by the same molecular mechanism or whether different pathways are employed. It is also possible that one tissue imposes segmentation on the others. Most recent studies have emphasized the importance of neural crest cells in patterning the head. Neural crest cells colonize the segmentally arranged arches according to their original position in the brain and convey positional information from the hindbrain into the periphery. During the screen for mutations that affect embryonic development of zebrafish, one mutant, called van gogh (vgo), in which segmentation of the pharyngeal region is absent, was isolated. In vgo, even though hindbrain segmentation is unaffected, the pharyngeal endoderm does not form reiterated pouches and surrounding mesoderm is not patterned correctly. Accordingly, migrating neural crest cells initially form distinct streams but fuse when they reach the arches. This failure to populate distinct pharyngeal arches is likely due to the lack of pharyngeal pouches. The results of our analysis suggest that the segmentation of the endoderm occurs without signaling from neural crest cells but that tissue interactions between the mesendoderm and the neural crest cells are required for the segmental appearance of the neural crest-derived cartilages in the pharyngeal arches. The lack of distinct patches of neural crest cells in the pharyngeal region is also seen in mutants of one-eyed pinhead and casanova, which are characterized by a lack of endoderm, as well as defects in mesodermal structures, providing evidence for the important role of the endoderm and mesoderm in governing head segmentation.

Control of her1 expression during zebrafish somitogenesis by a delta-dependent oscillator and an independent wave-front activity.

Somitogenesis has been linked both to a molecular clock that controls the oscillation of gene expression in the presomitic mesoderm (PSM) and to Notch pathway signaling. The oscillator, or clock, is thought to create a prepattern of stripes of gene expression that regulates the activity of the Notch pathway that subsequently directs somite border formation. Here, we report that the zebrafish gene after eight (aei) that is required for both somitogenesis and neurogenesis encodes the Notch ligand DeltaD. Additional analysis revealed that stripes of her1 expression oscillate within the PSM and that aei/DeltaD signaling is required for this oscillation. aei/DeltaD expression does not oscillate, indicating that the activity of the Notch pathway upstream of her1 may function within the oscillator itself. Moreover, we found that her1 stripes are expressed in the anlage of consecutive somites, indicating that its expression pattern is not pair-rule. Analysis of her1 expression in aei/DeltaD, fused somites (fss), and aei;fss embryos uncovered a wave-front activity that is capable of continually inducing her1 expression de novo in the anterior PSM in the absence of the oscillation of her1. The wave-front activity, in reference to the clock and wave-front model, is defined as such because it interacts with the oscillator-derived pattern in the anterior PSM and is required for somite morphogenesis. This wave-front activity is blocked in embryos mutant for fss but not aei/DeltaD. Thus, our analysis indicates that the smooth sequence of formation, refinement, and fading of her1 stripes in the PSM is governed by two separate activities.

The Drosophila SHC adaptor protein is required for signaling by a subset of receptor tyrosine kinases.

Receptor tyrosine kinases (RTKs) transduce signals via cytoplasmic adaptor proteins to downstream signaling components. We have identified loss-of-function mutations in dshc, the Drosophila homolog of the mammalian adaptor protein SHC. A point mutation in the phosphotyrosine binding (PTB) domain completely abolishes DSHC function and provides in vivo evidence for the function of PTB domains. Unlike other adaptor proteins, DSHC is involved in signaling by only a subset of RTKs: dshc mutants show defects in Torso and DER but not Sevenless signaling, which is confirmed by epistasis experiments. We show by double-mutant analysis that the adaptors DOS, DRK, and DSHC act in parallel to transduce the Torso signal. Our results suggest that DSHC confers specificity to receptor signaling.

Zebrafish vasa RNA but not its protein is a component of the germ plasm and segregates asymmetrically before germline specification.

Work in different organisms revealed that the vasa gene product is essential for germline specification. Here, we describe the asymmetric segregation of zebrafish vasa RNA, which distinguishes germ cell precursors from somatic cells in cleavage stage embryos. At the late blastula (sphere) stage, vasa mRNA segregation changes from asymmetric to symmetric, a process that precedes primordial germ cell proliferation and perinuclear localization of Vasa protein. Analysis of hybrid fish between Danio rerio and Danio feegradei demonstrates that zygotic vasa transcription is initiated shortly after the loss of unequal vasa mRNA segregation. Blocking DNA replication indicates that the change in vasa RNA segregation is dependent on a maternal program. Asymmetric segregation is impaired in embryos mutant for the maternal effect gene nebel. Furthermore, ultrastructural analysis of vasa RNA particles reveals that vasa RNA, but not Vasa protein, localizes to a subcellular structure that resembles nuage, a germ plasm organelle. The structure is initially associated with the actin cortex, and subsequent aggregation is inhibited by actin depolymerization. Later, the structure is found in close proximity of microtubules. We previously showed that its translocation to the distal furrows is microtubule dependent. We propose that vasa RNA but not Vasa protein is a component of the zebrafish germ plasm. Triggered by maternal signals, the pattern of germ plasm segregation changes, which results in the expression of primordial germ cell-specific genes such as vasa and, consequently, in germline fate commitment.

The molecular motor dynein is involved in targeting swallow and bicoid RNA to the anterior pole of Drosophila oocytes.

Localization of bicoid (bcd) messenger RNA to the anterior pole of the Drosophila oocyte requires the exuperantia ( exu), swallow (swa) and staufen (stau) genes. We show here that Swa protein transiently co-localizes with bcd RNA in mid-oogenesis. Swa also localizes to the anterior pole of the oocyte in the absence of bcd RNA. This localization does not require Exu, but depends on intact microtubules. In mutant ovaries with duplicated polarity of microtubules, Swa and bcd RNA are ectopically localized at the posterior pole, as well as being present at the anterior pole. We identify dynein light chain-1 (Ddlc-1), a component of the minus-end-directed microtubule motor cytoplasmic dynein, as a Swa-binding protein. We propose that Swa acts as an adaptor for the dynein complex and thereby enables dynein to transport bcd RNA along microtubules to their minus ends at the anterior pole of the oocyte.

Two distinct cell populations in the floor plate of the zebrafish are induced by different pathways.

The floor plate is a morphologically distinct structure of epithelial cells situated along the midline of the ventral spinal cord in vertebrates. It is a source of guidance molecules directing the growth of axons along and across the midline of the neural tube. In the zebrafish, the floor plate is about three cells wide and composed of cuboidal cells. Two cell populations can be distinguished by the expression patterns of several marker genes, including sonic hedgehog (shh) and the fork head-domain gene fkd4: a single row of medial floor plate (MFP) cells, expressing both shh and fkd4, is flanked by rows of lateral floor plate (LFP) cells that express fkd4 but not shh. Systematic mutant searches in zebrafish embryos have identified a number of genes, mutations in which visibly reduce the floor plate. In these mutants either the MFP or the LFP cells are absent, as revealed by the analysis of the shh and fkd4 expression patterns. MFP cells are absent, but LFP cells are present, in mutants of cyclops, one-eyed pinhead, and schmalspur, whose development of midline structures is affected. LFP cells are absent, but MFP cells are present, in mutants of four genes, sonic you, you, you-too, and chameleon, collectively called the you-type genes. This group of mutants also shows defects in patterning of the paraxial mesoderm, causing U- instead of V-shaped somites. One of the you-type genes, sonic you, was recently shown to encode the zebrafish Shh protein, suggesting that the you-type genes encode components of the Shh signaling pathway. It has been shown previously that in the zebrafish shh is required for the induction of LFP cells, but not for the development of MFP cells. This conclusion is supported by the finding that injection of shh RNA causes an increase in the number of LFP, but not MFP cells. Embryos mutant for iguana, detour, and umleitung share the lack of LFP cells with you-type mutants while somite patterning is not severely affected. In mutants that fail to develop a notochord, MFP cells may be present, but are always surrounded by LFP cells. These data indicate that shh, expressed in the notochord and/or the MFP cells, induces the formation of LFP cells. In embryos doubly mutant for cyclops (cyc) and sonic you (syu) both LFP and MFP cells are deleted. The number of primary motor neurons is strongly reduced in cyc;syu double mutants, while almost normal in single mutants, suggesting that the two different pathways have overlapping functions in the induction of primary motor neurons.

Somitogenesis in zebrafish.

Both genetic and embryological studies in the zebrafish, Danio rerio, have contributed to our general understanding of how somites form and differentiate. In the zebrafish, mutants have been isolated that have specific effects on virtually every aspect of somite development. The fss-type mutants, defining 5 genes, affect somite segmentation and epithelialization. The you-type mutants, comprising 7 genes, and mutants in another 13 genes defective in notochord formation, have somites with abnormal pattern and morphology. Eighteen genes have been identified that are required for the differentiation and maintenance of the somitic musculature, and 2 genes have been identified that are involved in the development of motoneurons that innervate the somitic musculature. The true utility of the zebrafish lies in the ability to combine genetic analysis with embryological experimentation. Such analysis of somite segmentation suggests that homologues of both the Drosophila pair-rule and segment polarity genes, her1 and Sonic hedge-hog, respectively, are involved generating periodicity during somitogenesis. The Sonic hedge-hog protein secreted from the notochord also induces the formation of specific muscle types including the slow muscle fibers which are initially induced in the medial somite and undergo a series of morphological transitions including migration through the somite to the lateral surface where they complete their differentiation. The role of the notochord in patterning the somite is also demonstrated by its involvement in regulating the permissiveness of the somite to the extension of axons of primary motoneurons.

A mutation in the zebrafish maternal-effect gene nebel affects furrow formation and vasa RNA localization.

BACKGROUND: In many animals, embryonic patterning depends on a careful interplay between cell division and the segregation of localized cellular components. Both of these processes in turn rely on cytoskeletal elements and motor proteins. A type of localized cellular component found in most animals is the germ plasm, a specialized region of cytoplasm that specifies the germ-cell fate. The gene vasa has been shown in Drosophila to encode an essential component of the germ plasm and is thought to have a similar function in other organisms. In the zebrafish embryo, the vasa RNA is localized to the furrows of the early cellular divisions. RESULTS: We identified the gene nebel in a pilot screen for zebrafish maternal-effect mutations. Embryos from females homozygous for a mutation in nebel exhibit defects in cell adhesion. Our analysis provides genetic evidence for a function of the microtubule array that normally develops at the furrow in the deposition of adhesive membrane at the cleavage plane. In addition, nebel mutant embryos show defects in the early localization of vasa RNA. The vasa RNA localization phenotype could be mimicked with microtubule-inhibiting drugs, and confocal microscopy suggests an interaction between microtubules and vasa-RNA-containing aggregates. CONCLUSIONS: Our data support two functions for the microtubule reorganization at the furrow, one for the exocytosis of adhesive membrane, and another for the translocation of vasa RNA along the forming furrow.

Transient establishment of anteroposterior polarity in the zebrafish pectoral fin bud in the absence of sonic hedgehog activity.

Sonic hedgehog (Shh) is expressed in the posterior vertebrate limb bud mesenchyme and directs anteroposterior patterning and growth during limb development. Here we report an analysis of the pectoral fin phenotype of zebrafish sonic you mutants, which disrupt the shh gene. We show that Shh is required for the establishment of some aspects of anteroposterior polarity, while other aspects of anteroposterior polarity are established independently of Shh, and only later come to depend on Shh for their maintenance. We also demonstrate that Shh is required for the activation of posterior HoxD genes by retinoic acid. Finally, we show that Shh is required for normal development of the apical ectodermal fold, for growth of the fin bud, and for formation of the fin endoskeleton.

A radiation hybrid map of the zebrafish genome.

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.

Oligomerisation of Tube and Pelle leads to nuclear localisation of dorsal.

In the Drosophila embryo the nuclear localisation of Dorsal, a member of the Rel family, is regulated by an extracellular signal, which is transmitted to the interior of the egg cell by a cascade of proteins involving the novel protein Tube and the protein kinase Pelle. Here we analyse the activation mechanism of Tube and Pelle and the interaction between these two components. We show that both proteins, although having different biochemical activities, are activated by the same mechanism. Membrane association alone is not sufficient, but oligomerisation is required for full activation of Tube and Pelle. By deletion analysis we determined the domains of Tube and Pelle mediating the physical interaction and the signalling to downstream components. In order to investigate the link between Pelle and the target of the signalling cascade, the Dorsal/Cactus complex, we isolated and characterised the novel, but evolutionary conserved protein Pellino, which associates with the kinase domain of Pelle.

Origin and development of the zebrafish endoderm.

The segregation of cells into germ layers is one of the earliest events in the establishment of cell fate in the embryo. In the zebrafish, endoderm and mesoderm are derived from cells that involute into an internal layer, the hypoblast, whereas ectoderm is derived from cells that remain in the outer layer, the epiblast. In this study, we examine the origin of the zebrafish endoderm and its separation from the mesoderm. By labeling individual cells located at the margin of the blastula, we demonstrate that all structures that are endodermal in origin are derived predominantly from the more dorsal and lateral cells of the blastoderm margin. Frequently marginal cells give rise to both endodermal and mesodermal derivatives, demonstrating that these two lineages have not yet separated. Cells located farther than 4 cell diameters from the margin give rise exclusively to mesoderm, and not to endoderm. Following involution, we see a variety of cellular changes indicating the differentiation of the two germ layers. Endodermal cells gradually flatten and extend filopodial processes forming a noncontiguous inner layer of cells against the yolk. At this time, they also begin to express Forkhead-domain 2 protein. Mesodermal cells form a coherent layer of round cells separating the endoderm and ectoderm. In cyclops-mutant embryos that have reduced mesodermal anlage, we demonstrate that by late gastrulation not only mesodermal but also endodermal cells are fewer in number. This suggests that a common pathway initially specifies germ layers together before a progressive sequence of determinative events segregate endoderm and mesoderm into morphologically distinct germ layers.

Zebrafish touch-insensitive mutants reveal an essential role for the developmental regulation of sodium current.

Developmental changes in neuronal connectivity and membrane properties underlie the stage-specific appearance of embryonic behaviors. The behavioral response of embryonic zebrafish to tactile stimulation first appears at 27 hr postfertilization. Because the touch response requires the activation of mechanosensory Rohon-Beard neurons, we have used whole-cell recordings in semi-intact preparations to characterize Rohon-Beard cell electrical membrane properties in several touch-insensitive mutants and then to correlate the development of excitability in these cells with changes in wild-type behavior. Electrophysiological analysis of mechanosensory neurons of touch-insensitive zebrafish mutants indicates that in three mutant lines that have been examined the sodium current amplitudes are reduced, and action potentials either have diminished overshoots or are not generated. In macho mutants the action potential never overshoots, and the sodium current remains small; alligator and steifftier show similar but weaker effects. The effects are specific to sodium channel function; resting membrane potentials are unaffected, and outward currents of normal amplitude are present. Developmental analysis of sodium current expression in mechanosensory neurons of wild-type embryos indicates that, during the transition from a touch-insensitive to a touch-sensitive embryo, action potentials acquire larger overshoots and briefer durations as both sodium and potassium currents increase in amplitude. However, in macho touch-insensitive mutants, developmental changes in action potential overshoot and sodium current are absent despite the normal regulation of action potential duration and potassium current. Thus, the maturation of a voltage-dependent sodium current promotes a behavioral response to touch. A study of these mutants will allow insight into the genes controlling the maturation of the affected sodium current.

spadetail-dependent cell compaction of the dorsal zebrafish blastula.

The dorsal marginal zone of the zebrafish blastula, equivalent to the amphibian Spemann organizer, is destined to become the tissues of the notochord and prechordal plate. Preceding gastrulation in the zebrafish, we find that these future mesendodermal cells acquire a cohesive cell behavior characterized by flattening and maximization of intercellular contacts, somewhat resembling cell compaction in mouse blastocysts. This behavior may suppress cell intermingling. Surprisingly, this blastula cell compaction requires normal function of spadetail, a gene known to be necessary for the dorsal convergent cell movement of paraxial mesoderm later in the gastrula. We propose that spadetail-dependent cell compaction subtly controls the early mixing and dispersal of dorsal cells that coalesce into the prospective organizer region. This early process may be necessary for the correct location of the boundary separating axial and paraxial cells.

Zebrafish segmentation and pair-rule patterning.

Segmentation in the vertebrate embryo is evident within the paraxial mesoderm in the form of somites, which are repeated structures that give rise to the vertebrae and muscle of the trunk and tail. In the zebrafish, our genetic screen identified two groups of mutants that affect somite formation and pattern. Mutations of one class, the fss-type mutants, disrupt the formation of the anterior-posterior somite boundaries during somitogenesis. However, segmentation within the paraxial mesoderm is not completely eliminated in these mutants. Irregular somite boundaries form later during embryogenesis and, strikingly, the vertebrae are not fused. Here, we show that formation of the irregular somite boundaries in these mutants is dependent upon the activity of a second group of genes, the you-type genes, which include sonic you, the zebrafish homologue of the Drosophila segment polarity gene, sonic hedgehog. Further to characterize the defects caused by the fss-type mutations, we examined their effects on the expression of her1, a zebrafish homologue of the Drosophila pair-rule gene hairy. In wild-type embryos, her1 is expressed in a dynamic, repeating pattern, remarkably similar to that of its Drosophila and Tribolium counterparts, suggesting that a pair-rule mechanism also functions in the segmentation of the vertebrate paraxial mesoderm. We have found that the fss-type mutants have abnormal pair-rule patterning. Although a her1 mutant could not be identified, analysis of a double mutant that abolishes most her1 expression suggests that a her1 mutant may not display a pair-rule phenotype analogous to the hairy phenotype observed in Drosophila. Cumulatively, our data indicate that zebrafish homologues of both the Drosophila segment polarity genes and pair-rule genes are involved in segmenting the paraxial mesoderm. However, both the relationship between these two groups of genes within the genetic heirarchy governing segmentation and the precise roles that they play during segmentation likely differ significantly between the two organisms.

fork head domain genes in zebrafish.

Nine members of the fork head domain gene family (fkd1-fkd9) were isolated from early cDNA libraries in the zebrafish. They show unique expression patterns in whole-mount RNA in situ hybridization during the first 24 h of embryonic development. These fkd genes fall into three of ten classes, based on sequence similarities within the DNA-binding domain, whereas members for the other seven classes described in other vertebrates were not found. In addition to conserved residues at certain positions in the fork head domain, characteristic transcription activation domains as well as similarities in expression patterns were found for members of the different classes. Members of class I (fkd1/axial, fkd2/Zffkh1, fkd4 and fkd7) are differentially transcribed in unsegmented dorsal axial structures such as the floor plate, the notochord, the hypochord and, in addition, the endoderm. Transcripts of fkd3 and fkd5 (class II) are mainly detected in the cells of the ectoderm which form neural tissues, as is the case for genes of this class in other species. RNAs of the three members of class V (fkd6, fkd8 and fkd9) are expressed in the paraxial mesoderm and transiently in the neuroectoderm. fkd6 is strongly expressed in neural crest cells from early stages on, whereas fkd2 and fkd7 are transcribed in individual neural crest cells in the pharyngula period.

Genetic analysis of vertebrate sensory hair cell mechanosensation: the zebrafish circler mutants.

The molecular basis of sensory hair cell mechanotransduction is largely unknown. In order to identify genes that are essential for mechanosensory hair cell function, we characterized a group of recently isolated zebrafish motility mutants. These mutants are defective in balance and swim in circles but have no obvious morphological defects. We examined the mutants using calcium imaging of acoustic-vibrational and tactile escape responses, high resolution microscopy of sensory neuroepithelia in live larvae, and recordings of extracellular hair cell potentials (microphonics). Based on the analyses, we have identified several classes of genes. Mutations in sputnik and mariner affect hair bundle integrity. Mutant astronaut and cosmonaut hair cells have relatively normal microphonics and thus appear to affect events downstream of mechanotransduction. Mutant orbiter, mercury, and gemini larvae have normal hair cell morphology and yet do not respond to acoustic-vibrational stimuli. The microphonics of lateral line hair cells of orbiter, mercury, and gemini larvae are absent or strongly reduced. Therefore, these genes may encode components of the transduction apparatus.

A sponge-like structure involved in the association and transport of maternal products during Drosophila oogenesis.

Localization of maternally provided RNAs during oogenesis is required for formation of the antero-posterior axis of the Drosophila embryo. Here we describe a subcellular structure in nurse cells and oocytes which may function as an intracellular compartment for assembly and transport of maternal products involved in RNA localization. This structure, which we have termed "sponge body," consists of ER-like cisternae, embedded in an amorphous electron-dense mass. It lacks a surrounding membrane and is frequently associated with mitochondria. The sponge bodies are not identical to the Golgi complexes. We suggest that the sponge bodies are homologous to the mitochondrial cloud in Xenopus oocytes, a granulo-fibrillar structure that contains RNAs involved in patterning of the embryo. Exuperantia protein, the earliest factor known to be required for the localization of bicoid mRNA to the anterior pole of the Drosophila oocyte, is highly enriched in the sponge bodies but not an essential structural component of these. RNA staining indicates that sponge bodies contain RNA. However, neither the intensity of this staining nor the accumulation of Exuperantia in the sponge bodies is dependent on the amount of bicoid mRNA present in the ovaries. Sponge bodies surround nuage, a possible polar granule precursor. Microtubules and microfilaments are not present in sponge bodies, although transport of the sponge bodies through the cells is implied by their presence in cytoplasmic bridges. We propose that the sponge bodies are structures that, by assembly and transport of included molecules or associated structures, are involved in localization of mRNAs in Drosophila oocytes.

Left-right pattern of cardiac BMP4 may drive asymmetry of the heart in zebrafish.

The first evident break in left-right symmetry of the primitive zebrafish heart tube is the shift in pattern of BMP4 expression from radially symmetric to left-predominant. The midline heart tube then 'jogs' to the left and subsequently loops to the right. We examined 279 mutations, affecting more than 200 genes, and found 21 mutations that perturb this process. Some cause BMP4 to remain radially symmetric. Others randomize the asymmetric BMP4 pattern. Retention of BMP4 symmetry is associated with failure to jog: right-predominance of the BMP4 pattern is associated with reversal of the direction of jogging and looping. Raising BMP4 diffusely throughout the heart, via sonic hedgehog injection, or the blocking of its action by injection of a dominant negative BMP4 receptor, prevent directional jogging or looping. The genes crucial to directing cardiac asymmetry include a subset of those needed for patterning the dorsoventral axis and for notochord and ventral spinal cord development. Thus, the pattern of cardiac BMP4 appears to be in the pathway by which the heart interprets lateralizing signals from the midline.