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), memo (mom), no tail (ntl), and dec, that are required for early notochord formation. Mutations in flh and ntl have been described previously, while mom and doe 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 morn 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 Nd and other notochord markers in the axial mesodermal region is disrupted. Nd expression is normal in doc embryos until early semitic 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 al 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 dec, 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.
We thank Bob Riggleman for providing the twist probe prior to publication, William Talbot, Anne Melby, Marnie Halpern and Chuck Kimmel for communicating results prior to publication, Bill Trevarrow for the flhn1 allele, Stefan Schulte-Merker for providing the ntl antibody, and N. H. Patel for providing the Eng antibody (4D9). We thank Klaus Trummler, Frank Uhlmann and Mathias Metz for assistance in the analysis of the ntl alleles, Silke Rudolph for technical assistance, Heike Schauerte for helping with the in situ hybridization, and Joel Wilson and Cornelia Fricke for their help with the fish work, and finally Tanya Whitfield, Francisco Pelegri, Darren Gilmour and Stefan Schulte-Merker for discussion and help with the manuscript.
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Odenthal J, Haffter P, Vogelsang E, et al. Mutations affecting the formation of the notochord in the zebrafish, Danio rerio. Development. 1996;123(1):103-115. doi:10.1242/dev.123.1.103
Odenthal, J., Haffter, P., Vogelsang, E., Brand, M., Van Eeden, F., Furutani Seiki, M., … Nüsslein Volhard, C. (1996). Mutations affecting the formation of the notochord in the zebrafish, Danio rerio. Development. Company of Biologists. https://doi.org/10.1242/dev.123.1.103
Odenthal, Jörg, Pascal Haffter, Elisabeth Vogelsang, Michael Brand, Fredericus Van Eeden, Makoto Furutani Seiki, Michael Granato, et al. “Mutations Affecting the Formation of the Notochord in the Zebrafish, Danio Rerio.” Development. Company of Biologists, 1996. https://doi.org/10.1242/dev.123.1.103.
J. Odenthal et al., “Mutations affecting the formation of the notochord in the zebrafish, Danio rerio,” Development, vol. 123, no. 1. Company of Biologists, pp. 103–115, 1996.
Odenthal J, Haffter P, Vogelsang E, Brand M, Van Eeden F, Furutani Seiki M, Granato M, Hammerschmidt M, Heisenberg C-PJ, Jiang Y, Kane D, Kelsh R, Mullins M, Warga R, Allende M, Weinberg E, Nüsslein Volhard C. 1996. Mutations affecting the formation of the notochord in the zebrafish, Danio rerio. Development. 123(1), 103–115.
Odenthal, Jörg, et al. “Mutations Affecting the Formation of the Notochord in the Zebrafish, Danio Rerio.” Development, vol. 123, no. 1, Company of Biologists, 1996, pp. 103–15, doi:10.1242/dev.123.1.103.
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