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.