The Location of Dorsal Information in Frog Early Development

Dorsal information is necessary for the development of the dorsal axial structures which characterize the vertebrates. The nature of dorsal information in early embryos is not known, but its presence is required for the formation of dorsal mesoderm with Spermann organizer activity. In frogs, the dorsoventral axis is specified by a cortical/cytoplasmic rotation in the egg shortly after fertilization, and this dorsal information is limited to a few cells in the equatorial and vegetal region of early cleavage embryos. At the 8-cell stage, 2–4 cells can promote dorsal development, and at the 32-cell stage, 4–6 cells have dorsal information. Recent experiments have shown that growth factors can induce cells to form dorsal mesoderm and that lithium can act in those cells to enhance the induction. It will be important to determine the relationship between the location of dorsal information defined embryologically and factors involved in the development of dorsal mesoderm.     

The control of cell fate along the dorsal-ventral axis of the Drosophila embryo.

We have analyzed the contributions made by maternal and zygotic genes to the establishment of the expression patterns of four zygotic patterning genes: decapentaplegic (dpp), zerknüllt (zen), twist (twi), and snail (sna). All of these genes are initially expressed either dorsally or ventrally in the segmented region of the embryo, and at the poles. In the segmented region of the embryo, correct expression of these genes depends on cues from the maternal morphogen dorsal (dl). The dl gradient appears to be interpreted on three levels: dorsal cells express dpp and zen, but not twi and sna; lateral cells lack expression of all four genes; ventral cells express twi and sna, but not dpp and zen. dl appears to activate the expression of twi and sna and repress the expression of dpp and zen. Polar expression of dpp and zen requires the terminal system to override the repression by dl, while that of twi and sna requires the terminal system to augment activation by dl. The zygotic expression patterns established by the maternal genes appear to specify autonomous domains that carry out independent developmental programs, insofar as mutations in the genes that are expressed ventrally do not affect the initiation or ontogeny of the expression patterns of the genes that are expressed dorsally, and vice versa. However, interactions between the zygotic genes specific to a particular morphological domain appear to be important for further elaboration of the three levels specified by dl. Two of the genes, dpp and twi, are unaffected by mutations in any of the tested zygotic dorsal-ventral genes, suggesting that dpp and twi are the primary patterning genes for dorsal ectoderm and mesoderm, respectively.     

A role for maternal beta-catenin in early mesoderm induction in Xenopus

Mesoderm formation results from an inducing process that requires maternal and zygotic FGF/MAPK and TGFbeta activities, while maternal activation of the Wnt/beta-catenin pathway determines the anterior-dorsal axis. Here, we show a new role of Wnt/beta-catenin signaling in mesoderm induction. We find that maternal beta-catenin signaling is not only active dorsally but also all around the equatorial region, coinciding with the prospective mesoderm. Maternal beta-catenin function is required both for expression of dorsal genes and for activation of MAPK and the mesodermal markers Xbra and eomesodermin. beta-catenin acts in a non- cell-autonomous manner upstream of zygotic FGF and nodal signals. The Wnt/beta-catenin activity in the equatorial region of the early embryo is the first example of a maternally provided mesoderm inducer restricted to the prospective mesoderm.     

Altered mitotic domains reveal fate map changes in Drosophila embryos mutant for zygotic dorsoventral patterning genes.

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.     

Dorsal Blastomeres in the Equatorial Region of the 32-Cell Xenopus Embryo Autonomously Produce Progeny Committed to the Organizer

For testing the autonomic differentiation abilities of dorsal equatorial blastomeres of 32-cell Xenopus embryos, their roles in head formation in normal development and the organizer-inducing capabilities of the dorsal-most vegetal cells, interspecific transplantations were made using Xenopus borealis and X. laevis . When transplanted into the ventral region, the dorsal blastomeres produced descendants that differentiated into prechordal mesoderm, notochord and somites in the recipient according to their fates. They induced formation of the secondary embryo with the head and tail. The prechordal mesoderm and notochord in the secondary structure consisted of progeny of the graft, whereas somites and the CNS were chimeric and the pronephros was composed of host cells. Replacement of the dorsal blastomeres by ventral equatorial cells caused complete arrest of head formation in the recipient. Without exception, the notochord was completely absent or very thin. These results confirm the assumption that the presumptive head organizer in the Xenopus embryo is localized in the dorsal equatorial region at the 32-cell stage and comes into existence not under the inductive influence of the dorsal-most vegetal cells, but owing to allocation of morphogenetic determinants residing in the fertilized egg to the dorsal equatorial blastomeres of the 32-cell embryo.