Drosophila Embryogenesis - Dorsal-ventral Axis

Dorsal-ventral Axis

Formation of the Dorsal-Ventral Axis is dependent on a maternally synthesized transcription factor known as dorsal protein. The production of dorsal protein is stimulated by the localization of the embryonic nuclei. The nuclei secretes a protein called Gurken. Gurken inhibits the production of PIPE protein by interacting with Torpedo receptor on flanking oocyte follicle cells. PIPE positive cells are able to secrete dorsal protein and form the ventral side of the egg, while PIPE negative cells do not secrete dorsal protein and form the dorsal side of the egg.

Dorsal induces the transcription of two genes twist & snail while repressing zerknüllt & decapentaplegic.

Intra-membranous dorsal receptor proteins, known as Toll receptors are responsible for transporting dorsal protein into the embryonic nuclei. These Toll receptors are the product of Toll gene, and are uniformly spaced across the embryoinic plasma-membrane.

Since dorsal protein is secreted by PIPE positive-ventral follicular cells of an egg, dorsal protein enters the embryo to the ventral side. Once transported into the nuclei, dorsal protein is most concentrated at the ventral side of the embryo.

This process sets up a gradient differential between the ventral and dorsal side of an immature embryo, the repression or induction of these four genes are differentially regulated. For example;

• At the ventral end of the embryo, blastoderm nuclei exposed to high concentrations of dorsal protein induce the transcription of twist and snail while repressing zerknüllt & decapentaplegic.
• In the middle of the embryo, blastoderm nuclei exposed to mild concentrations of dorsal protein don't express any genes.
• At the dorsal end of the embryo, blastoderm nuclei exposed to little or no dorsal protein express only zerknüllt & decapentaplegic.