Sox neuro, a new Drosophila Sox gene expressed in the developing central nervous system

We describe the identification and detailed expression pattern of a second Drosophila Sox gene, SoxNeuro (SoxN), highly related to mammalian group B Sox1, 2, 3 genes. SoxN is expressed in a highly dynamic pattern during embyogenesis, being associated with the development of the central nervous system (CNS), from the early steps onwards. We present strong evidence that the early SoxN neuroectoderm expression is controlled by the zygotic dorso-ventral patterning genes (dpp, sog, brk, twi).     

Localized enhancement and repression of the activity of the TGF-beta family member, decapentaplegic, is necessary for dorsal-ventral pattern formation in the Drosophila embryo.

Seven zygotically active genes are required for normal patterning of the dorsal 40% of the Drosophila embryo. Among these genes, decapentaplegic (dpp) has the strongest mutant phenotype: in the absence of dpp, all cells in the dorsal and dorsolateral regions of the embryo adopt fates characteristic of more ventrally derived cells (Irish and Gelbart (1987) Genes Dev. 1, 868-879). Here we describe the phenotypes caused by alleles of another of this set of genes, tolloid, and show that tolloid is required for dorsal, but not dorsolateral, pattern. Extragenic suppressors of tolloid mutations were isolated that proved to be mutations that elevate dpp activity. We studied the relationship between tolloid and dpp by analyzing the phenotypes of tolloid embryos with elevated numbers of the dpp gene and found that doubling the dpp+ gene dosage completely suppressed weak tolloid mutants and partially suppressed the phenotypes of tolloid null mutants. We conclude that the function of tolloid is to increase dpp activity. We also examined the effect of doubling dpp+ gene dosage on the phenotypes caused by other mutations affecting dorsal development. Like tolloid, the phenotypes of mutant embryos lacking shrew gene function were suppressed by elevated dpp, indicating that shrew also acts upstream of dpp to increase dpp activity. In contrast, increasing the number of copies of the dpp gene enhanced the short gastrulation (sog) mutant phenotype, causing ventrolateral cells to adopt dorsal fates. This indicates that sog gene product normally blocks dpp activity ventrally. We propose that the tolloid, shrew and sog genes are required to generate a gradient of dpp activity, which directly specifies the pattern of the dorsal 40% of the embryo.     

Distinct functions of the Tribolium zerknüllt genes in serosa specification and dorsal closure

BACKGROUND: In the long-germ insect Drosophila, a single extraembryonic membrane, the amnioserosa, covers the embryo at the dorsal side. In ancestral short-germ insects, an inner membrane, the amnion, covers the embryo ventrally, and an outer membrane, the serosa, completely surrounds the embryo. An early differentiation step partitions the uniform blastoderm into the anterior-dorsal serosa and the posterior-ventral germ rudiment giving rise to amnion and embryo proper. In Drosophila, amnioserosa formation depends on the dorsoventral patterning gene zerknüllt (zen), a derived Hox3 gene. RESULTS: The short-germ beetle Tribolium castaneum possesses two zen homologs, Tc-zen1 and Tc-zen2. Tc-zen1 acts early and specifies the serosa. The loss of the serosa after Tc-zen1 RNAi is compensated by an expansion of the entire germ rudiment toward the anterior. Instead of the serosa, the amnion covers the embryo at the dorsal side, and later size regulation normalizes the early fate shifts, revealing a high degree of plasticity of short-germ development. Tc-zen2 acts later and initiates the amnion and serosa fusion required for dorsal closure. After Tc-zen2 RNAi, the amnion and serosa stay apart, and the embryo closes ventrally, assuming a completely everted (inside-out) topology. CONCLUSIONS: In Tribolium, the duplication of the zen genes was accompanied by subfunctionalization. One of the paralogues, Tc-zen1, acts as an early anterior-posterior patterning gene by specifying the serosa. In absence of the serosa, Tribolium embryogenesis acquires features of long-germ development with a single extraembryonic membrane. We discuss implications for the evolution of insect development including the origin of the zen-derived anterior determinant bicoid.     

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.     

Activation of the easter zymogen is regulated by five other genes to define dorsal-ventral polarity in the Drosophila embryo.

The product of the Drosophila easter gene, a member of the trypsin family of serine proteases, must be more active ventrally than dorsally to promote normal embryonic polarity. The majority of the easter protein in the embryo is present in the unprocessed zymogen form and appears to be evenly distributed in the extracellular space, indicating that the asymmetric activity of wild-type easter must arise post-translationally. A dominant mutant form of easter that does not require cleavage of the zymogen for activity (ea delta N) is active both dorsally and ventrally. The ea delta N mutant bypasses the requirement for five other maternal effect genes, indicating that these five genes exert their effects on dorsal-ventral patterning solely by controlling the activation of the easter zymogen. We propose that dorsal-ventral asymmetry is initiated by a ventrally-localized molecule in the vitelline membrane that nucleates an easter zymogen activation complex, leading to the production of ventrally active easter enzyme.     

Drosophila Tbx6-related gene, Dorsocross, mediates high levels of Dpp and Scw signal required for the development of amnioserosa and wing disc primordium

Regional differentiation along the dorsoventral (DV) axis of the Drosophila embryo primarily depends on a graded BMP signaling activity generated by Decapentaplegic (Dpp) and Screw (Scw). We have identified triplicated Dpp and Scw target genes Dorsocross1, 2 and 3 (Doc1, 2, 3) that have a conserved T-box domain related to the vertebrate Tbx6 subfamily and act redundantly to induce dorsal structures. Doc genes are expressed in the dorsal region in the early blastoderm. After gastrulation, newly expressed Doc appears in a segmental pattern in the ectoderm. This expression correlates spatially with the second phase of Dpp expression in the ectoderm. Doc expression in the early blastoderm is abolished in either dpp or scw mutant embryos, whereas the ectodermal segmented expression depends only on Dpp. Inactivation of Doc genes with RNAi dramatically affected the development of amnioserosa and wing disc primordia, both of which depend on high levels of BMP signaling, although leg disc primordium, which depends on low levels of BMP, remained intact. Doc1 mRNA expressed in Xenopus embryos induced ventral mesoderm, suppressed activin-induced events and induced Xvent genes, which are analogous to the effects of native Tbx6 and its upstream regulator, BMP-4. These results suggest that the Tbx6 subfamily act in the BMP signaling pathway required for embryonic patterning in both animals.     

Interaction between torso and dorsal, two elements of different transduction pathways in the Drosophila embryo.

Early development of the Drosophila embryo is under the control of some maternal genes responsible for the establishment of its general pattern. Three sets of genes determine the anteroposterior pattern; two distinct systems specify anterior and posterior development and a third one, the terminal system, is responsible for the development of the poles of the embryo. A different set of genes specifies dorsoventral polarity, which is established by the graded activity of the dorsal gene product. Here I analyze the effect of the terminal system on the expression of two zygotic genes involved in dorsoventral pattern, snail and decapentaplegic, and I show that this effect is mediated by a reduction on dorsal activity by the terminal system. Due to the interaction of these two systems, both of which use transmembrane signalling mechanisms, the poles adopt a more dorsalized fate than their counterparts in the middle of the embryo.     

The Integrator Complex Subunit 6 (Ints6) Confines the Dorsal Organizer in Vertebrate Embryogenesis

Dorsoventral patterning of the embryonic axis relies upon the mutual antagonism of competing signaling pathways to establish a balance between ventralizing BMP signaling and dorsal cell fate specification mediated by the organizer. In zebrafish, the initial embryo-wide domain of BMP signaling is refined into a morphogenetic gradient following activation dorsally of a maternal Wnt pathway. The accumulation of β-catenin in nuclei on the dorsal side of the embryo then leads to repression of BMP signaling dorsally and the induction of dorsal cell fates mediated by Nodal and FGF signaling. A separate Wnt pathway operates zygotically via Wnt8a to limit dorsal cell fate specification and maintain the expression of ventralizing genes in ventrolateral domains. We have isolated a recessive dorsalizing maternal-effect mutation disrupting the gene encoding Integrator Complex Subunit 6 (Ints6). Due to widespread de-repression of dorsal organizer genes, embryos from mutant mothers fail to maintain expression of BMP ligands, fail to fully express vox and ved, two mediators of Wnt8a, display delayed cell movements during gastrulation, and severe dorsalization. Consistent with radial dorsalization, affected embryos display multiple independent axial domains along with ectopic dorsal forerunner cells. Limiting Nodal signaling or restoring BMP signaling restores wild-type patterning to affected embryos. Our results are consistent with a novel role for Ints6 in restricting the vertebrate organizer to a dorsal domain in embryonic patterning.