Drosophila - patterning the embryo

Question 1

DV polarity

Answer 1

Gradient of nuclear localisation of the dorsal protein:
- High on ventral side
- Low on dorsal side
Promoters of important DV genes require diff levels dorsal protein to be activated
Diff levels nuclear D lead to diff genes expressed
Promoters of twist and snail genes have low affinity dorsal binding sites, only expressed when high level D present, make mesoderm
Rhomboid has high affinity dorsal binding sites in promoter, and is repressed by twist/snail proteins, expressed laterally on both sides of mesoderm, make neuroectoderm
In dorsal ectoderm lots of things act together in complex manner to set up second signalling centre on dorsal side embryo
Signal conserved in vertebrates, knows as BMP signal

Question 2

AP patterning

Answer 2

AP gradient of Bicoid and Nanos lead to activation number genes encoding transcription factors know as gap genes
Primary target is Hunchback gene, which with Bicoid sets up expressiong of other GAP genes
GAP genes create combinatory code defines different regions in embyro

Question 3

Gap genes responsible for striped pair-rule expression

Answer 3

even skipped (Eve) and fushi tarazu (ftz)
Striped expression the prefigures segments of embryo not driven by periodic wave-like process, each stripe driven by specific gap gene combination
PR gene expression stripes slightly shifted relative to visible segments
E.g. anterior part of even skipped expression stripe forms posterior of larval segment
PR gene expression stripes know as para segments

Question 4

Segmentation genes elaborate pair rule pattern into segmental pattern

Answer 4

High level Eve and Ftz drive expression transcription factor, leads to formation of 14 stripes one in each segment
The “14 stripped expression of transcription regulator Engrailed (En) and other genes directly driving pattern of cuticle, e.g. the denticles that form stripy pattern on larvae
Future visible segments formed with Engrailed positive cells forming posterior boundary of segment, by this time embryo has cellularised
As well as En two signaling proteins expressed in adjected cells:
- Wingless (Wg) and Hedgehog (Hh)
- First under influence of pair rule genes
- Then as result feedback loop expression of Wg is maintained by Hh and En+Hh by Wg

Question 5

Wingless and headgehog

Answer 5

Are protein signals that act across cell boundaries
After discovery, precise pathways they use to influence transcription in Drosophila, it found they also important in other MCO including humans
Reduced or aberrant activity leads to variety congenital defects
Inappropriate activation is basis many types cancer
Wingless can cause colon cancer Hh can cause Basal Cell Carcinoma

Question 6

What makes segments look different to each other

Answer 6

Homeotic selector genes - HOX genes
Discovered through bizarre mutation in Drosophila:
- Homeotic mutations - one structure replaced with another
- Antennapedia mutation - when antenna replaced by legs
- Bithorax mutation - when there two sets wings instead of one, and no balancing organs
In most organisms Hox genes single complex, in drosophila broken into 2 pieces
Order on genome reflect spatial and timing of expression
Zebrafish have 7 HOX complexes
Vertebrates have 4 HOX complexes
The bithorax complex is responsible for diversification of posterior segment (this might need to be its on card)

Question 7

Summary

Answer 7

D/V patterning
ventrolateral side; nuclear gradient of Dorsal sets up 2 domains
Mesoderm and neuroectoderm
Dpp acts in a complex manner on the dorsal side (conserved in Vertebrates)
A/P patterning
Gap genes create a unique combinatory code
This code drives striped expression of PAIR RULE genes
Pair-Rule genes activate Engrailed, Hedgehog, and Wingless in 14 stripes.
Hh and Wnt are expressed in adjacent cells and maintain each other’s expression.
Segment identity is specified by the highly conserved Homeotic Selector genes, also known as Hox genes which are localised in a complex