The Toll cascade is a regulatory pathway that controls the dorsal-ventral axis f

Question

The Toll cascade is a regulatory pathway that controls the dorsal-ventral axis formation during Drosophila embryonic development. Genetic analysis of mutants in three genes (toll, cactus, and dorsal) of the cascade has shown the following. 1) toll mutant embryos are dorsalized (lacking ventral structures). 2) cactus mutant embryos are ventralized (lacking dorsal structures). 3) dorsal mutant embryos are dorsalized. 4) toll, cactus double mutant embryos are ventralized. 5) cactus, dorsal double mutant embryos are dorsalized. Based on these observations, the pathway (from upstream to downstream) should be ordered as:

a) cactus-toll-dorsal

b) toll-dorsal-cactus

c) dorsal-cactus-toll

d) toll-cactus-dorsal

e) dorsal-toll-cactus

Explanation / Answer

d) toll-cactus-dorsal

Dorsal-ventral polarity is established by the gradient of a transcription factor called Dorsal. The critical step is the translocation of the Dorsal protein from the cytoplasm into the nuclei of the ventral cells during the fourteenth division cycle. The absence of another maternal effect gene, cactus, causes the ventralization of all cells. The protein that actually distinguishes dorsum (back) from ventrum (belly) is the product of the dorsal gene. When this protein is translated, it is found throughout the embryo, not just on the ventral or dorsal side. While Dorsal protein can be found throughout the syncytial blastoderm of the early Drosophila embryo, it is translocated into nuclei only in the ventral part of the embryo
If Dorsal does not enter the nucleus, the genes responsible for specifying ventral cell types (snail and twist) are not transcribed, the genes responsible for specifying dorsal cell types (decapentaplegic and zerknüllt) are not repressed, and all the cells of the embryo become specified as dorsal cells.
In Drosophila, the Toll protein binds the signal from the Spätzle protein and activates the kinase region of the Pelle protein. The Pelle protein phosphorylates Cactus and Dorsal, causing the two proteins to separate from each other. The Dorsal protein can then enter the nucleus and regulate the transcription of ventrally specific genes.

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