Lecture 7: Invertebrate early development

Question 1

Describe the role of maternal genes in patterning of the embryo.

Answer 1

Even the oocyte is patterned: bicoid in anterior end and Oskar in posterior end => positional info already present. Bicoid is a DNA binding transcriptional activator, maternally loaded into the anterior end of the developing oocyte. When the egg is laid, mRNA is translated into proteins and diffused out (no membranes).

Question 2

What is the experimental evidence for Bicoid protein.

Answer 2

• Bicoid LOF mutants lack anterior structures.
• This can be rescued by transplantation of WT cytosol because there is no genetic material in the cytosol at this point.
• If cytosol from the anterior is transplanted into the middle of the egg, head-like structures appear.

Complete Bicoid KO => no morphogen gradient => patterning of 5 most posterior segments present but appear more anterior
Increased morphogen = gradient extends towards the posterior. Empty structure at the anterior because Bicoid concentration so high that it inhibits the differentiation of the first stripe.

Question 3

Describe the main results of the saturation mutagenesis screen.

Answer 3

In a screen where a saturation mutagenesis identified the genes in patterning and development of the larva almost 27000 lines were identified.
In the screen they found three major classes of phenotype:
1 = knirps mutation = one big stripe fused segmentation meaning large chunk of the body is missing. Head connects to tail. These are called gap genes because there’s a gap in the structure.
2 = paired mutation = every second segment pair (naked cuticle and denticle belts) is gone. 4 segments instead of 8. Pair rule genes.
3 = gooseberry mutation = the naked cuticle that goes between denticle belts has gone. Polarity in segment has been destroyed. Segment polarity genes.

Question 4

Explain the role of gap genes in the Drosophila embryo.

Answer 4

_Gap Genes (embryo still acellular)_
Gap genes are the first zygotic genes to be expressed along the A-P axis and all code for TFs. Expression of gap genes is dependent on A-P gradient of Bicoid protein. Bicoid primarily activated the anterior gap gene Hunchback which then switches on other gap genes.
Transient gradients of transcription factors are formed which all have different affinities for different target genes, and gap gene proteins act in combination with other TFs.

Question 5

Explain the role of pair rule genes in the developing Drosophila embryo.

Answer 5

Pair Rule Genes

Downstream of Gap genes = pair rule genes = every second segment. The first signs of segmentation present as grooves on the surface of the embryo after gastrulation which form 14 parasegments. 7 transverse stripes of pair-rule gene expression delimit parasegments.

Each stripe is controlled by a complex system of gap gene activity. All the factors work together to produce a single stripe of gene expression. Every segment is controlled by different combos of maternal and gap genes.

Individual promotor controls each stripe

Question 6

Explain the role of segment polarity genes in the developing embryo.

Answer 6

Downstream of pair rule genes are segment polarity genes. Segment polarity = within each segment there is a posterior naked cuticle and an anterior hairy denticle belt. This involves Wnt and Hh signalling – when mutated cause loss of the naked cuticle, leaving only the hairy denticle belt. I

Hh AND Wg are needed to specify smooth cuticle and hairy denticle:

• Hh signalling occurs in cells that sit next to one cell that is competent to express Wg at the parasegment boundary
• Hh released by posterior cells induce naked cuticle and activate expression of Wg in adjacent cell
• Wg secreted and feeds back to Hh-expressing cells to sustain Hh expression
• Autoregulatory feedback loop between Wg and Hh expressing cells to create two distinct identities
• This is why both loss of Wg and Hh cause the same phenotype (loss of naked cuticle)

Question 7

Explain the role of hox genes.

Answer 7

Downstream of segment polarity genes is Hox genes which provide fate information to each segment along the A-P axis, in the same order as the genes are in the genome. Each segment has a unique identity. They are controlled by a combination of gap and pair rule genes. Each are TF and contain a homeobox binding domain.

Homologous genes found in many organisms with similar functions and an overall function to specify identity along the AP axis.

Question 8

State and explain the different type of insect with examples.

Answer 8

Long germ band = all segments defined at once by maternal gap and pair-rule genes e.g., Drosophila. Embryogenesis is very quick. The blastoderm corresponds to the whole of the future embryo. An evolutionary advanced group of insects.

Short/intermediate germ band = blastoderm is short and forms only the anterior segments e.g., centipede. Start with head made by a proto gap gene & thoracic segment, probably via an ancestral version of the system Drosophila now use. Add abdominal segments sequentially via a segmentation clock. Posterior disc (proctodeum) appears to bud off segments as it gets smaller. Moderate complexity and not too slow. Likely to represent the original ‘ancestral’ segmentation mechanism. Intermediate germ band insects have a few proto gap genes.

Question 9

Explain how segmentation occurs in Strigamia maritima.

Answer 9

Notch clock = cycling system stimulating the pathway.

• Proctodaeum have clear rings of Delta (ligand) and Notch (receptor) in the notch signalling pathway – this starts from a ‘dot’ and a wave of gene expression expands out from the dot
• Notch activated => expression of target genes, especially Her
• Her protein translated which switches on off its own transcription as well as delta translation
• Level of delta ligand decreases => level of activation of neighbour cells decreases => less Her made => no inhibition of delta => delta increases => cycle continues

The notch clock system means one cell has active notch and the other has delta = oscillations of signal going up and down until equilibria. The system causes segmentation definition.