The Dorsal protein is a pivotal maternal morphogen in Drosophila embryogenesis that directs the establishment of the dorsal-ventral (D-V) polarity axis. Its activity and localization dictate tissue fate by regulating gene expression in a concentration-dependent manner.

Dorsal Protein Localization and Gradient Formation

Dorsal protein is uniformly distributed in the cytoplasm of early Drosophila embryos but translocates into nuclei preferentially on the ventral side, forming a gradient essential for D-V patterning. This nuclear translocation is governed by Toll receptor signaling, which leads to the phosphorylation and degradation of Cactus, an inhibitor that otherwise retains Dorsal in the cytoplasm.

Statement Analysis

• Statement A: Incorrect. Dorsal protein translocates into the nuclei of embryonic cells, not follicle cells, and gurken deficiency affects dorsal follicle cell fate upstream, not direct nuclear import of Dorsal protein. Gurken mutants cause ventralized embryos due to lack of dorsalizing signal.

• Statement B: Correct. Dorsal protein is found uniformly in the cytoplasm throughout the syncytial blastoderm but forms a nuclear gradient through selective import into ventral nuclei.

• Statement C: Correct. In embryos lacking Cactus, Dorsal is found constitutively in nuclei throughout the embryo, including ventral cells, owing to loss of cytoplasmic retention.

• Statement D: Correct. If Dorsal cannot enter nuclei, dorsal cell specification genes are not transcribed, resulting in loss of proper D-V patterning.

Correct Combinations

Among the options, the correct statements regarding Dorsal protein activity in D-V polarity formation are (2) B and C.