Publications in 1996

Baier, H., Klostermann, S., Trowe, T., Karlstrom, R. O., Nüsslein-Volhard, C. and Bonhoeffer, F. (1996). Genetic dissection of the retinotectal projection. Development 123, 415-25.

A systematic search for mutations affecting the retinotectal projection in zebrafish larvae was performed, as part of the large-scale Tübingen screen for homozygous diploid mutants in embryonic development. 2,746 inbred lines (F2 families) from males mutagenized with ethylnitroso urea were screened. In wild-type larvae, developing retinal axons travel along a stereotyped route to the contralateral optic tectum. Here, their terminals form a highly ordered retinotopic map. To detect deviations from this pattern, an axon tracing assay was developed that permits screening of large numbers of mutagenized fish. Two fluorescent tracer dyes (DiI and DiO) were injected at opposite poles of the eyes of day-5 aldehyde-fixed larvae. 12 hours later, retinal axons were labelled over their entire length, and could be observed through the intact skin. The assay procedure (aldehyde fixation, mounting, injection of dyes, microscopic analysis) took about 1 minute per fish. In total, 125,000 individual fish larvae were processed.

During the screen, 114 mutations in approx. 35 genes were discovered. For the mutants subjected to complementation testing, the number of alleles per locus ranges from 1 to 15. The mutations affect distinct steps in the retinotectal pathway, from pathfinding between eye and tectum to map formation along the dorsal-ventral and the anterior-posterior axis of the tectum. Mutations that disturb axon pathfinding to the tectum for the most part do not disrupt retinotopic mapping, and vice versa. The majority of the mutants display associated defects in other tissues and die before day 10. These mutants provide new tools for studying the formation of neuronal maps. The results of this screen show that a large-scale genetic approach can be applied to relatively late and circumscribed developmental processes in the vertebrate brain.

Bergmann, A., Stein, D., Geisler, R., Hagenmaier, S., Schmid, B., Fernandez, N., Schnell, B. and Nüsslein-Volhard, C. (1996). A gradient of cytoplasmic Cactus degradation establishes the nuclear localization gradient of the dorsal morphogen in Drosophila. Mech Dev 60, 109-23.

Dorsoventral axis formation in the Drosophila embryo is established by a signal transduction pathway that comprises the products of at least 12 maternal genes. Two of these genes, dorsal and cactus, show homology to the mammalian transcription factor NF-kappa B and its inhibitor I kappa B, respectively. As in the case for I kappa B and NF-kappa B, Cactus inhibits Dorsal by retaining it in the cytoplasm. In response to the signal produced and transmitted by the products of the other genes, Dorsal translocates to the nucleus preferentially on the ventral side of the embryo. Here, we show that Cactus forms a cytoplasmic concentration gradient inversely correlated to the nuclear translocation gradient of Dorsal. Deletions of the N-terminus and C-terminus of Cactus reveal that two modes of degradation control cactus activity: signal-induced degradation and signal-independent degradation, respectively. Genetic evidence indicates that the degradation of Cactus is required, but not sufficient to translocate Dorsal completely into the nucleus.

Brand, M., Heisenberg, C.-P., Jiang, Y.-J., Beuchle, D., Lun, K., Furutani-Seiki, M., Granato, M., Haffter, P., Hammerschmidt, M., Kane, D., Kelsh, R., Mullins, M., Odenthal, J., van Eeden, F. J. M. and Nüsslein-Volhard, C. (1996). Mutations in zebrafish genes affecting the formation of the boundary between midbrain and hindbrain. Development 123, 179-90.

Mutations in two genes affect the formation of the boundary between midbrain and hindbrain (MHB): no isthmus (noi) and acerebellar (ace). noi mutant embryos lack the MHB constriction, the cerebellum and optic tectum, as well as the pronephric duct. Analysis of noi mutant embryos with neuron-specific antibodies shows that the MHB region and the dorsal and ventral midbrain are absent or abnormal, but that the rostral hindbrain is unaffected with the exception of the cerebellum. Using markers that are expressed during its formation (eng, wnt1 and pax-b), we find that the MHB region is already misspecified in noi mutant embryos during late gastrulation. The tectum is initially present and later degenerates. The defect in ace mutant embryos is more restricted: MHB and cerebellum are absent, but a tectum is formed. Molecular organisation of the tectum and tegmentum is disturbed, however, since eng, wnt1 and pax-b marker gene expression is not maintained. We propose that noi and ace are required for development of the MHB region and of the adjacent mid- and hindbrain, which are thought to be patterned by the MHB region.

Presence of pax-b RNA, and absence of pax-b protein, together with the observation of genetic linkage and the occurrence of a point mutation, show that noi mutations are located in the pax-b gene. pax-b is a vertebrate orthologue of the Drosophila gene paired, which is involved in a pathway of cellular interactions at the posterior compartment boundary in Drosophila. Our results confirm and extend a previous report, and show that at least one member of this conserved signalling pathway is required for formation of the boundary between midbrain and hindbrain in the zebrafish.

Brand, M., Heisenberg, C.-P., Warga, R., Pelegri, F., Karlstrom, R. O., Beuchle, D., Picker, A., Jiang, Y.-J., Furutani-Seiki, M., van Eeden, F. J. M., Granato, M., Haffter, P., Hammerschmidt, M., Kane, D., Kelsh, R., Mullins, M., Odenthal, J. and Nüsslein-Volhard, C. (1996a). Mutations affecting development of the midline and general body shape during zebrafish embryogenesis. Development 123, 129-42.

Tissues of the dorsal midline of vertebrate embryos, such as notochord and floor plate, have been implicated in inductive interactions that pattern the neural tube and somites. In our screen for embryonic visible mutations in the zebrafish we found 113 mutations in more than 27 genes with altered body shape, often with additional defects in CNS development. We concentrated on a subgroup of mutations in ten genes (the midline-group) that cause defective development of the floor plate. By using floor plate markers, such as the signaling molecule sonic hedgehog, we show that the schmalspur (sur) gene is needed for early floor plate development, similar to one-eyed-pinhead (oep) and the previously described cyclops (cyc) gene. In contrast to oep and cyc, sur embryos show deletions of ventral CNS tissue restricted to the mid- and hindbrain, whereas the forebrain appears largely unaffected. In the underlying mesendodermal tissue of the head, sur is needed only for development of the posterior prechordal plate, whereas oep and cyc are required for both anterior and posterior prechordal plate development. Our analysis of sur mutants suggests that defects within the posterior prechordal plate may cause aberrant development of ventral CNS structures in the mid- and hindbrain. Later development of the floor plate is affected in mutant chameleon, you-too, sonic-you, iguana, detour, schmalhans and monorail embryos; these mutants often show additional defects in tissues that are known to depend on signals from notochord and floor plate. For example, sur, con and yot mutants show reduction of motor neurons; median deletions of brain tissue are seen in sur, con and yot embryos; and cyc, con, yot, igu and dtr mutants often show no or abnormal formation of the optic chiasm. We also find fusions of the ventral neurocranium for all midline mutants tested, which may reveal a hitherto unrecognized function of the midline in influencing differentiation of neural crest cells at their destination. As a working hypothesis, we propose that midline-group genes may act to maintain proper structure and inductive function of zebrafish midline tissues.

Chen, J.-N., Haffter, P., Odenthal, J., Vogelsang, E., Brand, M., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Hammerschmidt, M., Heisenberg, C.-P., Jiang, Y.-J., Kane, D. A., Kelsh, R. N., Mullins, M. C. and Nüsslein-Volhard, C. (1996). Mutations affecting the cardiovascular system and other internal organs in zebrafish. Development 123, 293-302.

In a screen for early developmental mutants of the zebrafish, we have identified mutations specifically affecting the internal organs. We identified 53 mutations affecting the cardiovascular system. Nine of them affect specific landmarks of heart morphogenesis. Mutations in four genes cause a failure in the fusion of the bilateral heart primordia, resulting in cardia bifida. In lonely atrium, no heart venticle is visible and the atrium is directly fused to the outflow tract. In the overlooped mutant, the relative position of the two heart chambers is distorted. The heart is enormously enlarged in the santa mutant. In two mutants, scotch tape and superglue, the cardiac jelly between the two layers of the heart is significantly reduced. We also identified a number of mutations affecting the function of the heart. The mutations affecting heart function can be subdivided into two groups, one affecting heart contraction and another affecting the rhythm of the heart beat. Among the contractility group of mutants are 5 with no heart beat at all and 15 with a reduced heart beat of one or both chambers. 6 mutations are in the rhythmicity group and specifically affect the beating pattern of the heart. Mutations in two genes, bypass and kurzschluss, cause specific defects in the circulatory system. In addition to the heart mutants, we identified 23 mutations affecting the integrity of the liver, the intestine or the kidney. In this report, we demonstrate that it is feasible to screen for genes specific for the patterning or function of certain internal organs in the zebrafish. The mutations presented here could serve as an entrypoint to the establishment of a genetic hierarchy underlying organogenesis.

van Eeden, F. J. M., Granato, M., Schach, U., Brand, M., Furutani-Seiki, M., Haffter, P., Hammerschmidt, M., Heisenberg, C.-P., Jiang, Y.-J., Kane, D. A., Kelsh, R. N., Mullins, M. C., Odenthal, J., Warga, R. M., Allende, M. L., Weinberg, E. S. and Nüsslein-Volhard, C. (1996). Mutations affecting somite formation and patterning in the zebrafish, Danio rerio. Development 123, 153-64.

Somitogenesis is the basis of segmentation of the mesoderm in the trunk and tail of vertebrate embryos. Two groups of mutants with defects in this patterning process have been isolated in our screen for zygotic mutations affecting the embryonic development of the zebrafish (Danio rerio). In mutants of the first group, boundaries between individual somites are invisible early on, although the paraxial mesoderm is present. Later, irregular boundaries between somites are present. Mutations in fused somites (fss) and beamter (bea) affect all somites, whereas mutations in deadly seven (des), after eight (aei) and white tail (wit) only affect the more posterior somites. Mutants of all genes but wit are homozygous viable and fertile. Skeletal stainings and the expression pattern of myoD and snail1 suggest that anteroposterior patterning within individual somites is abnormal. In the second group of mutants, formation of the horizontal myoseptum, which separates the dorsal and ventral part of the myotome, is reduced. Six genes have been defined in this group (you-type genes). you-too mutants show the most severe phenotype; in these the adaxial cells, muscle pioneers and the primary motoneurons are affected, in addition to the horizontal myoseptum.

The horizontal myoseptum is also missing in mutants that lack a notochord. The similarity of the somite phenotype in mutants lacking the notochord and in the you-type mutants suggests that the genes mutated in these two groups are involved in a signaling pathway from the notochord, important for patterning of the somites.

van Eeden, F. J. M., Granato, M., Schach, U., Brand, M., Furutani-Seiki, M., Haffter, P., Hammerschmidt, M., Heisenberg, C.-P., Jiang, Y.-J., Kane, D. A., Kelsh, R. N., Mullins, M. C., Odenthal, J., Warga, R. M. and Nüsslein-Volhard, C. (1996a). Genetic analysis of fin formation in the zebrafish, Danio rerio. Development 123, 255-62.

In the zebrafish, Danio rerio, a caudal and pectoral fin fold develop during embryogenesis. At larval stages the caudal fin fold is replaced by four different fins, the unpaired anal, dorsal and tail fins. In addition the paired pelvic fins are formed. We have identified a total of 118 mutations affecting larval fin formation. Mutations in 11 genes lead to abnormal morphology or degeneration of both caudal and pectoral fin folds. Most mutants survive to adulthood and form a surprisingly normal complement of adult fins.

Mutations in nine genes result in an increased or reduced size of the pectoral fins. Interestingly, in mutants of one of these genes, dackel (dak), pectoral fin buds form initially, but later the fin epithelium fails to expand. Expression of sonic hedgehog mRNA in the posterior mesenchyme of the pectoral fin bud is initiated in dak embryos, but not maintained.

Mutations in five other genes affect adult fin but not larval fin development. Two mutants, longfin (lof) and another longfin (alf) have generally longer fins. Stein und bein (sub) has reduced dorsal and pelvic fins, whereas finless (fls) and wanda (wan) mutants affect all adult fins. Finally, mutations in four genes causing defects in embryonic skin formation will be briefly reported.

Furutani-Seiki, M., Jiang, Y.-J., Brand, M., Heisenberg, C.-P., Houart, C., Beuchle, D., van Eeden, F. J. M., Granato, M., Haffter, P., Hammerschmidt, M., Kane, D. A., Kelsh, R. N., Mullins, M. C., Odenthal, J. and Nüsslein-Volhard, C. (1996). Neural degeneration mutants in the zebrafish, Danio rerio. Development 123, 229-39.

Forty zebrafish mutants with localized or general neural degeneration are described. The onset and duration of degeneration and the distribution of ectopically dying cells are specific characteristics of each mutant. Mutants are classified into four groups by these parameters. Class I: late focal neural degeneration mutants. These 18 mutants have restricted cell death mainly in the tectum and the dorsal hindbrain after 36 hours. The degeneration does not spread and disappears at later stages of development. Class II: early focal neural degeneration mutants. Ten mutants in this class exhibit transient restricted degeneration affecting mainly the diencephalon, the hindbrain and the spinal cord at 20 hours. The midbrain is less affected. The degeneration shifts to the dorsal diencephalon and the tectum at 36 hours. Class III: late spreading neural degeneration mutants. The 8 mutants in this class display a degeneration that is first seen in the tectum and subsequently spreads throughout the nervous system from 36 hours on. Class IV: early general neural degeneration mutants. This class of four mutants already shows overall cell degeneration in the nervous system at the 15-somite stage.

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.

Granato, M. and Nüsslein-Volhard, C. (1996). Fishing for genes controlling development. Curr Opin Genet Dev 6, 461-8.

In recent years, the zebrafish has become a popular system for studying vertebrate development. Large scale mutant searches have led to the identification of >400 genes with unique functions during embryonic and larval development. A number of these genes play important roles in well studied processes, such as dorsoventral patterning of the early embryo, notochord formation and signaling, somitogenesis and neural specification. Other newly identified genes offer opportunities to investigate less well understood processes, including locomotion behavior, organogenesis, neural crest development and axonal pathfinding.

Granato, M., van Eeden, F. J. M., Schach, U., Trowe, T., Brand, M., Furutani-Seiki, M., Haffter, P., Hammerschmidt, M., Heisenberg, C.-P., Jiang, Y.-J., Kane, D. A., Kelsh, R. N., Mullins, M. C., Odenthal, J. and Nüsslein-Volhard, C. (1996a). Genes controlling and mediating locomotion behavior of the zebrafish embryo and larva. Development 123, 399-413.

Zebrafish embryos and larvae have stage-specific patterns of motility or locomotion. Two embryonic structures accomplish this behavior: the central nervous system (CNS) and skeletal muscles. To identify genes that are functionally involved in mediating and controlling different patterns of embryonic and larval motility, we included a simple touch response test in our zebrafish large-scale genetic screen. In total we identified 166 mutants with specific defects in embryonic motility. These mutants fall into 14 phenotypically distinct groups comprising at least 48 genes. Here we describe the various phenotypic groups including mutants with no or reduced motility, mechanosensory defective mutants, 'spastic' mutants, circling mutants and motor circuit defective mutants.

In 63 mutants, defining 18 genes, striation of somitic muscles is reduced. Phenotypic analysis provides evidence that these 18 genes have distinct and consecutive functions during somitic muscle development. The genes sloth (slo) and frozen (fro) already act during myoblast differentiation, while 13 genes appear to function later, in the formation of myofibers and the organization of sarcomeres. Mutations in four other genes result in muscle-specific degeneration.

103 mutations, defining at least 30 genes, cause no obvious defects in muscle formation and may instead affect neuronal development. Analysis of the behavioral defects suggests that these genes participate in the diverse locomotion patterns observed, such as touch response, rhythmic tail movements, equilibrium control, or that they simply confer general motility to the animal. In some of these mutants specific defects in the developing nervous system are detected. Mutations in two genes, nevermind (nev) and macho (mao), affect axonal projection in the optic tectum, whereas axon formation and elongation of motorneurons are disrupted by mutations in the diwanka (diw) and the unplugged (unp) genes.

Haffter, P. and Nüsslein-Volhard, C. (1996). Large-scale genetics in a small vertebrate, the zebrafish. Int J Dev Biol 40, 221-7.

The systematic isolation and characterization of mutants in Drosophila has enormously facilitated the analysis of molecular mechanisms underlying developmental pathways in the embryo. A similar approach is presently being used to study embryonic development of the zebrafish, which is becoming a mainstream model organism for vertebrate development. With its genetic versatility and sophisticated embryology, zebrafish offers the possibility to rapidly increase our knowledge of vertebrate development and add to what we have learned from other vertebrate model organisms.

Haffter, P., Granato, M., Brand, M., Mullins, M. C., Hammerschmidt, M., Kane, D. A., Odenthal, J., van Eeden, F. J. M., Jiang, Y.-J., Heisenberg, C.-P., Kelsh, R. N., Furutani-Seiki, M., Vogelsang, E., Beuchle, D., Schach, U., Fabian, C. and Nüsslein-Volhard, C. (1996a). The identification of genes with unique and essential functions in the development of the zebrafish, Danio rerio. Development 123, 1-36.

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.

Genes identified in the Tübingen screen: A-H | I-P | Q-Z | Phenotypic groups

Haffter, P., Odenthal, J., Mullins, M. C., Lin, S., Farrell, M. J., Vogelsang, E., Haas, F., Brand, M., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Hammerschmidt, M., Heisenberg, C.-P., Jiang, Y.-J., Kane, D. A., Kelsh, R. N., Hopkins, N. and Nüsslein-Volhard, C. (1996b). Mutations affecting pigmentation and shape of the adult zebrafish. Dev Genes Evol 206, 260-76.

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.

Hammerschmidt, M., Pelegri, F., Mullins, M. C., Kane, D. A., Brand, M., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Haffter, P., Heisenberg, C.-P., Jiang, Y.-J., Kelsh, R. N., Odenthal, J., Warga, R. M. and Nüsslein-Volhard, C. (1996). Mutations affecting morphogenesis during gastrulation and tail formation in the zebrafish, Danio rerio. Development 123, 143-51.

We have identified several genes that are required for various morphogenetic processes during gastrulation and tail formation. Two genes are required in the anterior region of the body axis: one eyed pinhead (oep) and dirty nose (dns). oep mutant embryos are defective in prechordal plate formation and the specification of anterior and ventral structures of the central nervous system. In dns mutants, cells of the prechordal plate, such as the prospective hatching gland cells, fail to specify.

Two genes are required for convergence and extension movements. In mutant trilobite embryos, extension movements on the dorsal side of the embryo are affected, whereas in the formerly described spadetail mutants, for which two new alleles have been isolated, convergent movements of ventrolateral cells to the dorsal side are blocked.

Two genes are required for the development of the posterior end of the body axis. In pipetail mutants, the tailbud fails to move ventrally on the yolk sac after germ ring closure, and the tip of the tail fails to detach from the yolk tube. Mutants in kugelig (kgg) do not form the yolk tube at the posterior side of the yolk sac.

Hammerschmidt, M., Pelegri, F., Mullins, M. C., Kane, D. A., van Eeden, F. J. M., Granato, M., Brand, M., Furutani-Seiki, M., Haffter, P., Heisenberg, C.-P., Jiang, Y.-J., Kelsh, R. N., Odenthal, J., Warga, R. M. and Nüsslein-Volhard, C. (1996a). dino and mercedes, two genes regulating dorsal development in the zebrafish embryo. Development 123, 95-102.

We describe two genes, dino and mercedes, which are required for the organization of the zebrafish body plan. In dino mutant embryos, the tail is enlarged at the expense of the head and the anterior region of the trunk. The altered expression patterns of various marker genes reveal that, with the exception of the dorsal most marginal zone, all regions of the early dino mutant embryo acquire more ventral fates. These alterations are already apparent before the onset of gastrulation. mercedes mutant embryos show a similar but weaker phenotype, suggesting a role in the same patterning processes. The phenotypes suggests that dino and mercedes are required for the establishment of dorsal fates in both the marginal and the animal zone of the early gastrula embryo. Their function in the patterning of the ventrolateral mesoderm and the induction of the neuroectoderm is similar to the function of the Spemann organizer in the amphibian embryo.

Heisenberg, C.-P., Brand, M., Jiang, Y.-J., Warga, R. M., Beuchle, D., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Haffter, P., Hammerschmidt, M., Kane, D. A., Kelsh, R. N., Mullins, M. C., Odenthal, J. and Nüsslein-Volhard, C. (1996). Genes involved in forebrain development in the zebrafish, Danio rerio. Development 123, 191-203.

We identified four zebrafish mutants with defects in forebrain induction and patterning during embryogenesis. The four mutants define three genes: masterblind (mbl), silberblick (slb), and knollnase (kas). In mbl embryos, the anterior forebrain acquires posterior forebrain characteristics: anterior structures such as the eyes, olfactory placodes and the telencephalon are missing, whereas the epiphysis located in the posterior forebrain is expanded. In slb embryos, the extension of the embryonic axis is initially delayed and eventually followed by a partial fusion of the eyes. Finally, in kas embryos, separation of the telencephalic primordia is incomplete and dorsal midline cells fail to form a differentiated roof plate. Analysis of the mutant phenotypes indicates that we have identified genes essential for the specification of the anterior forebrain (mbl), positioning of the eyes (slb) and differentiation of the roof plate (kas).

In an appendix to this study we list mutants showing alterations in the size of the eyes and abnormal differentiation of the lenses.

Jiang, Y.-J., Brand, M., Heisenberg, C.-P., Beuchle, D., Furutani-Seiki, M., Kelsh, R. N., Warga, R. M., Granato, M., Haffter, P., Hammerschmidt, M., Kane, D. A., Mullins, M. C., Odenthal, J., van Eeden, F. J. M. and Nüsslein-Volhard, C. (1996). Mutations affecting neurogenesis and brain morphology in the zebrafish, Danio rerio. Development 123, 205-16.

In a screen for embryonic mutants in the zebrafish a large number of mutants were isolated with abnormal brain morphology. We describe here 26 mutants in 13 complementation groups that show abnormal development of large regions of the brain. Early neurogenesis is affected in white tail (wit). During segmentation stages, homozygous wit embryos display an irregularly formed neural keel, particularly in the hindbrain. Using a variety of molecular markers, a severe increase in the number of various early differentiating neurons can be demonstrated. In contrast, late differentiating neurons, radial glial cells and some non-neural cell types, such as the neural crest-derived melanoblasts, are much reduced. Somitogenesis appears delayed. In addition, very reduced numbers of melanophores are present posterior to the mid-trunk. The wit phenotype is reminiscent of neurogenic mutants in Drosophila, such as Notch or Delta. In mutant parachute (pac) embryos the general organization of the hindbrain is disturbed and many rounded cells accumulate loosely in the hindbrain and midbrain ventricles. Mutants in a group of 6 genes, snakehead(snk), natter (nat), otter (ott), fullbrain (ful), viper (vip) and white snake (wis) develop collapsed brain ventricles, before showing signs of general degeneration. atlantis (atl), big head (bid), wicked brain (win), scabland (sbd) and eisspalte (ele) mutants have different malformation of the brain folds. Some of them have transient phenotypes, and mutant individuals may grow up to adults.

Kane, D. A., Hammerschmidt, M., Mullins, M. C., Maischein, H.-M., Brand, M., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Haffter, P., Heisenberg, C.-P., Jiang, Y.-J., Kelsh, R. N., Odenthal, J., Warga, R. M. and Nüsslein-Volhard, C. (1996). The zebrafish epiboly mutants. Development 123, 47-55.

Epiboly, the enveloping of the yolk cell by the blastoderm, is the first zebrafish morphogenetic movement. We isolated four mutations that affect epiboly: half baked, avalanche, lawine and weg. Homozygous mutant embryos arrest the vegetal progress of the deep cells of the blastoderm; only the yolk syncytial layer of the yolk cell and the enveloping layer of the blastoderm reach the vegetal pole of the embryo. The mutations half baked, avalanche and lawine produce a novel dominant effect, termed a zygotic-maternal dominant effect: heterozygous embryos produced from heterozygous females slow down epiboly and accumulate detached cells over the neural tube; a small fraction of these mutant individuals are viable. Heterozygous embryos produced from heterozygous males crossed to homozygous wild-type females complete epiboly normally and are completely viable. Additionally, embryos heterozygous for half baked have an enlarged hatching gland, a partial dominant phenotype. The phenotypes of these mutants demonstrate that, for the spreading of cells during epiboly, the movement of the deep cells of the blastoderm require the function of genes that are not necessary for the movement of the enveloping layer or the yolk cell. Furthermore, the dominant zygotic-maternal effect phenotypes illustrate the maternal and zygotic interplay of genes that orchestrate the early cell movements of the zebrafish.

Kane, D. A., Maischein, H.-M., Brand, M., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Haffter, P., Hammerschmidt, M., Heisenberg, C.-P., Jiang, Y.-J., Kelsh, R. N., Mullins, M. C., Odenthal, J., Warga, R. M. and Nüsslein-Volhard, C. (1996a). The zebrafish early arrest mutants. Development 123, 57-66.

This report describes mutants of the zebrafish having phenotypes causing a general arrest in early morphogenesis. These mutants identify a group of loci making up about 20% of the loci identified by mutants with visible morphological phenotypes within the first day of development. There are 12 Class I mutants, which fall into 5 complementation groups and have cells that lyse before morphological defects are observed. Mutants at three loci, speed bump, ogre and zombie, display abnormal nuclei. The 8 Class II mutants, which fall into 6 complementation groups, arrest development before cell lysis is observed. These mutants seemingly stop development in the late segmentation stages, and maintain a body shape similar to a 20 hour embryo.

Mutations in speed bump, ogre, zombie, specter, poltergeist and troll were tested for cell lethality by transplanting mutant cells into wild-type hosts. With poltergeist, transplanted mutant cells all survive. The remainder of the mutants tested were autonomously but conditionally lethal: mutant cells, most of which lyse, sometimes survive to become notochord, muscles, or, in rare cases, large neurons, all cell types which become postmitotic in the gastrula. Some of the genes of the early arrest group may be necessary for progression though the cell cycle; if so, the survival of early differentiating cells may be based on having their terminal mitosis before the zygotic requirement for these genes.

Karlstrom, R. O., Trowe, T., Klostermann, S., Baier, H., Brand, M., Crawford, A. D., Grunewald, B., Haffter, P., Hoffmann, H., Meyer, S. U., Muller, B. K., Richter, S., van Eeden, F. J. M., Nüsslein-Volhard, C. and Bonhoeffer, F. (1996). Zebrafish mutations affecting retinotectal axon pathfinding. Development 123, 427-38.

We have isolated mutants in the zebrafish Danio rerio that have defects in axonal connectivity between the retina and tectum. 5-day-old fish larvae were screened by labeling retinal ganglion cells with DiI and DiO and observing their axonal projections to and on the tectum.

82 mutations, representing 13 complementation groups and 6 single allele loci, were found that have defects in retinal ganglion cell axon pathfinding to the tectum. These pathfinding genes fall into five classes, based on the location of pathfinding errors between eye and tectum. In Class I mutant larvae (belladonna, detour, you-too, iguana, umleitung, blowout) axons grow directly to the ipsilateral tectal lobe after leaving the eye. Class II mutant larvae (chameleon, bashful) have ipsilaterally projecting axons and, in addition, pathfinding mistakes are seen within the eye. In Class III mutant larvae (esrom, tilsit, tofu) fewer axons than normal cross the midline, but some axons do reach the contralateral tectal lobe. Class IV mutant larvae (boxer, dackel, pinscher) have defects in axon sorting after the midline and retinal axons occasionally make further pathfinding errors upon reaching the contralateral tectal lobe. Finally, Class V mutant larvae (bashful, grumpy, sleepy, cyclops, astray) have anterior-posterior axon trajectory defects at or after the midline.

The analysis of these mutants supports several conclusions about the mechanisms of retinal axon pathfinding from eye to tectum. A series of sequential cues seems to guide retinal axons to the contralateral tectal lobe. Pre-existing axon tracts seem not to be necessary to guide axons across the midline. The midline itself seems to play a central role in guiding retinal axons. Axons in nearby regions of the brain seem to use different cues to cross the ventral midline. Mutant effects are not all-or-none, as misrouted axons may reach their target, and if they do, they project normally on the tectum. The retinotectal pathfinding mutants reveal important choice points encountered by neuronal growth cones as they navigate between eye and tectum.

Karlstrom, R. O. and Kane, D. A. (1996a). A flipbook of zebrafish embryogenesis. Development 123, 461.

Kelsh, R. N., Brand, M., Jiang, Y.-J., Heisenberg, C.-P., Lin, S., Haffter, P., Odenthal, J., Mullins, M. C., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Hammerschmidt, M., Kane, D. A., Warga, R. M., Beuchle, D., Vogelsang, L. and Nüsslein-Volhard, C. (1996). Zebrafish pigmentation mutations and the processes of neural crest development. Development 123, 369-89.

Neural crest development involves cell-fate specification, proliferation, patterned cell migration, survival and differentiation. Zebrafish neural crest derivatives include three distinct chromatophores, which are well-suited to genetic analysis of their development. As part of a large-scale mutagenesis screen for embryonic/early larval mutations, we have isolated 285 mutations affecting all aspects of zebrafish larval pigmentation. By complementation analysis, we define 94 genes. We show here that comparison of their phenotypes permits classification of these mutations according to the types of defects they cause, and these suggest which process of neural crest development is probably affected. Mutations in eight genes affect the number of chromatophores: these include strong candidates for genes necessary for the processes of pigment cell specification and proliferation. Mutations in five genes remove part of the wild-type pigment pattern, and suggest a role in larval pigment pattern formation. Mutations in five genes show ectopic chromatophores in distinct sites, and may have implications for chromatophore patterning and proliferation. 76 genes affect pigment or morphology of one or more chromatophore types: these mutations include strong candidates for genes important in various aspects of chromatophore differentiation and survival. In combination with the embryological advantages of zebrafish, these mutations should permit cellular and molecular dissection of many aspects of neural crest development.

Mullins, M. C., Hammerschmidt, M., Kane, D. A., Odenthal, J., Brand, M., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Haffter, P., Heisenberg, C.-P., Jiang, Y.-J., Kelsh, R. N. and Nüsslein-Volhard, C. (1996). Genes establishing dorsoventral pattern formation in the zebrafish embryo: the ventral specifying genes. Development 123, 81-93.

We identified 6 genes that are essential for specifying ventral regions of the early zebrafish embryo. Mutations in these genes cause an expansion of structures normally derived from dorsal-lateral regions of the blastula at the expense of ventrally derived structures. A series of phenotypes of varied strengths is observed with different alleles of these mutants. The weakest phenotype is a reduction in the ventral tail fin, observed as a dominant phenotype of swirl, piggytail, and somitabun and a recessive phenotype of mini fin, lost-a-fin and some piggytail alleles. With increasing phenotypic strength, the blood and pronephric anlagen are also reduced or absent, while the paraxial mesoderm and anterior neuroectoderm is progressively expanded. In the strong phenotypes, displayed by homozygous embryos of snailhouse, swirl and somitabun, the somites circle around the embryo and the midbrain region is expanded laterally.

Several mutations in this group of genes are semidominant as well as recessive indicating a strong dosage sensitivity of the processes involved. Mutations in the piggytail gene display an unusual dominance that depends on both a maternal and zygotic heterozygous genotype, while somitabun is a fully penetrant dominant maternal-effect mutation. The similar and overlapping phenotypes of mutants of the 6 genes identified suggest that they function in a common pathway, which begins in oogenesis, but also depends on factors provided after the onset of zygotic transcription, presumably during blastula stages. This pathway provides ventral positional information, counteracting the dorsalizing instructions of the organizer, which is localized in the dorsal shield.

Nüsslein-Volhard, C. (1996). Gradients that organize embryo development. Sci Am 275, 54-9.

Odenthal, J., Haffter, P., Vogelsang, E., Brand, M., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Hammerschmidt, M., Heisenberg, C.-P., Jiang, Y.-J., Kane, D. A., Kelsh, R. N., Mullins, M. C., Warga, R. M., Allende, M. L., Weinberg, E. S. and Nüsslein-Volhard, C. (1996). Mutations affecting the formation of the notochord in the zebrafish, Danio rerio. Development 123, 103-15.

In a large scale screen for mutants with defects in the embryonic development of the zebrafish we identified mutations in four genes, floating head (flh), momo (mom), no tail (ntl), and doc, that are required for early notochord formation. Mutations in flh and ntl have been described previously, while mom and doc are newly identified genes. Mutant mom embryos lack a notochord in the trunk, and trunk somites from the right and left side of the embryo fuse underneath the neural tube. In this respect mom appears similar to flh. In contrast, notochord precursor cells are present in both ntl and doc embryos.

In order to gain a greater understanding of the phenotypes, we have analysed the expression of several axial mesoderm markers in mutant embryos of all four genes. In flh and mom, Ntl expression is normal in the germ ring and tailbud, while the expression of Ntl and other notochord markers in the axial mesodermal region is disrupted. Ntl expression is normal in doc embryos until early somitic stages, when there is a reduction in expression which is first seen in anterior regions of the embryo. This suggests a function for doc in the maintenance of ntl expression. Other notochord markers such as twist, sonic hedgehog and axial are not expressed in the axial mesoderm of ntl embryos, their expression parallels the expression of ntl in the axial mesoderm of mutant doc, flh and mom embryos, indicating that ntl is required for the expression of these markers. The role of doc in the expression of the notochord markers appears indirect via ntl.

Floor plate formation is disrupted in most regions in flh and mom mutant embryos but is present in mutant ntl and doc embryos. In mutant embryos with strong ntl alleles the band of cells expressing floor plate markers is broadened. A similar broadening is also observed in the axial mesoderm underlying the floor plate of ntl embryos, suggesting a direct involvement of the notochord precursor cells in floor plate induction.

Mutations in all of these four genes result in embryos lacking a horizontal myoseptum and muscle pioneer cells, both of which are thought to be induced by the notochord. These somite defects can be traced back to an impairment of the specification of the adaxial cells during early stages of development. Transplantation of wild-type cells into mutant doc embryos reveals that wild-type notochord cells are sufficient to induce horizontal myoseptum formation in the flanking mutant tissue. Thus doc, like flh and ntl, acts cell autonomously in the notochord.

In addition to the four mutants with defects in early notochord formation, we have isolated 84 mutants, defining at least 15 genes, with defects in later stages of notochord development. These are listed in an appendix to this study.

Odenthal, J., Roßnagel, K., Haffter, P., Kelsh, R. N., Vogelsang, E., Brand, M., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Hammerschmidt, M., Heisenberg, C.-P., Jiang, Y.-J., Kane, D. A., Mullins, M. C. and Nüsslein-Volhard, C. (1996a). Mutations affecting xanthophore pigmentation in the zebrafish, Danio rerio. Development 123, 391-8.

In a large-scale screen for mutants with defects in embryonic development we identified 17 genes (65 mutants) specifically required for the development of xanthophores. We provide evidence that these genes are required for three different aspects of xanthophore development. (1) Pigment cell formation and migration (pfeffer and salz); (2) pigment synthesis (edison, yobo, yocca and brie) and (3) pigment translocation (esrom, tilsit and tofu). The number of xanthophore cells that appear in the body is reduced in embryos with mutations in the two genes, salz and pfeffer. In heterozygous and homozygous salz and pfeffer adults, the melanophore stripes are interrupted, indicating that xanthophore cells have an important function in adult melanophore pattern formation. Most other genes affect only larval pigmentation. In embryos mutant for edison, yobo, yocca and brie, differences in pteridine synthesis can be observed under UV light and by thin-layer chromatography. Homozygous mutant females of yobo show a recessive maternal effect. Embryonic development is slowed down and embryos display head and tail truncations. Xanthophores in larvae mutant in the three genes esrom, tilsit and tofu appear less spread out. In addition, these mutants display a defect in retinotectal axon pathfinding. These mutations may affect xanthophore pigment distribution within the cells or xanthophore cell shape. Mutations in seven genes affecting xanthophore pigmentation remain unclassified.

Piotrowski, T., Schilling, T. F., Brand, M., Jiang, Y.-J., Heisenberg, C.-P., Beuchle, D., Grandel, H., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Haffter, P., Hammerschmidt, M., Kane, D. A., Kelsh, R. N., Mullins, M. C., Odenthal, J., Warga, R. M. and Nüsslein-Volhard, C. (1996). Jaw and branchial arch mutants in zebrafish II: anterior arches and cartilage differentiation. Development 123, 345-56.

In a large scale screen for mutants that affect the early development of the zebrafish, 109 mutants were found that cause defects in the formation of the jaw and the more posterior pharyngeal arches. Here we present the phenotypic description and results of the complementation analysis of mutants belonging to two major classes: (1) mutants with defects in the mandibular and hyoid arches and (2) mutants with defects in cartilage differentiation and growth in all arches. Mutations in four of the genes identified during the screen show specific defects in the first two arches and leave the more posterior pharyngeal arches largely unaffected (schmerle, sucker, hoover and sturgeon). In these mutants ventral components of the mandibular and hyoid arches are reduced (Meckel's cartilage and ceratohyal cartilage) whereas dorsal structures (palatoquadrate and hyosymplectic cartilages) are of normal size or enlarged. Thus, mutations in single genes cause defects in the formation of first and second arch structures but also differentially affect development of the dorsal and ventral structures within one arch.

In 27 mutants that define at least 8 genes, the differentiation of cartilage and growth is affected. In hammerhead mutants particularly the mesodermally derived cartilages are reduced, whereas jellyfish mutant larvae are characterized by a severe reduction of all cartilaginous elements, leaving only two pieces in the position of the ceratohyal cartilages. In all other mutant larvae all skeletal elements are present, but consist of smaller and disorganized chondrocytes. These mutants also exhibit shortened heads and reduced pectoral fins. In homozygous knorrig embryos, tumor-like outgrowths of chondrocytes occur along the edges of all cartilaginous elements. The mutants presented here may be valuable tools for elucidating the genetic mechanisms that underlie the development of the mandibular and the hyoid arches, as well as the process of cartilage differentiation.

Ransom, D. G., Haffter, P., Odenthal, J., Brownlie, A., Vogelsang, E., Kelsh, R. N., Brand, M., van Eeden, F. J. M., Furutani-Seiki, M., Granato, M., Hammerschmidt, M., Heisenberg, C.-P., Jiang, Y.-J., Kane, D. A., Mullins, M. C. and Nüsslein-Volhard, C. (1996). Characterization of zebrafish mutants with defects in embryonic hematopoiesis. Development 123, 311-9.

As part of a large scale chemical mutagenesis screen of the zebrafish (Danio rerio) genome, we have identified 33 mutants with defects in hematopoiesis. Complementation analysis placed 32 of these mutants into 17 complementation groups. The allelism of the remaining 1 blood mutant is currently unresolved. We have categorized these blood mutants into four phenotypic classes based on analyses of whole embryos and isolated blood cells, as well as by in situ hybridization using the hematopoietic transcription factors GATA-1 and GATA-2. Embryos mutant for the gene moonshine have few if any proerythroblasts visible on the day circulation begins and normal erythroid cell differentiation is blocked as determined by staining for hemoglobin and GATA-1 expression. Mutations in five genes, chablis, frascati, merlot, retsina, thunderbird and two possibly unique mutations cause a progressive decrease in the number of blood cells during the first 5 days of development. Mutations in another seven genes, chardonnay, chianti, grenache, sauternes, weißherbst and zinfandel, and two additional mutations result in hypochromic blood cells which also decrease in number as development proceeds. Several of these mutants have immature cells in the circulation, indicating a block in normal erythroid development. The mutation in zinfandel is dominant, and 2-day old heterozygous carriers fail to express detectable levels of hemoglobin and have decreasing numbers of circulating cells during the first 5 days of development. Mutations in two genes, freixenet and yquem, result in the animals that are photosensitive with autofluorescent blood, similar to that found in the human congenital porphyrias. The collection of mutants presented here represent several steps required for normal erythropoiesis. The analysis of these mutants provides a powerful approach towards defining the molecular mechanisms involved in vertebrate hematopoietic development.

Schilling, T. F., Piotrowski, T., Grandel, H., Brand, M., Heisenberg, C.-P., Jiang, Y.-J., Beuchle, D., Hammerschmidt, M., Kane, D. A., Mullins, M. C., van Eeden, F. J. M., Kelsh, R. N., Furutani-Seiki, M., Granato, M., Haffter, P., Odenthal, J., Warga, R. M., Trowe, T. and Nüsslein-Volhard, C. (1996). Jaw and branchial arch mutants in zebrafish I: branchial arches. Development 123, 329-44.

Jaws and branchial arches together are a basic, segmented feature of the vertebrate head. Seven arches develop in the zebrafish embryo (Danio rerio), derived largely from neural crest cells that form the cartilaginous skeleton. In this and the following paper we describe the phenotypes of 109 arch mutants, focusing here on three classes that affect the posterior pharyngeal arches, including the hyoid and five gill-bearing arches. In lockjaw, the hyoid arch is strongly reduced and subsets of branchial arches do not develop. Mutants of a large second class, designated the flathead group, lack several adjacent branchial arches and their associated cartilages. Five alleles at the flathead locus all lead to larvae that lack arches 4-6. Among 34 other flathead group members complementation tests are incomplete, but at least six unique phenotypes can be distinguished. These all delete continuous stretches of adjacent branchial arches and unpaired cartilages in the ventral midline. Many show cell death in the midbrain, from which some neural crest precursors of the arches originate. lockjaw and a few mutants in the flathead group, including pistachio, affect both jaw cartilage and pigmentation, reflecting essential functions of these genes in at least two neural crest lineages. Mutants of a third class, including boxer, dackel and pincher, affect pectoral fins and axonal trajectories in the brain, as well as the arches. Their skeletal phenotypes suggest that they disrupt cartilage morphogenesis in all arches. Our results suggest that there are sets of genes that: (1) specify neural crest cells in groups of adjacent head segments, and (2) function in common genetic pathways in a variety of tissues including the brain, pectoral fins and pigment cells as well as pharyngeal arches.

Trowe, T., Klostermann, S., Baier, H., Granato, M., Crawford, A. D., Grunewald, B., Hoffmann, H., Karlstrom, R. O., Meyer, S. U., Muller, B., Richter, S., Nüsslein-Volhard, C. and Bonhoeffer, F. (1996). Mutations disrupting the ordering and topographic mapping of axons in the retinotectal projection of the zebrafish, Danio rerio. Development 123, 439-50.

Retinal ganglion cells connect to their target organ, the tectum, in a highly ordered fashion. We performed a large-scale screen for mutations affecting the retinotectal projection of the zebrafish, which resulted in the identification of 114 mutations. 44 of these mutations disturb either the order of RGC axons in the optic nerve and tract, the establishment of a topographic map on the tectum, or the formation of proper termination fields. Mutations in three genes, boxer, dackel and pinscher, disrupt the sorting of axons in the optic tract but do not affect mapping on the tectum. In these mutants, axons from the dorsal retina grow along both the ventral and the dorsal branch of the optic tract. Mutations in two genes, nevermind and who-cares, affect the dorsoventral patterning of the projection. In embryos homozygous for either of these mutations, axons from dorsal retinal ganglion cells terminate ventrally and dorsally in the tectum. In nevermind, the retinotopic order of axons along the optic nerve and tract is changed in a characteristic way as well, while it appears to be unaffected in who-cares. Two mutations in two complementation groups, gnarled and macho, affect the anteroposterior patterning of the projection. In these mutants, nasodorsal axons branch and terminate too soon in the anterior tectum. In 27 mutants belonging to six complementation groups, retinal axons do not form normal termination fields. Some implications for models concerning the formation of topographic projections are discussed.

Whitfield, T. T., Granato, M., van Eeden, F. J. M., Schach, U., Brand, M., Furutani-Seiki, M., Haffter, P., Hammerschmidt, M., Heisenberg, C.-P., Jiang, Y.-J., Kane, D. A., Kelsh, R. N., Mullins, M. C., Odenthal, J. and Nüsslein-Volhard, C. (1996). Mutations affecting development of the zebrafish inner ear and lateral line. Development 123, 241-54.

Mutations giving rise to anatomical defects in the inner ear have been isolated in a large scale screen for mutations causing visible abnormalities in the zebrafish embryo (Haffter, P., Granato, M., Brand, M. et al. (1996a) Development 123, 1-36). 58 mutants have been classified as having a primary ear phenotype; these fall into several phenotypic classes, affecting presence or size of the otoliths, size and shape of the otic vesicle and formation of the semicircular canals, and define at least 20 complementation groups. Mutations in seven genes cause loss of one or both otoliths, but do not appear to affect development of other structures within the ear. Mutations in seven genes affect morphology and patterning of the inner ear epithelium, including formation of the semicircular canals and, in some, development of sensory patches (maculae and cristae). Within this class, dog-eared mutants show abnormal development of semicircular canals and lack cristae within the ear, while in van gogh, semicircular canals fail to form altogether, resulting in a tiny otic vesicle containing a single sensory patch. Both these mutants show defects in the expression of homeobox genes within the otic vesicle. In a further class of mutants, ear size is affected while patterning appears to be relatively normal; mutations in three genes cause expansion of the otic vesicle, while in little ears and microtic, the ear is abnormally small, but still contains all five sensory patches, as in the wild type. Many of the ear and otolith mutants show an expected behavioural phenotype: embryos fail to balance correctly, and may swim on their sides, upside down, or in circles. Several mutants with similar balance defects have also been isolated that have no obvious structural ear defect, but that may include mutants with vestibular dysfunction of the inner ear (Granato, M., van Eeden, F. J. M., Schach, U. et al. (1996a) Development 123, 399-413). Mutations in 19 genes causing primary defects in other structures also show an ear defect. In particular, ear phenotypes are often found in conjunction with defects of neural crest derivatives (pigment cells and/or cartilaginous elements of the jaw). At least one mutant, dog-eared, shows defects in both the ear and another placodally derived sensory system, the lateral line, while hypersensitive mutants have additional trunk lateral line organs.