Quiz 4 - 103/104 Answer Key 1. Below are 3 general regulatory strategies downstream of β-catenin that allow for organizer formation. For each strategy, name an affected target gene, and describe (briefly!) how that gene’s product then contributes to organizer formation. (3 pts) Strategy Target gene Function 1) Activation chordin, noggin, dkk, frzb BMP or Wnt antagonists 2) Repression 3) Derepression mir-15,16 repress Nodal receptor translation Iroquois, Siamois Iroquois: repress BMP, Wnt transcription Siamois: activate Xnr3, chordin transcription 2. How does VegT contribute to both endoderm and mesoderm tissue differentiation? (2 pts) VegT activates Nodal signaling in the presumptive endoderm. Diffusible Nodal protein then signals to the presumptive mesoderm, as well as the presumptive endoderm. Cells that do not express VegT but receive Nodal signals become mesoderm. Cells that express VegT and receive Nodal signals become endoderm. 3. Name 3 morphogenetic movements that occur during gastrulation, and the cells or tissues that execute these movements. (3 pts) Movement Cells or tissues 1) Epiboly pre-ectoderm 2) Apical constriction to cause invagination bottle cells (endodermal) 3) Convergent extension pre-notochord (trunk/tail org) Spreading migration pre-head mesoderm (head org) Involution pre-meso- and –endoderm (endomesoderm) Vegetal inrolling pre-endoderm (yolky cells) 4. What would you get from a Nieuwkoop recombinant whose vegetal tissue had been injected with Nodal morpholino? (Don’t just give me a phenotype…describe the pathway in 2-3 sentences.)(2pt) Nodal signaling is required for both mesoderm formation and endoderm formation, but not for ectoderm formation. A Nodal MO would greatly reduce the amount of Nodal protein (likely through blocking the translation of Nodal protein). Therefore, levels of active Smad2/3 would be very low everywhere, and a subset of the genes required for mesoderm and endoderm differentiation would not be activated. The embryo would therefore be primarily ectoderm, and would have a “ventralized” phenotype. 4. Regarding posteriorization: If an egg is injected in the ventral side with mRNA for noggin or chordin, it develops a secondary trunk-tail axis, but it doesn’t develop a secondary head containing anterior nervous tissue. Why not? (1-2 sentences) (2pts) Noggin and chordin are BMP antagonists. Low BMP signaling allows for neural ectoderm specification. (This is why you get some posterior neural ectoderm) However, to get anterior neural ectoderm, low Wnt signaling is also required. In order to specify anterior neural ectoderm, an organizer must also secret Wnt antagonists.