1. Genetics of Axis Specification in Drosophila: Dorsal-Ventral Axis Specification
Gilbert - Chapter 9

2. Today’s Goals
Describe the use of a Genetic screen to identify functionality of individual gene products
Identify Eric Weischaus and Christianne Nusslein-Volhard
Identify key molecular players in establishing polarity and segments in various types of embryos
Define the term “positional information”
Describe how evolutionarily conserved genes play similar roles in development of a wide variety of species
Define transcription factor

3. Generating Dorsal-Ventral Pattern in the D. melanogaster embryo
During the genetic screen for fruit fly embryonic genes
Discovered a gene called Dorsal
Mutation is SEVERE - no VENTRAL side!
Maternal Effect gene
Dorsal mRNA Placed in the oocyte by nurse cells

6. Dorsal Protein
Although mRNA is placed in the egg by mother, dorsal does not work like bicoid
When the protein is translated, there is NO gradient formed
Rather, dorsal protein is found throughout the embryo!
How can this serve as positional information to make a ventral side??

7. The Answer! Dorsal is a transcription factor
Where in the cell should it function??

9. Dorsal Protein become Translocated to the Nuclei
For this transcription factor to function, it must enter the nucleus
Although Dorsal protein is expressed throughout the egg cytoplasm, once cells have begun to form . . .
Dorsal protein enters the nuclei of JUST THE VENTRAL-most cells!

10. In the Dorsal Mutant
Dorsal doesn’t enter the nuclei in the ventral cells
No genes to specify “Ventral” are activated
Genes that specify “Dorsal” are not repressed

11. Answers always lead to more questions!
How does Dorsal protein enter ONLY the nuclei of the ventral cells??
When first produced, Dorsal is complexed with a protein called Cactus in the cytoplasm of the developing embryo
As long as Cactus Binds to dorsal, dorsal cannot enter the nuclei

13. How does Cactus release Dorsal?
In response to a cell signaling pathway
What kinds of proteins will be involved here???
Part of the Dorsal - Toll Pathway

14. Ventral Cell
Follicle Cell

15. Let’s take a look the Pathway!
Ventral Follicle cells (nurse cells) release paracrine signals
Pipe along with a protein called “Factor X”
These bind to Nudel outside the cell
In the space between the follicle cells and the ventral embryonic cells:
Nudel enzymatically splits the Gd protein (gastrulation deficient)

16. Ventral Cell
Follicle Cell

17. MORE! Gd cleaves the Snake protein Snake cleaves Easter
Easter cleaves Spätzle
Spätzle is the ligand for the Toll receptor
Binds to Toll
Toll activates Pelle - A protein Kinase
Pelle phosphorylates Cactus
Cacus gets degraded, releasing dorsal
Dorsal enters the nucleus

18. What does Dorsal do once it enters the nucleus?
What are the DOWNSTREAM events that are activated by Dorsal transcription factor?
Cells that express the highest amounts of dorsal in their nuclei become mesoderm!
Helps to initiate gastrulation!

20. Dorsal - Downstream Factors
Dorsal initiates transcription of genes involved in mesoderm specification, and gastrulation
Twist, snail, Fgf-8
OF COURSE, it is always more complicated than just the examples I am showing :)

24. Dorsal-Ventral Patterning Summary
D-V patterning in the fruit fly embryo has similarities to A-P patterning, yet is unique
Uses positional information to specify regions of the embryo
Incorporates cell-cell signaling to convey positional information in some cases