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

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

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

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??

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

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!

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

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

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

Ventral Cell Follicle Cell

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)

Ventral Cell Follicle Cell

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

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!

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 :)

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