DORSAL-VENTRAL AXIS FORMATION IN DROSOPHILA

DORSAL-VENTRAL AXIS FORMATION IN DROSOPHILA

 4.7.        Dorsal-Ventral axis formation:

After fertilization, the nucleus of zygote moves to future dorsal surface. The nucleus transcribes the Gurken mRNA in the dorsal side. Gurken mRNA translate into Gurken protein. This Gurken protein is secreted by oocytes. Gurken protein work as the ligand for torpedo receptor present on the dorsal follicular cell. Torpedo receptor blocks the pipe protein synthesis after binding with Gurken protein. This pipe is not formed at the dorsal surface. Pipe gene expression gets inhibited by torpedo protein at dorsal side, pipe gene plays important role in ventralization of the oocyte surface. Ventralization of the embryo occurs if the mutation or maternal deficiencies take place within either the gurken or the torpedo gene. Through the transplantation experiment, two results come. First, the torpedo mutant eggs were able to produce normal embryos if they transplant into wild type ovary. Second, wild type egg develops mutant, ventralized embryos if they transplanted into the mother's egg chamber, which is torpedo mutant.

4.8.         Ventral axis formation:

Only the ventral follicle cells were able to make the pipe. A protein torpedo does not block pipe synthesis because gurken protein is absent in ventral surface. Slightly later stage of development unknown protein (x) gets modified by pipe protein and both of them get the bind and get secreted from the ventral follicle cells. Pipe activates the Nudel protein. Nudel protein is secreted by ventral embryonic cells and present in the perivitelline fluid.

Three serine proteases which are the product of gastrulation defective (gd), snake (snk), and easter (en) genes and these are secreted within inactive form into the perivitelline fluid, get activated in a cascade manner by nudel.

Activated Nudel activates the Gastrulation-defective protease. The cleaved Gd works as protease and cleaved the snake protein. The cleaved snake works as protease and becomes active. The cleaved snake cleaved the ester protein. Nudel cleaved the inactive Gd. Cleaved Easter protease cause the cleavage of the Spatzle protein. The ventral portion of the embryo is the place where the cleavage of these three proteases is limited.

Spatzle protein is now able to bind to its receptor, which is toll protein (maternal product) present in the oocyte cell membrane throughout and it is the product of the toll gene. Thus, Toll signal only gets receive by the ventral cell. When Spatzle binds to and activates the Toll protein, Toll cause the activation of Pelle, which is a protein kinase. Pelle with the help of Tube, get bring on to the plasma membrane and Tube also get activated here. In the cytoplasm of the syncytial blastoderm, Dorsal-Cactus complex is present. Pelle a protein Kinase cause the phosphorylation of Cactus, as a result, Cactus undergo ubiquitinylation and get degraded. Dorsal this causes the release of ventral side and Dorsal protein moves to nuclei of the ventral part of the embryo. It is a transcription factor, cause the transcription of the gene specifying ventral cell types, as a result, surface get ventralizes.

About 90 minutes after fertilization synthesis of Dorsal protein takes place and present throughout the syncytial blastoderm of Drosophila’s early embryo, as dorsal protein produce instantly form complex with cactus, which is also present in the cytoplasm of the syncytial blastoderm.


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