Lecture 9 Flashcards
What are the two main problems faced during segmentation
Organism needs to increase from one cells to many cells and makes these cells different from each other
Segmentation is highly conserved in all organisms, T or F
T
Where is segmentation easily visible in the Drosophila embryo
IN the anterior abdomen
When does segmentation occur in Drosophila embryos
Around 24 hours after fertilisation
The Drosophila embryo must set up all of the body axes in order to undergo segmentation, T or F
F – the anterior-posterior and dorsoventral axes have already been partially established in the oocyte by the adult fly. The embryo just needs to define and redefine this pattern
What was the contribution of Christiane Nüsslein-Volhard and Eric Wischaus to the study of Drosophila
They undertook a saturation genetic screen to identify all the genes involved in development and patterning of the larval cuticle. This lead to the identification of 4332 embryonic lethal mutations and 139 complementation groups involved in patterning
Most genes in development require both alleles to be functional to be expressed properly and provide normal function (haploinsufficient), T or F
F – most genes involved in development are haplosufficient
Describe how complementation testing can be implemented to determine if mutations lie in the same gene or different ones
If you cross two parent individuals that both have the same mutant phenotype you can determine if these mutations lie in the same gene or different genes. If the progeny produced by breeding these two heterozygotes do not show the mutant phenotype, then the mutations are said to complement each other. If 25% of the progeny do show the mutant phenotype, then the mutations of the parents must lie in the same gene and thus fail to complement each other
Recall the hierarchy of genes that dictate patterning in the Drosophila embryo
Maternal genes–> Gap genes–> Paired-rule genes–> Segment polarity genes
What type of gene is gooseberry
Segment polarity gene
What type of gene is bicoid
Maternal gene
What type of gene are knirps, giant, Krupel and tailess
Gap genes
Paired and fushi tarazu are hox genes, T or F
F – they are paired-rule genes
Where is paired expressed in the Drosophila embryo
In alternating parasegments
Gooseberry is expressed in alternating parasegments, T or F
F – it is expressed in all parasegments
Genes higher up the patterning hierarchy have a greater influence over segmentation, T or F
T
Give an example of another maternal gene, other than bicoid and explain where its expressed and its role in development
Nanos is another maternal gene. It is expressed in posterior embryo and is responsible for patterning the posterior larvae
What is seen in bicoid mutants and why is this
Bicoid mutants develop without head structures. This is because bicoid is at its highest concentration at the anterior end and dictates formation of the head structures
What is unique about bicoid and its role in development
Bicoid is a morphogen but is also in itself a transcription factor, unlike other maternal and patterning genes that are usually just transcription factors.
What is the role of bicoid in establishing the anterior-posterior axis prior to fertilisation
Bicoid mRNA is deposited in the anterior oocyte by the adult female fly in a process called maternal loading. This leads to the localisation of bicoid protein at the anterior region of the embryo
Describe the bicoid gradient in the early Drosophila embryo
Bicoid has its highest concentration at the anterior region of the embryo and drops away towards the middle
What unique feature of the Drosophila embryo allows the bicoid gradient to be established easily and accounts for the simple early patterning
At this stage in development the Drosophila embryo is a syncytial blastoderm whereby many nuclei are contained in the same cytoplasm. This allows the bicoid protein to diffuse easily through the embryo and establish a gradient easily
Explain the transplantation experiments carried out on the Drosophila embryo and what they showed
Transplantation of wild type Drosophila embryo cytoplasm into a bicoid mutant was sufficient to rescue some of the head structures. Transplantation of wild type cytoplasm to an ectopic site in the middle of the embryo lead to a duplication of the embryo with head structures developing in the middle flanked by thoracic segments either side. This shows that bicoid is both necessary and sufficient to dictate head structure formation
What happens if both copies of the bicoid gene are mutated, why is this
If both copies of bicoid are mutated then the embryo will try to pattern normally using other maternal genes that are functional such as nanos. However the resultant patterning would still be abnormal
What is the effects of forced overexpression of bicoid
Shunting of the segments towards the posterior end
With morphogens, the ligand doesn’t act as the signal to dictate differentiation as such but it is in fact the gradient of the morphogen that encodes information on cell fate, T or F
T
How can bicoid expression be visualise in situ
Fusing reporter genes for chaperones such as Hsp70 or genes easily stained for such as LacZ to the promoter sequence for bicoid allows easy visualisation. Wherever bicoid is expressed the reporter gene will be to as its under the control of the same promoter sequence. These reporter genes can then be easily visualised
What attribute of morphogens accounts for there ability to be used to carry out multiple different functions
The there are different affinity sites for transcription factor morphogens such as bicoid. Some sites in the DNA have low affinity for the transcription factor ligand and so will only elicit binding at really high concentrations. However, other sites will have high affinity and will always elicit binding of the morphogen regardless of concentration
What is the role of gap genes
Subdivide the embryo into different parts once the basic pattern is established
What is significant about gap gene interactions
The gap genes actually also interact and repress each other as well as their targets
Hunchback is a gap gene that regulates points in the embryo where other gap genes are switched on, what is significant about its expression with regards to other maternal gene(s)
Hunchback expression directly mirrors bicoid
How are the paired-rule genes expressed in the Drosophila embryo
Expressed in alternating parasegments whereby their expression is controlled stripe by stripe
What is paired-rule gene expression dependant on
Interactions of positively and negatively acting transcriptional regulators, many of which are gap genes
Even-skipped is a paired-rule gene only expressed in parasegments 3. What conditions are required for even-skipped expression
Very low concentrations of giant and kruppel, high concentrations of hunchback and a little bit of bicoid
Describe the expression pattern of segment polarity genes
Segment polarity genes are expressed in all 14 parasegments
What has happened in the Drosophila embryo by the time that the segment polarity genes are expressed, and what are the implications of this
Cellularisation has occurred meaning that signalling pathways dictating patterning and development now need to get more complicated. Factors used for signalling now need to be secreted
Engrailed is a segment polarity gene that is always expressed in the very anterior part of the segment, T or F
F – this was initially thought to be the case but in fact, engrailed is aways expressed most posteriorly in the segment
What common phenotype is seen in wingless and hedgehog mutants in Drosophila and why is this
Both mutants are said to have a lawn of denticles in the larvae. This is because both genes maintain each other, hh maintains wg which supresses denticle development
What is the function of wingless and hedgehog together
They work together to inhibit the formation of the denticles
The segment polarity gene engrailed is switched on by wingless, what is the role of engrailed
Engrailed then switches on hedgehog
Explain how wingless and hedgehog interact
Hedgehog signals in a paracrine manner to the adjacent cell to switch on wingless expression. Wingless then acts on its own receptors in an autocrine manner to upregulate is expression but also acts via engrailed to upregulate hedgehog too
Hh and wg feedback onto each other to maintain their expression and refine segment borders, T or F
T
The hedgehog-wingless signalling loop is present in in the posterior parts of the segments, T or F
T
Wg is expressed uniformly throughout the Drosophila embryo, T or F
F – its expressed as a gradient
What is the role of the Hox genes (homeobox domain containing genes)
Hox genes provide identity once the segments have already formed
Hox genes are transcription factors, T or F
T
From which genes do the hox genes receive input from
Pair rule and gap genes
What is unique about the expression of the hox genes
They are expressed along the anterior-posterior axis in the same order in which they lie in the genome
What happens due to a of loss of function in a hox gene
The segment will adopt a different identity, this is known as a homeotic transformation
What is meant when Drosophila are referred to as long germ band organisms
All 14 segments are defined at once. All tissues are present they just need defining during development. This process is relatively quick (24hours in Drosophila)
How do short germ band organisms develop differently to long germ bands
Similar process to short band organisms in the anterior head and thoracic segments. However, the abdominal segments are added sequentially by a region known as the posterior proliferative disc budding off segments as it gets smaller
Short germ band segmentation is more complex and slower than that of long band organisms, T or F
T
What signalling pathway mediates the segmentation of short band organisms
Delta-Notch
Explain how the segmentation clock of delta, notch and Hes1 accounts for segmentation in short germ band organisms
Notch and delta signal to each other in a juxtacrine manner. Activation of the Notch receptor leads to the expression of Her/Hes1 which inhibits delta expression in the nucleus of that cell. At high concentrations Her also inhibits itself. This mechanisms acts as a molecular oscillator or clock whereby notch activity rises and then triggers the expression of Her. Her then switches off delta and then itself. This causes notch to rise again and the process repeats. This pattern the segments at exactly the right time as the organism grows based on the time taken to produce Her mRNA and then translate it and switch delta off and then itself
How is the segmentation clock relevant to vertebrates
Humans are short band organisms. The primitive streak in vertebrates also use the notch pathway and its oscillations to produce and pattern body segments
