Lecture 7- Development of the Drosophila body plan
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Briefly outline the segmentation of Drosophila
• Segmentation occurs very quickly from the unpatterned egg and further grooves develop between segments. • Segmentation is easily observed in drosophila larvae where they have 8 clear abdominal sections • Additional segments in the head and thorax • Each thoracic segment has a pair of legs attached to it
Why is complementation testing completed?
To determine if two mutations associated with a specific phenotype represent two different forms of the same genes (alleles) or are variations of two different genes
What occurs if a mutation occurs in the same genes in complementation testing?
Phenotype was observed due to failure to complement
What occurs if a mutation occurs in different genes in complementation testing?
No phenotype was observed as complementation occurred
What were the 3 major classes of phenotype discovered in the drosophila complementation testing?
• Knirps • Paired • Goosebury
Describe the Knirps phenotype and the type of gene that causes it
• Normal head and tail but only one abdominal stripe • Large middle chunk is missing • Type of gap gene
Describe the Paired phenotype and the type of gene that causes it
• Every second segment pair is missing * Type of pair-rule gene
Describe the Gooseberry phenotype and the type of gene that causes it
• Naked cuticle which goes between the denticle belts is missing • Belts are fused together • Polarity between the segments has been destroyed • Type of segment polarity gene
Outline the order of genes involved in segmentation
• Maternal genes • Gap genes • Pair rule genes • Segment polarity genes (however these do not directly feed into the process) • Selector genes
Give examples of 3 maternal genes, there location and what happens if mutated
• Bicoid: located anteriorly so anterior structures are missing when mutated • Nanos: located posteriorly so tip remains but most posterior structures are missing when mutated • Torso: located in the middle, when mutated anterior and posterior are missing by torso remains
Describe the expression of Bicoid
• Bicoid is a DNA binding transcriptional activator which is maternally loaded into the developing oocyte • Bicoid mRNA is deposited into the anterior part of the embryo by the nurse cells • When the egg is laid, bicoid mRNA is translated into protein which diffuses throughout the syncytial blastoderm
Describe the phenotype when an anterior cytosol from a wild type egg is transplanted to the anterior end of a bicoid mutant egg
The anterior structures are partially rescued
Describe the phenotype when an anterior cytosol from a wild type egg is transplanted into the middle of a bicoid mutant egg
Ectopic head structures are formed and a mirror image of the thoracic segment forms either side of the central head
What happens to the drosophila segmentation pattern when there are 0 gene copies of bicoid?
No bicoid protein so the patterning has been pushed to the anterior side
What happens to the drosophila segmentation pattern when there is 1 gene copy/wiltype of bicoid?
Clear gradient of bicoid protein so segmentation patterning is normal
What happens to the drosophila segmentation pattern when there are 4 gene copies/overexpression of bicoid?
More bicoid protein anteriorly so the bicoid gradient extends further. The segmentation patterning is pushed more posteriorly and the high levels of bicoid anteriorly inhibits the differentiation/formation of head structures
How are low and high affinity binding sites activated by different concentrations of bicoid?
High affinity binding sites: activated at lower thresholds of biocoid Low affinity binding sites: activated/require high concentrations of bicoid
Describe the expression of gap genes
• Expression of gap genes is dependant on the maternal genes * Gap genes read maternal gene gradients to define broad blocks/domains of gene expression
Describe the expression of pair rule genes
• Expression of pair rules genes is controlled stripe by stripe • Each stripe is controlled by a complex system (e.g bicoid/giant/hunchback/kruppel) which work together to produce a singe stripe of gene expression • Expression is dependant on the interaction of positively and negatively acting acting transcriptional regulators (e.g gap genes)
Describe the expression of segment polarity genes
• Repeated pattern where each half of each segment is deleted and a mirror-image is duplicated and reversed to replace the missing half segment
Describe the expression of Wnt and Hh as an example of segment polarity genes
• Both Wnt and Hh cause loss of naked cuticle and replaced with hairy cuticle • Hh and Wnt feedback to each other to maintain each others expression and refine segment borders • In the posterior end, Hh is on and is secreted and diffuses to the new cells anterior to it • Hh causes the expression of Wnt. Wnt is secreted and diffuses backwards to the more posterior cell to maintain Hh • Hh maintains Wnt expression which surporsess denticle development • Wnt diffuses anteriorly, where there is Wnt there is naked cuticle, as the distance away increases, hairs start to develop and get bigger
Which genes pass information and enable the expression of hox/selector genes?
Gap genes and pair rule genes
Describe the expression of hox/selector genes
• Provide the ‘who am I’ information to each segment • Expression of hox genes along the A/P body axis occurs in the same order as genes that are within the genome • Encode for DNA binding TFs
Is drosophila a long or short germ band insect and why?
• Long germ band as all 14 segments are defined at once * Provides an explanation for why drosophila embryogenesis occurs so quickly