Lecture 2: DV patterning

Question 1

How does the egg develop?

Answer 1

• Nurse cells are left at the anterior while they pump their contents into the oocyte.
• Follicle cells migrate over the oocyte.
• After 13 cellular divisions, the membranes move from the outer membrane to enclose each separate nucleus. This forms a layer of cells over the embryo called the cellular blastoderm.
• Pole cells form at the posterior pole.

Question 2

What is gurken? How does it work?

Answer 2

• In contrast the egg is already polarised for Drosophila.
• Microtubules move the oocyte nucleus to the anterior of the oocyte cell membrane.
• The nurse cells create gurken mRNA. It is moved to the anterior of the oocyte.
• Grk protein is made. It does not diffuse much and it signals to the follicle cells which take on dorsal fates. They become more columnar dorsal follicle cells.
• Deficiencies of Gurken or Torpedo cause ventralization of the embryo. Gurken is active in the oocyte, whereas torpedo is active in somatic follicle cells.
• Gurken protein activates follicle cells by interacting with the Torpedo receptor.
• Torpedo activates mirror, a TF which represses the pipe gene. Pipe is therefore only made in the ventral follicle cells.

Question 3

What is pipe? How does it work?

Answer 3

• The ventral vitelline envelope is sulphated by the pipe protein.
• Vitelline membrane like (VML) is one of the targets.
• Gastrulation-defective protein binds to the vitelline envelope.
• GD cleaves snake into its active form. GD then forms a complex with Snake and uncleaved Easter.
• Easter is cleaved into its active form.
• Easter then cleaves Spatzle.
• Spatzle cleavage is limited to the most ventral portion of the embryo. This is done with a protease inhibitor from the follicle cells from the ovary. Spatzle is found throughout the blastoderm.
• Spatzle activates Toll, a transmembrane protein found in all cells.

Question 4

How does the dorsal-ventral axis form in embryos?

Answer 4

An axis also needs to be established in the embryo.
• Dorsal is a TF that activates genes for the ventral axis.
• The mRNA for Dorsal is distributed by the nurse cells.
• Dorsal may be found everywhere, but it is only translocated into the nucleus in the ventral side.
• Dorsal is first produced and complexed with Cactus, which prevents nuclear translocation.
• Toll activates a kinase called Pelle when it is turned on. Pelle phosphorylates Cactus. Cactus becomes degraded.
• Dorsal enters the nucleus and ventralises the cell.
• Different concentrations of Dorsal specify different fates for cells.
• Dorsal activates genes for the mesodermal phenotype (nervecord, muscles etc) and proper gastrulation. For example, the genes twist and snail.
• Dorsal represses dorsalising genes such as dpp and zen.
• Sog diffuses into the dorsal region and antagonizes the activity of Dpp, providing the position of the dorsal region.