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Publications in 1992

K. Arora and C. Nüsslein-Volhard (1992). Altered mitotic domains reveal fate map changes in Drosophila embryos mutant for zygotic dorsoventral patterning genes. Development 114, 1003-24.

The spatial and temporal pattern of mitoses during the fourteenth nuclear cycle in a Drosophila embryo reflects differences in cell identities. We have analysed the domains of mitotic division in zygotic mutants that exhibit defects in larval cuticular pattern along the dorsoventral axis. This is a powerful means of fate mapping mutant embryos, as the altered position of mitotic domains in the dorsoventral pattern mutants correlate with their late cuticular phenotypes. In the mutants twist and snail, which fail to differentiate the ventrally derived mesoderm, mitoses specific to the mesoderm are absent. The lateral mesectodermal domain shows a partial ventral shift in twist mutants but a proportion of ventral cells do not behave characteristically, suggesting that twist has a positive role in the establishment of the mesoderm. In contrast, snail is required to repress mesectodermal fates in cells of the presumptive mesoderm. In the absence of both genes, the mesodermal and the mesectodermal anlage are deleted. Mutations at five loci delete specific pattern elements in the dorsal half of the embryo and cause partial ventralization. Mutations in the genes zerknüllt and shrew affect cell division only in the dorsalmost cells corresponding to the amnioserosa, while the genes tolloid, screw and decapentaplegic (dpp) affect divisions in both the prospective amnioserosa and the dorsal epidermis. We demonstrate that in each of these mutants dorsally placed mitotic domains are absent and this effect is correlated with an expansion and dorsal shift in the position of more ventral domains. The loss of activity in each of the five genes results in qualitatively similar alterations in the mitotic pattern; mutations with stronger ventralizing phenotypes affect increasingly greater subsets of the dorsal cells. Double mutant analysis indicates that these genes act in a concerted manner to specify dorsal fates. The correlation between phenotypic strength and the progressive loss of dorsal pattern elements in the ventralized mutants, suggests that one of these gene products, perhaps dpp, may provide positional information in a graded manner.

R. Geisler, A. Bergmann, Y. Hiromi and C. Nüsslein-Volhard (1992). cactus, a gene involved in dorsoventral pattern formation of Drosophila, is related to the I kappa B gene family of vertebrates. Cell 71, 613-21.

Among the maternally active genes of Drosophila, cactus is the only one whose loss of function mutations specifically produce ventralized embryos. Its product inhibits nuclear translocation of the dorsal morphogen in the dorsal region of the embryo. Here we report the cloning of cactus and the sequencing of its maternal transcript. The identity of our clones was verified by induction of phenocopies with antisense RNA and rescue of the mutant phenotype with sense RNA. cactus is predicted to encode an acidic, cytoplasmic protein with seven ankyrin repeats. The sequence has similarity to the I kappa B proteins that inhibit the vertebrate transcription factor NF-kappa B. In analogy to results obtained with I kappa B and NF-kappa B, bacterially expressed cactus protein can inhibit DNA binding of dorsal protein in vitro.

GenBank accession no.: L04964

K. Isoda, S. Roth and C. Nüsslein-Volhard (1992). The functional domains of the Drosophila morphogen dorsal: evidence from the analysis of mutants. Genes Dev 6, 619-30.

The dorsal (dl) protein is a member of the Rel family of transcription factors. It is distributed in a nuclear concentration gradient along the dorsoventral axis of Drosophila embryos and activates or represses a discrete set of zygotic genes in a concentration-dependent manner. The nuclear uptake of the dl protein is stimulated by products of the dorsal group genes but inhibited by the cactus (cact) product. To analyze the functional domains of the dl protein, we sequenced 11 dl alleles and studied their interaction with cact. Four of these alleles were found to result in carboxy-terminal truncations of the protein. A deletion of 80 carboxy-terminal amino acids abolishes the ability of dl protein to activate the expression of mesodermal genes. Larger deletions also affect the repressor function of dl. However, a protein consisting only of the Rel homologous region still acts as a weak repressor of zerknüllt transcription. A missense mutation in the presumptive DNA-binding domain causes a complete lack-of-function phenotype in trans to a deficiency but exerts a dominant-negative effect in trans to a wild-type copy of dl. These and genetic data with the alleles that produce truncated proteins indicate that dl oligomerizes. The proteins truncated at the carboxy-terminal end show increased levels of nuclear uptake dorsally, but they still respond to the cact-mediated inhibition of nuclear transport. Therefore, carboxy-terminal sequences influence the cytoplasmic retention, although a domain of dl-cact interaction residues in the amino-terminal portion.

S. Schulte-Merker, R. K. Ho, B. G. Herrmann and C. Nüsslein-Volhard (1992). The protein product of the zebrafish homologue of the mouse T gene is expressed in nuclei of the germ ring and the notochord of the early embryo. Development 116, 1021-32.

Embryos mutant for the T gene, in mice, make insufficient mesoderm and fail to develop a notochord. We report the cloning and sequencing of the T gene in the zebrafish (Brachydanio rerio) and show the nuclear localization of the protein product. Both RNA and protein are found in cells of the germ ring, including enveloping layer cells, prior to and during gastrulation of zebrafish embryos. Nuclei of the yolk syncytial layer do not express Zf-T. High levels of expression are maintained throughout early development in the notochord, while in paraxial mesoderm cells the gene is turned off during gastrulation. Exposure of animal cap cells to activinA induces Zf-T expression, as does transplantation into the germ ring.

GenBank accession no.: S57147

F. Sprenger and C. Nüsslein-Volhard (1992). Torso receptor activity is regulated by a diffusible ligand produced at the extracellular terminal regions of the Drosophila egg. Cell 71, 987-1001.

torso encodes a receptor tyrosine kinase (torso) required for anterior and posterior terminal development of the Drosophila embryo. Injecting eggs with in vitro synthesized torso mRNAs revealed that torso activation is governed by an extracellular molecule, presumably the torso ligand, produced at terminal regions of the egg during early embryogenesis. In the absence of torso, the ligand shows no apparent localization, indicating that it is diffusible and normally bound by an excess of torso receptor at the egg poles. Mutant ligand-binding torso proteins can suppress telson formation in a dominant negative manner, suggesting that the ligand is limited in amount. Analysis of torso mutations indicates that torso functions as a tyrosine kinase and that gain-of-function mutations causing ligand-independent activation are located in the extracellular domain.

D. St Johnston and C. Nüsslein-Volhard (1992). The origin of pattern and polarity in the Drosophila embryo. Cell 68, 201-19.

D. Stein and C. Nüsslein-Volhard (1992). Multiple extracellular activities in Drosophila egg perivitelline fluid are required for establishment of embryonic dorsal-ventral polarity. Cell 68, 429-40.

Twelve maternal effect genes (the dorsal group and cactus) are required for the establishment of the embryonic dorsal-ventral axis in the Drosophila embryo. Embryonic dorsal-ventral polarity is defined within the perivitelline compartment surrounding the embryo by the ventral formation of a ligand for the Toll receptor. Here, by transplantation of perivitelline fluid we demonstrate the presence of three separate activities present in the perivitelline fluid that can restore dorsal-ventral polarity to mutant easter, snake, and spätzle embryos, respectively. These activities are not capable of defining the polarity of the dorsal-ventral axis; instead they restore structures according to the intrinsic dorsal-ventral polarity of the mutant embryos. They appear to be involved in the ventral formation of a ligand for the Toll protein. This process requires serine proteolytic activity; the injection of serine protease inhibitors into the perivitelline space of wild-type embryos results in the formation of dorsalized embryos.