Zebrafish acetylcholinesterase is encoded by a single gene localized on linkage group 7. Gene structure and polymorphism; molecular forms and expression pattern during development

We cloned and sequenced the acetylcholinesterase gene and cDNA of zebrafish, Danio rerio. We found a single gene (ache) located on linkage group LG7. The relative organization of ache, eng2, and shh genes is conserved between zebrafish and mammals and defines a synteny. Restriction fragment length polymorphism analysis was allowed to identify several allelic variations. We also identified two transposable elements in non-coding regions of the gene. Compared with other vertebrate acetylcholinesterase genes, ache gene contains no alternative splicing at 5' or 3' ends where only a T exon is present. The translated sequence is 60-80% identical to acetylcholinesterases of the vertebrates and exhibits an extra loop specific to teleosts. Analysis of molecular forms showed a transition, at the time of hatching, from the globular G4 form to asymmetric A12 form that becomes prominent in adults. In situ hybridization and enzymatic activity detection on whole embryos confirmed early expression of the acetylcholinesterase gene in nervous and muscular tissues. We found no butyrylcholinesterase gene or activity in Danio. These findings make zebrafish a promising model to study function of acetylcholinesterase during development and regulation of molecular forms assembly in vivo.

A comparative map of the zebrafish genome

Zebrafish mutations define the functions of hundreds of essential genes in the vertebrate genome. To accelerate the molecular analysis of zebrafish mutations and to facilitate comparisons among the genomes of zebrafish and other vertebrates, we used a homozygous diploid meiotic mapping panel to localize polymorphisms in 691 previously unmapped genes and expressed sequence tags (ESTs). Together with earlier efforts, this work raises the total number of markers scored in the mapping panel to 2119, including 1503 genes and ESTs and 616 previously characterized simple-sequence length polymorphisms. Sequence analysis of zebrafish genes mapped in this study and in prior work identified putative human orthologs for 804 zebrafish genes and ESTs. Map comparisons revealed 139 new conserved syntenies, in which two or more genes are on the same chromosome in zebrafish and human. Although some conserved syntenies are quite large, there were changes in gene order within conserved groups, apparently reflecting the relatively frequent occurrence of inversions and other intrachromosomal rearrangements since the divergence of teleost and tetrapod ancestors. Comparative mapping also shows that there is not a one-to-one correspondence between zebrafish and human chromosomes. Mapping of duplicate gene pairs identified segments of 20 linkage groups that may have arisen during a genome duplication that occurred early in the evolution of teleosts after the divergence of teleost and mammalian ancestors. This comparative map will accelerate the molecular analysis of zebrafish mutations and enhance the understanding of the evolution of the vertebrate genome.

Zebrafish comparative genomics and the origins of vertebrate chromosomes

To help understand mechanisms of vertebrate genome evolution, we have compared zebrafish and tetrapod gene maps. It has been suggested that translocations are fixed more frequently than inversions in mammals. Gene maps showed that blocks of conserved syntenies between zebrafish and humans were large, but gene orders were frequently inverted and transposed. This shows that intrachromosomal rearrangements have been fixed more frequently than translocations. Duplicated chromosome segments suggest that a genome duplication occurred in ray-fin phylogeny, and comparative studies suggest that this event happened deep in the ancestry of teleost fish. Consideration of duplicate chromosome segments shows that at least 20% of duplicated gene pairs may be retained from this event. Despite genome duplication, zebrafish and humans have about the same number of chromosomes, and zebrafish chromosomes are mosaically orthologous to several human chromosomes. Is this because of an excess of chromosome fissions in the human lineage or an excess of chromosome fusions in the zebrafish lineage? Comparative analysis suggests that an excess of chromosome fissions in the tetrapod lineage may account for chromosome numbers and provides histories for several human chromosomes.

Hhex and scl function in parallel to regulate early endothelial and blood differentiation in zebrafish

During embryogenesis, endothelial and blood precursors are hypothesized to arise from a common progenitor, the hemangioblast. Several genes that affect the differentiation of, or are expressed early in, both the endothelial and blood lineages may in fact function at the level of the hemangioblast. For example, the zebrafish cloche mutation disrupts the differentiation of both endothelial and blood cells. The transcription factor gene scl is expressed in both endothelial and blood lineages from an early stage and can regulate their differentiation. Here we report that in zebrafish the homeobox gene hhex (previously called hex) is also expressed in endothelial and blood lineages from an early stage. We find that hhex expression in these lineages is significantly reduced in cloche mutant embryos, indicating that hhex functions downstream of cloche to regulate endothelial and blood differentiation. Ectopic expression of hhex through injection of a DNA construct leads to the premature and ectopic expression of early endothelial and blood differentiation genes such as fli1, flk1 and gata1, indicating that Hhex can positively regulate endothelial and blood differentiation. However, analysis of a hhex deficiency allele shows that hhex is not essential for early endothelial and blood differentiation, suggesting that another gene, perhaps scl, compensates for the absence of Hhex function. Furthermore, we find that hhex and scl can induce each other's expression, suggesting that these two genes cross-regulate each other during early endothelial and blood differentiation. Together, these data provide the initial framework of a pathway that can be used to further integrate the molecular events regulating hemangioblast differentiation.

Zebrafish genes for neuropeptide Y and peptide YY reveal origin by chromosome duplication from an ancestral gene linked to the homeobox cluster

Neuropeptide Y (NPY) and peptide YY (PYY) are related 36-amino acid peptides. NPY is widely distributed in the nervous system and has several physiological roles. PYY serves as an intestinal hormone as well as a neuropeptide. We report here cloning of the npy and pyy genes in zebrafish (Danio rerio). NPY differs at only one to four amino acid positions from NPY in other jawed vertebrates. Zebrafish PYY differs at three positions from PYY from other fishes and at 10 positions from mammals. In situ hybridization showed that neurons containing NPY mRNA have a widespread distribution in the brain, particularly in the telencephalon, optic tectum, and rhombencephalon. PYY mRNA was found mainly in brainstem neurons, as reported previously for vertebrates as divergent as the rat and the lamprey, suggesting an essential role for PYY in these neurons. PYY mRNA was observed also in the telencephalon. These results were confirmed by immunocytochemistry. As in the human, the npy gene is located adjacent to homeobox (hox) gene cluster A (copy a in zebrafish), whereas the pyy gene is located close to hoxBa. This suggests that npy and pyy arose from a common ancestral gene in a chromosomal duplication event that also involved the hox gene clusters. As zebrafish has seven hox clusters, it is possible that additional NPY family genes exist or have existed. Also, the NPY receptor system seems to be more complex in zebrafish than in mammals, with at least two receptor genes without known mammalian orthologues.

Genetic linkage mapping of zebrafish genes and ESTs

Genetic screens in zebrafish (Danio rerio) have isolated mutations in hundreds of genes essential for vertebrate development, physiology, and behavior. We have constructed a genetic linkage map that will facilitate the identification of candidate genes for these mutations and allow comparisons among the genomes of zebrafish and other vertebrates. On this map, we have localized 771 zebrafish genes and expressed sequence tags (ESTs) by scoring single-stranded conformational polymorphisms (SSCPs) in a meiotic mapping panel. Of these sequences, 642 represent previously unmapped genes and ESTs. The mapping panel was comprised of 42 homozygous diploid individuals produced by heat shock treatment of haploid embryos at the one-cell stage (HS diploids). This "doubled haploid" strategy combines the advantages of mapping in haploid and standard diploid systems, because heat shock diploid individuals have only one allele at each locus and can survive to adulthood, enabling a relatively large quantity of genomic DNA to be prepared from each individual in the mapping panel. To integrate this map with others, we also scored 593 previously mapped simple-sequence length polymorphisms (SSLPs) in the mapping panel. This map will accelerate the molecular analysis of zebrafish mutations and facilitate comparative analysis of vertebrate genomes.

Expression of sox11 gene duplicates in zebrafish suggests the reciprocal loss of ancestral gene expression patterns in development

To investigate the role of sox genes in vertebrate development, we have isolated sox11 from zebrafish (Danio rerio). Two distinct classes of sox11-related cDNAs were identified, sox11a and sox11b. The predicted protein sequences shared 75% identity. In a gene phylogeny, both sox11a and sox11b cluster with human, mouse, chick, and Xenopus Sox11, indicating that zebrafish, like Xenopus, has two orthologues of tetrapod Sox11. The work reported here investigates the evolutionary origin of these two gene duplicates and the consequences of their duplication for development. The sox11a and sox11b genes map to linkage groups 17 and 20, respectively, together with other loci whose orthologues are syntenic with human SOX11, suggesting that during the fish lineage, a large chromosome region sharing conserved syntenies with mammals has become duplicated. Studies in mouse and chick have shown that Sox11 is expressed in the central nervous system during development. Expression patterns of zebrafish sox11a and sox11b confirm that they are expressed in the developing nervous system, including the forebrain, midbrain, hindbrain, eyes, and ears from an early stage. Other sites of expression include the fin buds and somites. The two sox genes, sox11a and sox11b, are expressed in both overlapping and distinct sites. Their expression patterns suggest that sox11a and sox11b may share the developmental domains of the single Sox11 gene present in mouse and chick. For example, zebrafish sox11a is expressed in the anterior somites, and zebrafish sox11b is expressed in the posterior somites, but the single Sox11 gene of mouse is expressed in all the somites. Thus, the zebrafish duplicate genes appear to have reciprocally lost expression domains present in the sox11 gene of the last common ancestor of tetrapods and zebrafish. This splitting of the roles of Sox11 between two paralogues suggests that regulatory elements governing the expression of the sox11 gene in the common ancestor of zebrafish and tetrapods may have been reciprocally mutated in the zebrafish gene duplicates. This is consistent with duplicate gene evolution via a duplication-degeneration-complementation process.

A recessive mutation leading to vertebral ankylosis in zebrafish is associated with amino acid alterations in the homologue of the human membrane-associated guanylate kinase DLG3

We describe the characterization of the zebrafish homologue of the human gene DLG3. The zebrafish dlg3 gene encodes a membrane-associated guanylate kinase containing a single PDZ domain. This gene was cloned using a gene-trap construct inserted in the gene's first intron. The insertion co-segregates with a viable mutation called humpback (hmp), which leads to formation of ankylotic vertebrae in adult fishes. Insertion and mutation have both been mapped to chromosome 12, in a segment which is syntenic with region p12 to q12 of human chromosome 17. The hmp mutant phenotype, however, appears to be due to two point mutations in the guanylate kinase domain rather than to the transgene insertion itself. The results of this study are discussed in the light of the possible function of the guanylate kinase domain.

Neuropeptide Y receptor subtype with unique properties cloned in the zebrafish: the zYa receptor

Neuropeptide Y (NPY) belongs to a family of structurally related neuroendocrine peptides for which five different G-protein-coupled receptor subtypes have been cloned in mammals. To identify additional subtypes we have performed PCR with degenerate primers in different species. We describe here the cloning and pharmacological profile of a unique NPY receptor subtype in the zebrafish that has tentatively been called the zYa receptor. It has 46-50% amino acid identity to the mammalian Y1, Y4 and y6 receptors and the previously cloned zebrafish receptors zYb and zYc, and only about 27% to Y2 and Y5. The zYa receptor binds NPY and PYY from mammals as well as zebrafish with high affinities and has a K(d) of 28 pM for porcine (125)I-PYY. It has a unique binding profile displaying some features in common with each of the mammalian Y1, Y2 and Y5 receptors. In a microphysiometer assay the receptor responds with extracellular acidification. Chromosomal mapping in the zebrafish genome of zYa, zYb and zYc receptor genes indicates a possible orthologous relationship between zYc and mammalian y6, but identifies no obvious mammalian ortholog for zYa (zYb is a recent copy of zYc in the fish lineage). These results imply that previous studies of NPY in fishes, which have striven to interpret the effects within the framework of mammalian Y1, Y2, and Y5 receptors, need to be reevaluated. Thus, the sequence comparisons, pharmacological properties, and chromosomal localization suggest that the zYa receptor is a novel NPY receptor subtype which is likely to be present also in mammals.

The BMP-related protein radar: a maintenance factor for dorsal neuroectoderm cells?

We have previously cloned several members of the TGF-beta superfamily of growth factors in zebrafish, one of which, Radar, belongs to the Dpp-Vg1-related (DVR) subgroup, with highest homology to GDF6. The pattern of expression of Radar suggested a possible involvement in several induction steps during embryogenesis including in the dorsal neural tube, red blood cells, the dorsal fin and the retina. We have analyzed the pattern of expression of Radar in comparison with that of a marker of dorsal neural tube structures, msxC and show that Radar and msxC are expressed in similar and/or adjacent tissues throughout embryogenesis. In order to demonstrate a functional relationship between these two proteins, we have generated a full-length cDNA for Radar and shown that Radar overexpression by DNA injection maintains expression of msxC in tissues where it is normally expressed then turned off, in particular in the dorsal neurectoderm. Study of the phenotype of a mutant carrying a deletion of Radar shows a loss of identity and death of the cells of the dorsal neural tube. Taken together these results suggest that Radar could be involved in maintaining the identity of cells of the dorsal-most neural tube and of at least a subset of neural crest cells.

Preservation of duplicate genes by complementary, degenerative mutations

The origin of organismal complexity is generally thought to be tightly coupled to the evolution of new gene functions arising subsequent to gene duplication. Under the classical model for the evolution of duplicate genes, one member of the duplicated pair usually degenerates within a few million years by accumulating deleterious mutations, while the other duplicate retains the original function. This model further predicts that on rare occasions, one duplicate may acquire a new adaptive function, resulting in the preservation of both members of the pair, one with the new function and the other retaining the old. However, empirical data suggest that a much greater proportion of gene duplicates is preserved than predicted by the classical model. Here we present a new conceptual framework for understanding the evolution of duplicate genes that may help explain this conundrum. Focusing on the regulatory complexity of eukaryotic genes, we show how complementary degenerative mutations in different regulatory elements of duplicated genes can facilitate the preservation of both duplicates, thereby increasing long-term opportunities for the evolution of new gene functions. The duplication-degeneration-complementation (DDC) model predicts that (1) degenerative mutations in regulatory elements can increase rather than reduce the probability of duplicate gene preservation and (2) the usual mechanism of duplicate gene preservation is the partitioning of ancestral functions rather than the evolution of new functions. We present several examples (including analysis of a new engrailed gene in zebrafish) that appear to be consistent with the DDC model, and we suggest several analytical and experimental approaches for determining whether the complementary loss of gene subfunctions or the acquisition of novel functions are likely to be the primary mechanisms for the preservation of gene duplicates. For a newly duplicated paralog, survival depends on the outcome of the race between entropic decay and chance acquisition of an advantageous regulatory mutation. Sidow 1996(p. 717) On one hand, it may fix an advantageous allele giving it a slightly different, and selectable, function from its original copy. This initial fixation provides substantial protection against future fixation of null mutations, allowing additional mutations to accumulate that refine functional differentiation. Alternatively, a duplicate locus can instead first fix a null allele, becoming a pseudogene. Walsh 1995 (p. 426) Duplicated genes persist only if mutations create new and essential protein functions, an event that is predicted to occur rarely. Nadeau and Sankoff 1997 (p. 1259) Thus overall, with complex metazoans, the major mechanism for retention of ancient gene duplicates would appear to have been the acquisition of novel expression sites for developmental genes, with its accompanying opportunity for new gene roles underlying the progressive extension of development itself. Cooke et al. 1997 (p. 362)

Isolation of zebrafish gdf7 and comparative genetic mapping of genes belonging to the growth/differentiation factor 5, 6, 7 subgroup of the TGF-beta superfamily

The Growth/differentiation factor (Gdf) 5, 6, 7 genes form a closely related subgroup belonging to the TGF-beta superfamily. In zebrafish, there are three genes that belong to the Gdf5, 6, 7 subgroup that have been named radar, dynamo, and contact. The genes radar and dynamo both encode proteins most similar to mouse GDF6. The orthologous identity of these genes on the basis of amino acid similarities has not been clear. We have identified gdf7, a fourth zebrafish gene belonging to the Gdf5, 6, 7 subgroup. To assign correct orthologies and to investigate the evolutionary relationships of the human, mouse, and zebrafish Gdf5, 6, 7 subgroup, we have compared genetic map positions of the zebrafish and mammalian genes. We have mapped zebrafish gdf7 to linkage group (LG) 17, contact to LG9, GDF6 to human chromosome (Hsa) 8 and GDF7 to Hsa2p. The radar and dynamo genes have been localized previously to LG16 and LG19, respectively. A comparison of syntenies shared among human, mouse, and zebrafish genomes indicates that gdf7 is the ortholog of mammalian GDF7/Gdf7. LG16 shares syntenic relationships with mouse chromosome (Mmu) 4, including Gdf6. Portions of LG16 and LG19 appear to be duplicate chromosomes, thus suggesting that radar and dynamo are both orthologs of Gdf6. Finally, the mapping data is consistent with contact being the zebrafish ortholog of mammalian GDF5/Gdf5.

Ectopic expression of hoxb2 after retinoic acid treatment or mRNA injection: disruption of hindbrain and craniofacial morphogenesis in zebrafish embryos

To investigate pattern formation in the vertebrate hindbrain, we isolated a full length hoxb2 cDNA clone from zebrafish. In a gene phylogeny, zebrafish hoxb2 clusters with human HOXB2, and it maps on linkage group 3 along with several other loci whose orthologues are syntenic with human HOXB2. In the hindbrain, hoxb2 is expressed at high levels in rhombomere 3 (r3), lower levels in r4, still lower in r5, and at undetectable levels in r6. In r7, r8, and the rostral spinal cord, hoxb2 is expressed at a lower level than in r5. Lateral cells appearing to emanate from r4 express both hoxb2 and dlx2, suggesting that they are neural crest. Overexpression of hoxb2 by mRNA injections into early cleavage stage embryos resulted in abnormal morphogenesis of the midbrain and rostral hindbrain, abnormal patterning in r4, fusion of cartilage elements arising from pharyngeal arches 1 and 2, and ectopic expression of krx20 and valentino (but not pax2, rtk1, or hoxb1) in the rostral hindbrain, midbrain, and, surprisingly, the eye. Treatments with retinoic acid produced a phenotype similar to that of ectopic hoxb2 expression, including ectopic krx20 (but not valentino) expression in the eye, and fusion of cartilages from pharyngeal arches 1 and 2. The results suggest that hoxb2 plays an important role in the patterning of hindbrain and pharyngeal arches in the zebrafish.

Zebrafish hox clusters and vertebrate genome evolution

HOX genes specify cell fate in the anterior-posterior axis of animal embryos. Invertebrate chordates have one HOX cluster, but mammals have four, suggesting that cluster duplication facilitated the evolution of vertebrate body plans. This report shows that zebrafish have seven hox clusters. Phylogenetic analysis and genetic mapping suggest a chromosome doubling event, probably by whole genome duplication, after the divergence of ray-finned and lobe-finned fishes but before the teleost radiation. Thus, teleosts, the most species-rich group of vertebrates, appear to have more copies of these developmental regulatory genes than do mammals, despite less complexity in the anterior-posterior axis.

Treatment planning optimization by quasi-Newton and simulated annealing methods for gamma unit treatment system

The gamma unit is used to irradiate a target within the brain. During such a treatment many parameters, including the number of shots, the coordinates, the collimator size and the weight associated with each shot, affect the amount of dose delivered to the target volume and to the surrounding normal tissues. Hence it is not easy to determine an appropriate set of these parameters by a trial and error method. For this reason, we present here an optimization method to determine mathematically those parameters. This method is composed of two steps: firstly, a quasi-Newton method is used to deal with the continuous variables such as position and weight of shots; the result obtained at the end of this step then serves as the initial configuration for the next step, in which a simulated annealing method is applied to optimize all the aforementioned parameters. Application of the proposed methods to two examples shows that our optimization algorithm runs in a satisfactory way.

Intrasynaptosomal free calcium and nitric oxide metabolism in central nervous system oxygen toxicity

BACKGROUND

Central nervous system (CNS) oxygen (O2) toxicity is complex, and the etiology of its most severe manifestation, O2 convulsions, is yet to be determined. A role for nitric oxide (NO) has been proposed, although recent data have indicated that NO is synthesized from L-arginine by an enzyme, NO synthase (NOS). The enzyme is dependent on free calcium (Ca2+) concentration, therefore increases in intracellular Ca2+ may constitute the physiological and pathophysiological mechanisms for stimulating the synthesis of NO.

METHODS

In this study, the intrasynaptosomal free calcium concentration ([Ca2+]i) was measured by the fluorescence of fura-2/AM, and cGMP (as an indirect marker of NO levels) was by radioimmunoassay (RIA) in the rat hippocampus after hyperbaric oxygen (HBO) exposure. We also investigated the effects of daurisoline (DSL, calcium channel blocker) and N-nitro-L-arginine (LNNA, NOS inhibitor) on the above biochemical parameters and the development of oxygen toxicity.

RESULTS

The results show that when the rats were exposed to HBO at 0.5 MPa the intrasynaptosomal Ca2+ and cGMP levels increased by two and three times, respectively, whereas with the use of DSL prior to HBO, the accumulation of [Ca2+]i and cGMP dropped to 56% and 60%, correspondingly. In the rats medicated with LNNA prior to HBO. [Ca2+]i and cGMP levels dropped to 70% and 36% of the HBO group. At the same time, the appearance of CNS oxygen toxicity was delayed and the survival rate increased. The protective effects of LNNA were reversed by L-arginine pretreatment. These findings suggest that the neuronal Ca2+ overload during HBO exposure is a major factor in the pathogenesis of CNS O2 toxicity, and cGMP-NO pathways may be directly involved in HBO-induced seizures.

Mutant rescue by BAC clone injection in zebrafish

Genes essential for vertebrate body plan specification, organ development, and organ function are likely to be shared between mammals and zebrafish, but only in zebrafish have large-scale, genome-wide mutagenesis screens been conducted to isolate embryonic lethal mutations. Discovering the roles played by these disrupted genes requires their molecular characterization, which would be facilitated by assaying large cloned genomic DNAs for their potential to rescue mutant phenotypes. Here we demonstrate that bacterial artificial chromosomes can rescue the phenotype of floating head (flh) mutants. Homozygous flh embryos lack a differentiated notochord and have a reduced, discontinuous floor plate. Mutant embryos injected with genomic clones containing the flh+ gene often had stretches of several to many notochord cells overlaid by a row of floor-plate cells. In contrast, control mutant embryos injected with artificial chromosomes lacking the flh+ gene failed to form notochord. We conclude that the injection of large-insert genomic clones will speed the isolation of zebrafish genes disrupted by mutation and hence the identification of gene functions necessary for development of vertebrate embryos.

Vertebrate genome evolution and the zebrafish gene map

In chordate phylogeny, changes in the nervous system, jaws, and appendages transformed meek filter feeders into fearsome predators. Gene duplication is thought to promote such innovation. Vertebrate ancestors probably had single copies of genes now found in multiple copies in vertebrates and gene maps suggest that this occurred by polyploidization. It has been suggested that one genome duplication event occurred before, and one after the divergence of ray-finned and lobe-finned fishes. Holland et al., however, have argued that because various vertebrates have several HOX clusters, two rounds of duplication occurred before the origin of jawed fishes. Such gene-number data, however, do not distinguish between tandem duplications and polyploidization events, nor whether independent duplications occurred in different lineages. To investigate these matters, we mapped 144 zebrafish genes and compared the resulting map with mammalian maps. Comparison revealed large conserved chromosome segments. Because duplicated chromosome segments in zebrafish often correspond with specific chromosome segments in mammals, it is likely that two polyploidization events occurred prior to the divergence of fish and mammal lineages. This zebrafish gene map will facilitate molecular identification of mutated zebrafish genes, which can suggest functions for human genes known only by sequence.

Genetic interactions in zebrafish midline development

Mutational analyses have shown that the genes no tail (ntl, Brachyury homolog), floating head (flh, a Not homeobox gene), and cyclops (cyc) play direct and essential roles in the development of midline structures in the zebrafish. In both ntl and flh mutants a notochord does not develop, and in cyc mutants the floor plate is nearly entirely missing. We made double mutants to learn how these genes might interact. Midline development is disrupted to a greater extent in cyc;flh double mutants than in either cyc or flh single mutants; their effects appear additive. Both the notochord and floor plate are completely lacking, and other phenotypic disturbances suggest that midline signaling functions are severely reduced. On the other hand, trunk midline defects in flh;ntl double mutants are not additive, but are most often similar to those in ntl single mutants. This finding reveals that loss of ntl function can suppress phenotypic defects due to mutation at flh, and we interpret it to mean that the wild-type allele of ntl (ntl+) functions upstream to flh in a regulatory hierarchy. Loss of function of ntl also strongly suppresses the floor plate deficiency in cyc mutants, for we found trunk floor plate to be present in cyc;ntl double mutants. From these findings we propose that ntl+ plays an early role in cell fate choice at the dorsal midline, mediated by the Ntl protein acting to antagonize floor plate development as well as to promote notochord development.

[Study on the organization of chromatin and chromosome in mouse spermatogenic cells by scanning electron microscopy]

It remains unclear about the intermediate construction of chromosome due to its highly compact nature and the limitation in methods. The present study was designed to investigate the construction of chromatin and mitotic chromosome in situ with scanning electron microscopy. Mouse testes were selected as the material, because of in which the spermatogenic cells divide actively and successively to form the sperm. Such a feature would be able to study the structure of mammalian chromatin and chromosomes along with the change of nuclear cycle. The animal were perfused with 200 ml of 0.075 mol/L KCl hypotonic solution to remove blood and placed for 15-20 min on ice followed by 0.5% glutaraldehyde and 0.5% formaldehyde for fixing. Through treated by the routine process of fractured and freeze dried with t-butyl alcohol, the specimens were then coated with a 3 nm thick platinum and observed with Hitachi S-430 scanning electron microscopy. It was found that the hypotonic treatment with 0.075 mol/L KCl solution was suit for demonstrating the nuclear structure, when the organelles were well preserved. The chromatin fibers of 10-30 nm and 80-125 nm in diameter could be recognized in the interphase nuclei, which were arranged losely at the region of euchromatin, and folded with each other into chromatin masses at the region of heterochromatin, while the chromatin fibers with the diameter of 80-125 nm often could be viewed on the mitotic chromosomes. Since its presence in interphase nuclei and mitotic chromosomes, it was considered that the chromatin fibers with 80-125 nm in diameter might play a role in the condensation of chromosome, serve as a type of the intermediate structure.

valentino: a zebrafish gene required for normal hindbrain segmentation

Mutational analysis can serve both to identify new genes essential for patterning embryonic development and to determine their functions. Here we describe the identification and phenotypic characterization of alleles of valentino, which we recovered in a genetic screen that sought to identify mutations in the zebrafish that disrupt region-specific gene expression patterns in the embryonic brain. valentino is required for normal hindbrain segmentation and the hindbrain of valentino mutant embryos is shortened by the length of one rhombomere. We demonstrate that valentino is required cell-autonomously in the development of rhombomeres 5 and 6, and propose that valentino functions in the subdivision and expansion of a common precursor region in the presumptive hindbrain into the definitive rhombomeres 5 and 6. These results provide genetic evidence for a two-segment periodicity in the hindbrain and suggest that this periodicity arises sequentially, through the specification and later subdivision of a two-rhombomere unit, or ‘protosegment’.

Holoprosencephaly and chromosomal anomalies

Holoprosencephaly is a rare cerebral malformation resulting from failure or incomplete cleavage of the forebrain. The sonographic diagnosis consists of monoventricle, fused thalami and absent cavum septum pellucidi. Chromosomal anomalies, diabetes mellitus, alcohol, autosomal recessive inheritance and toxins have been implicated. We describe seven cases of holoprosencephaly diagnosed in the antenatal and postnatal periods. The chromosomal anomalies included trisomy 13, triploidy, trisomy 13 with an unbalanced 13; 14 translocation and isochromosome of the long arm of 18. The clinicopathological findings and chromosomal anomalies are correlated.