Dorsal-ventral (D-V) axis specification in Drosophila embryogenesis is an intricate process involving coordinated signaling between the oocyte and surrounding follicle cells. A critical component is the Gurken protein, which mediates dorsal follicle cell fate determination via the Torpedo receptor. Mutations or deficiencies in the maternal gurken gene disrupt this system, leading to altered molecular event patterns.

Impact of Gurken Deficiency on Dorsal-Ventral Specification

In a wild-type embryo, Gurken signals dorsal follicle cells via Torpedo receptor activation, preventing expression of ventralizing factors such as Pipe. However, in a gurken-deficient embryo, the failure of this dorsalizing signal causes the entire follicle cell epithelium to adopt a ventral fate. Consequently, the Pipe protein is synthesized ubiquitously across follicle cells, including presumptive dorsal regions.

Key Molecular Events in Gurken-Deficient Presumptive Dorsal Side

• Torpedo receptor activation (A) is absent on the dorsal follicle cells due to lack of Gurken signal.

• Pipe synthesis (B) occurs ventrally normally but expands dorsally because of failed dorsal specification.

• Cascade of protease activity (C) is triggered downstream of Pipe modifications to eggshell components, activating the Toll signaling pathway in the embryo.

• Cactus dephosphorylation (D) is part of the intracellular response to Toll activation. However, in ventalized embryos, Cactus is degraded rather than dephosphorylated, allowing Dorsal to enter nuclei.

• Entry of Dorsal protein into nuclei (E) marks activation of the D-V transcriptional program; in gurken mutants, Dorsal enters nuclei across the entire circumference due to uniform Toll activation, eliminating dorsal identity and causing ventralized phenotype.

Correct Combination for Gurken-Deficient Embryo’s Presumptive Dorsal Side

Since Torpedo is not activated (A is absent), Pipe synthesis (B), protease cascade (C), and Dorsal nuclear entry (E) occur dorsally in a gurken-deficient embryo. Cactus undergoes degradation due to constitutive Toll signaling rather than dephosphorylation; thus, D is not part of the correct events.

Hence, the correct combination of events is (3) B, C, and E only.