5.1 Drosophilla Flashcards
The steps in fly cleavage to cellular blastoderm stage
•fusion of egg and sperm
•nuclear division creating syncytium (cleavage is on the surface of the yolk mass and is syncytial)
•nuncle migrate to periphery of cytoplasm (nuclei migrate to the periphery of the yolk mass, giving a syncytial blastoderm, at about 2 hours post-fertilization)
•syncytial blastoderm
•cellular blastoderm (this cellularizes by three hours post-fertilization to give the cellular blastoderm around the outside of the yolk mass) this is when nuclei get covered by PM to form distinct cells
■ some nuclei become pole cells, which become the
germ cell line at the posterior end of the embryo)
-karokinesis but no cytokinesis
How do sperm enter?
sperm enter the anterior end via a micropyle
Mid-Blastula Transition
After the nuclei have reached the periphery,
characterised by slowing down of nuclear division, cellularisation and increase in RNA transcription…
■ Zygotic gene transcription is enhanced… Maternal-to Zygotic transition.
■ Phase controlled by
■ Ratio of chromatin to cytoplasm
Formation of Cellular blastoderm
- Nuclei migrate to the periphery and undergo change in shape.
- Microtubules surround the nucleus
- Nuclei and microtubules together called
- energids.
- Plasma membrane folds inwards to partition energids forming individual cells
Drosophila gastrulation
- A furrow of presumptive mesoderm folds inwards to form the ventral furrow which then pinches off to become a ventral tube (no dorsal tube)
- the endoderm at each end folds inwards to form the cephalic furrow and the posterior transverse folds respectively
- the ectoderm along the ventral midline forms the germ band, which extends posteriorly, so that it wraps around the dorsal side of the embryo towards the head (confined by the egg case which doesn’t grow)-this retracts later
- segmentation lines begin to appear on the surface of the embryo–can now distinguish the head, thorax, and abdominal segmental regions
Basic 3 part thorax
Prothorax
- one pair of legs
- T1
Mesothrorax
- one pair of legs
- one pair of wins
- T2
Metathorax
- one pair of legs
- one paring blanking organs (halters)
- T3
How many segments on Abdominal
8
Anterior-posterior axis development in oocyte egg chamber
- Oocyte moves in the posterior region within the egg chamber.
- Nucleus moves towards the terminal follicle cells and synthesizes Gurken. Gurken binds to Torpedo (expressed by the terminal follicle cells), and activates PKA, leads to differentiation of follicle cells into posterior follicle cells.
Oogonium
-divides 4 times
-16 cells
-15 nurse cells, 1 oocyte
nurse cells connected to each other
-together everything is the egg chamber: follicle, oocyte, nurse cells
-oocyte always posterior of egg chamber
Axis plan in oocyte
- Bicoid mRNA binds dyenin, (a motor protein, -ve) of the microtubule. Dyenin moves bicoid mRNA to the anterior end.
- Nanos diffuses to the posterior end of the oocyte
Experiment:
Interchange pole ells (germ cell precursors) between wt embryos and wmbeuos from mothers monozygous for a mutation of the torpedo gene
- torpedo diffident germ cells in wt. female –>normal D-V axis
- wt. gern cells in torpedo deficient female –>no D-V axis (entire embryo ventral)
- Torpedo protein required in follicle cells but not oocyte for normal Dorsal-Ventral Patterning of embryo
Dorsal-Ventral polarity in Drosophila Pathway - 10 steps
- Oocyte nucleus travels to anterior dorsal side of oocyte where it localized gurken mRNA
- transplanted turkmen is received by torpedo proteins
3a. torpedo signal causes follicle cells to differentiate to a dorsal morphology
3b. pipe synthesis is inhibited in dorsal follicle cells - turkmen does not diffuse to ventral follicle cells
5a. ventral follicle cells synthesis pipe.
5b. pipe signals sulfate ventral vitelline proteions - sulfated viteline membrane proteions bind gastrulation- defective (GD)
7a. GD cleaves snake to its active form and forms a complex with snake and uncleaved easter proteins
7b. easter protein is cleaved into its active form - clever easter binds to and cleaves Spatzle activated spatzle binds to Toll receptor protein
- Toll activation activates Tube and Pelle which phosphorylate the Cactus protein. Cactus is degraded, releasing it from Dorsal
- Dorsal protein enters the nucleus and centralizes the cell.
Specification of cell fate by gradient of dorsal protein
Dorsal protein accumulates in the nucleus in the ventral side of the embryo whereby these cells become the mesoderm.
Dorsal enters in ventral (forms normal embryo). No dorsal in embryo = dorsalized embryo. Dorsal protein induced to express everywhere, centralized embryo.
What triggers the invagination of the mesodermal cells
- The mesodermal cells at the ventral side invaginate and buckel inwards to form the ventral tube.
- Staining of nuclei with a protein that is marker for mesoderm.
- Fgf8 receptors present in the mesoderm cells, fgf8 ligands expressed by the cells above mesoderm (mesectoerm).
- Fgf8 binds to fgf8 receptors and triggers the invagination of the mesodermal cells within the embryo.
- fgf= fibreglass growth factor
Pre-Gradient: Pattern formation in Drosophila in the syncytial
cytoplasm-by a hierarchy of gene action that sets up
gradients of morphogenetic proteins
- Maternal mRNA molecules, from maternal genes are deposited (even tethered) at the anterior and posterior ends of the egg and are translated into proteins, which diffuse throughout the syncytial blastoderm, setting up a gradients
- These proteins enter the cleavage stage nuclei and activate or repress other maternal or zygotic genes, establishing another round of patterning
- Broad regions are established first, then these are refined to to produce segmental identities
- There is a strict temporal sequence of gene action-a real hierarchy
