22. The following statements regarding the generation of dorsal/ventral axis in Drosophila was made:
    A. Gurken protein moves along with the oocyte nucleus and signals follicle cells to adopt the ventral fate.
    B. Maternal deficiencies of either the gurken or torpedo gene cause ventralization of the embryo.
    C. Gurken is active only in the oocytes, Torpedo is active only in the somatic follicle cells.
    D. The Pipe protein is made in the dorsal follicle cells.
    E. The highest concentration of Dorsal is in the dorsal cell nuclei, which becomes the mesoderm.
    Which one of the following combination of the above statements is true?
    (1) A and E (2) C and D
    (3) B and C (4) B and E

The formation of the dorsal-ventral (D-V) axis in Drosophila melanogaster embryos is a vital early developmental event that establishes the body plan along the back-to-belly orientation. This axis formation is governed by a series of molecular signals involving maternal genes and proteins that coordinate oocyte and follicle cell interactions.

The Role of Gurken and Torpedo

During oogenesis, the oocyte nucleus migrates to the anterior-dorsal side of the oocyte. It produces the Gurken protein, a transforming growth factor-alpha (TGF-α) homologue. Gurken is secreted locally and signals to the adjacent somatic follicle cells by binding to the Torpedo receptor, an EGF receptor on these cells. This signaling induces the follicle cells to adopt a dorsal fate, thus establishing the future dorsal side of the embryo.

Effects of Maternal Mutations

Maternal deficiencies in either the gurken or torpedo genes disrupt this dorsal fate specification, resulting in embryos that are ventralized—that is, their dorsal structures fail to develop properly. This underlines the importance of the Gurken-Torpedo signaling axis in D-V patterning.

Pipe Protein Expression in Ventral Follicle Cells

Pipe, a sulfotransferase, is expressed specifically in the ventral follicle cells—those that do not receive the Gurken signal. Pipe modifies components of the eggshell, creating a ventral cue that is essential for subsequent activation of the Toll signaling pathway in the embryo, which transduces the D-V axis information inside the embryonic nuclei.

Dorsal Protein Gradient and Mesoderm Formation

The maternal transcription factor Dorsal is uniformly present in the cytoplasm but its nuclear localization is restricted to ventral nuclei due to Toll signaling. The highest concentrations of Dorsal protein accumulate in ventral nuclei, specifying mesodermal cell fate. Dorsal forms a ventral-to-dorsal gradient influencing gene expression along the D-V axis, governing tissue differentiation.

Evaluation of Statements

• Statement A: Incorrect. Gurken signals follicle cells to adopt a dorsal, not ventral, fate.

• Statement B: Correct. Maternal mutations in gurken or torpedo cause loss of dorsal structures, leading to ventralization.

• Statement C: Correct. Gurken is active in oocytes; Torpedo is active in somatic follicle cells.

• Statement D: Incorrect. Pipe protein is produced in ventral, not dorsal, follicle cells.

• Statement E: Incorrect. The highest concentration of Dorsal is in ventral nuclei that give rise to mesoderm, not dorsal nuclei.

Correct Combination

Given the analysis, the correct combination is (3) B and C.