8. Invertebrate early development II: The drosophila body plan/segmentation

slide 8

The Heidelberg screen identified both maternal and zygotic screens required for body patterning

several mutations possibly relating to each other i.e in smae gene or not similar in any way

rewatch lecture on this part about mutations leading to death or survival

complementation grouos - how many genes required basically to build the embryo

Maternal vs zygotic genes

• What are maternal genes?

• What are zygotic genes?

• How do they differ?

• How did they pattern the embryo?

Different classes of patterning genes

knirps mutations - large amount of body plan missing - gap genes

paired mutations - larvae missing repeated segments - pair rule genes

gooseberry mutations - segment polarity genes

Maternal genes lay down the coordinates

polarised already in the egg

Maternal genes Gap genes Pair rule genes Segment polarity genes

when u dont have torso - mutant embryo lack ends - no head/backend

Bicoid is a maternal gene which determines anterior

Bicoid is a DNA binding transcriptional activator - maternally loaded into developing oocyte

Classic experiments which show that Bicoid acts as a morphogen

whatever this gene is it must be in the cytoplasm

- normal development

- loss of anterior structures

- partial rescue

- ectopic head structures and mirror image thoracic segments

Linking Bicoid morphogen activity to segmentation

were able to show without bicoid. 1 copy of bicoid and 4 copies of bicoid (expansion of anterior region)

Able to find bicoid is a DNA binding transcription factor

• high affinity binding sites - activated at lower threshold conc of bicoid

• low affinity

Maternal genes activate ’Gap genes’

Read maternal gene gradients to define broad ‘blocks’ or domains of gene expression

Gap genes activate ‘Pair rule genes’

Expression of pair rule genes is controlled stripe by stripe (!)

Dependent on the interaction of positively and negatively acting transcriptional regulators (many of which are gap genes)

Patterning is further refined through ‘Segment polarity genes’

Parasegments lay the coordinates for future segments

wingless (wg)

hedgehog (hh)

hedgehogs receptor is ‘patched’ - produces wingless

secretion factors - when cell recieves wingless it creates more hedgehog - feedback onto each other to maintiain their expression

Hh and Wg feedback onto each other to maintain each others expression and refine segment borders

when engrailed gets turned on it stays turned on - called a selector gene - important for determining cell types

Interactions between Hh, Wg and Eg establish parasegment boundaries – controlling denticle pattern

Hh maintains Wg expression which suppresses denticle development

Selector genes give segments identity – informing where things like legs and arms will go

Hox genes / Homeotic genes

Provide ‘who am I’ information to each segment. Expression of homeotic genes along the a/p body axis occurs in the same order as the genes are within the genome. Controlled by a combination of gap and pair-rule genes Homeobox containing, DNA binding, transcription factors

antennapedia

gap genes ——> selector genes

How does this work in other animals ?

Short, intermediate and long germ band insects… and centipedes!#

Drosophila is a ‘long germ band’ insect - all 14 segments are defined at once

Quick - embryogenesis complete in just 24 hours

Complicated - maternal, gap, pair rule genes interact for every segment

Short and intermediate germ band insects

Tribolium (beetle)

Start with head & thoracic segments - probably via an ancestral version of the system Drosophila now uses

Add abdominal segments sequentially - 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

watch videos - wildtype vs Tc-TI7+10^RNAi

Segment addition in Strigamia maritima

Segment addition in Strigamia maritima – clock mechanism

Delta (ligand)

Notch (receptor)

Adjacent stripes of Delta and Hes4 set up feedback loop necessary for oscillation

her = Hes4

The Segmentation clock

Feedback loops - Notch activation causes down regulation of Notch ligand - time lag in response causes oscillation between strong and weak signalling levels - propagation of signal between cells causes wave of activation

Zebrafish somitogenesis Aei= after eight A mutant for Delta From Liao et al, 2016

Segmentation and Evolution

Segmentation in Vertebrates

zebrafish

chick

short germ band insect

The majority of known candidate pacemaker genes lie in the Notch pathway

Zebrafish DeltaC in situ

Evolution of Segmentation

- Notch / Delta segmentation clock

- ancient evolutionary ‘idea’