Question

Question 4 4 pts What event CAUSES the dorsal identity of one side of a vertebrate embryo? Endoderm interaction with ectoderm during gastrulation Sperm fusion relocation of vesicles carrying disheveled protein Endoderm interaction with mesoderm during gastrulation Sperm fusion relocation of vesicles carrying b-catenin protein

Answer 1

Correct answer is Option b. Sperm fusion relocation of vesicles carrying dishiveled protein.

Explanation:

Option a. is incorrect since endoderm interaction with the ectoderm during gastrulation occurs only after the specification of Dorso-ventral axis. In addition, the interaction between endoderm and ectoderm results in the specification of the anterior-posterior axis.

Option b. is correct since sperm fusion causes cortical rotation. Cortical rotation is a phenomenon observed in Xenopus eggs which results from a 30 degree rotation and relocation of the dense,yolky vegetal cytoplasm relative to the to the cortex. This produces a gray crescent. Due to cortical rotation, the cortical granules or vesicles are relocated. These cortical vesicles contain dishivelled protein which lead to downstream signalling through Wnt signally pathway. The Dishiveled protein activates GSK - 3 beta Binding Protein (GBP). Generally GBP protein binds to GSK -3 beta protein and inhibits it. GSK -3 beta is now bound and inhibited by GBP. So beta-catenin is set free which carries out downstream signalling and specification of the dorsal side. On the ventral side, due to the absence of cortical vesicles there is no Wnt -signalling , GBP protein is not activated. Due to this GSK - 3 beta protein is active which inhibits beta- catening thus specifying the ventral side.

Option c. is incorrect since Endoderm interaction with Mesoderm during gastrulation leads to the specification of Notochord, connective tissue,etc.

Option d. is incorrect since beta-catenin is present througout the fertilized egg. Beta- catenin is not contained in the cortical vesicles. It is just inhibited on the ventral side and uninhibited on the dorsal side.