Cui

(a) 2-hour embryo

— Single 2n nucleus

(b) Multinucleate syncytium r

| Multiple nuclear divisions create a single multinucleate cell, the syncytium.

| Multiple nuclear divisions create a single multinucleate cell, the syncytium.

^ The nuclei migrate to the periphery of the embryo and divide several more times, creating the syncytial blastoderm.

Syncytial blastoderm

^ The nuclei migrate to the periphery of the embryo and divide several more times, creating the syncytial blastoderm.

Pole nuclei

Pole nuclei

Cellular blastoderm

^ The cell membrane grows around each nucleus, producing a layer of cells that surrounds the embryo. The resulting structure is the cellular blastoderm.

Pole cells

Pole cells

^ Nuclei at one end of the blastoderm develop into pole cells, which become the primordial germ cells.

21.4 Early development of a Drosophila embryo.

The nuclear uptake of Dorsal protein is thought to be governed by a protein called Cactus, which binds to Dorsal protein and traps it in the cytoplasm. The presence of yet another protein, called Toll, can alter Dorsal, allowing it to dissociate from Cactus and move into the nucleus. Together, Cactus and Toll regulate the nuclear distribution of Dorsal protein, which in turn determines the dorsal-ventral axis of the embryo.

Inside the nucleus, Dorsal protein acts as a transcription factor, binding to regulatory sites on the DNA and activating or repressing the expression of other genes (Table 21.2). High nuclear concentration of Dorsal protein (as on the ventral side of the embryo) activates a gene called twist, which causes mesoderm to develop. Low concentrations of

Dorsal

The anterior-posterior and dorsal-ventral axes of the embryo are established.

Anterior

Dorsal

Anterior

Posterior

Ventral

Ventral

Posterior

Early embryo

(b) 10-hour embryo

^ The number and orientation of the body segments are established.

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