Question

Briefly explain how the Dorsal protein is redistributed in the formation of the drosophila embryo and how this redistribution helps to establish the dorsal ventral axis of the early embryo.

Im just kinda confused on this can someone explain it to me?

Answer 1

The Morphogenetic Agent for Dorsal-Ventral Polarity

Dorsal-ventral polarity is established by the gradient of a transcription factor called Dorsal. Unlike Bicoid, whose gradient is established within a syncytium, Dorsal forms a gradient over a field of cells that is established as a consequence of cell-to-cell signaling events.

The specification of the dorsal-ventral axis takes place in several steps. The critical step is the translocation of the Dorsal protein from the cytoplasm into the nuclei of the ventral cells during the fourteenth division cycle. Anderson and Nüsslein-Volhard 1984 isolated 11 maternal effect genes, each of whose absence is associated with a lack of ventral structures (Figure 9.33). The absence of another maternal effect gene, cactus, causes the ventralization of all cells. The proteins encoded by these maternal genes are critical for making certain that the Dorsal protein gets into only those nuclei on the ventral surface of the embryo.† After its translocation, the Dorsal protein acts on cell nuclei to specify the different regions of the embryo. Different concentrations of Dorsal protein in the nuclei appear to specify different fates in those cells.

Figure 9.33

Effect of mutations affecting the distribution of the Dorsal protein. (A) Deformed larva consisting entirely of dorsal cells. Larvae like these developed from the eggs of a female homozygous for a mutation of the snake gene, one of the maternal effect genes involved in the signaling cascade that establishes a gradient of Dorsal in the embryo. (B) Larvae developed from snake mutant eggs that received injections of mRNA from wild-type eggs. These larvae have a wild-type appearance. (From Anderson and Nüsslein-Volhard 1984; photographs courtesy of C. Nüsslein-Volhard.)

The Translocation of Dorsal Protein

The protein that actually distinguishes dorsum (back) from ventrum (belly) is the product of the dorsal gene. The RNA transcript of the mother's dorsal gene is placed in the oocyte by her ovarian cells. However, Dorsal protein is not synthesized from this maternal message until about 90 minutes after fertilization. When this protein is translated, it is found throughout the embryo, not just on the ventral or dorsal side. How, then, can this protein act as a morphogen if it is located everywhere in the embryo?

In 1989, the surprising answer was found (Roth et al. 1989; Rushlow et al. 1989; Steward 1989). While Dorsal protein can be found throughout the syncytial blastoderm of the early Drosophila embryo, it is translocated into nuclei only in the ventral part of the embryo (Figure 9.34A, B).In the nucleus, Dorsal binds to certain genes to activate or suppress their transcription. If Dorsal does not enter the nucleus, the genes responsible for specifying ventral cell types (snail and twist) are not transcribed, the genes responsible for specifying dorsal cell types (decapentaplegic and zerknüllt) are not repressed, and all the cells of the embryo become specified as dorsal cells.

Figure 9.34

Translocation of Dorsal protein into ventral, but not lateral or dorsal, nuclei. (A) Fate map of a cross section through the Drosophila embryo. The most ventral part becomes the mesoderm; the next higher portion becomes the neurogenic (ventral) ectoderm. The lateral and epidermal ectoderm can be distinguished in the cuticle, and the dorsalmost region becomes the amnioserosa, the extraembryonic layer that surrounds the embryo. (B-D) Cross sections of embryos stained with antibody to show the presence of Dorsal protein (dark-stained area). (B) A wild-type embryo, showing Dorsal protein in the ventralmost nuclei. (C) A dorsalized mutant, showing no localization of Dorsal protein in any nucleus. (D) A ventralized mutant, in which Dorsal protein has entered the nucleus of every cell. (A from Rushlow et al. 1989; B-D from Roth et al. 1989, photographs courtesy of the authors.)

This model of dorsal-ventral axis formation in Drosophila is supported by analyses of mutations that give rise to an entirely dorsalized or an entirely ventralized phenotype (see Figures 9.33A and 9.34). In those mutants in which all the cells are dorsalized (as is evident by their dorsal cuticle), Dorsal protein does not enter the nucleus in any cell. Conversely, in those mutants in which all cells have a ventral phenotype, Dorsal protein is found in every cell nucleus.

The signal cascade

Signal from the oocyte nucleus to the follicle cells

If Dorsal protein is found throughout the embryo, but gets translocated into the nuclei of only ventral cells, then something else must be providing asymmetrical cues (Figure 9.35). It appears that this signal is mediated through a complex interaction between the oocyte and its surrounding follicle cells.

Figure 9.35

Schematic representation of the generation of dorsal-ventral polarity in Drosophila. (A) The oocyte develops in an ovarian follicle consisting of 15 nurse cells (which supply maternal proteins and messages to the developing egg) and numerous follicle cells. (B) The nucleus of the oocyte travels to what will become the dorsal side of the embryo. The gurken genes of the oocyte synthesize mRNA that becomes localized between the oocyte nucleus and the cell membrane, where it is translated into Gurken protein. The Gurken signal is received by the receptor protein made by the torpedo gene of the follicle cells. Given the short diffusibility of the signal, only the follicle cells closest to the oocyte nucleus (i.e., the dorsal follicle cells) receive this signal. The signal from the Torpedo receptor causes the follicle cells to take on a characteristic dorsal follicle morphology and (somehow) inhibit the synthesis of Pipe protein. Therefore, this protein is made only by the ventral follicle cells. (C) Pipe modifies an unknown protein (X) and allows it to be secreted from the ventral follicle cells. Nudel protein interacts with this modified factor to split the products of the gastrulation defective and snake genes to create an active enzyme that will split the zymogen form of the Easter protein into an active Easter protease. The Easter protease splits the Spätzle protein into a form that can bind to the Toll receptor (which is found throughout the embryonic cell membrane). Thus, only the ventral cells receive the Toll signal. This signal separates the Cactus protein from the Dorsal protein, allowing Dorsal to be translocated into the nuclei and ventralize the cells. (After van Eeden and St. Johnston 1999.)