Axes of Development 2 Flashcards
What drives early developmental changes?
2 types of genes:
Maternal effect genes(not to be confused with maternal inheritance) At this stage genotype of the mother determines the phenotype of progeny.
Zygotically acting genes expressed in embryo
What effect do maternal effect genes have on embryo?
They function in early development and directly influence phenotype.
How do maternal effect genes carry out their function?
Pattern formation takes place in oocyte while developing in ovary of mother.
Maternal expression of genes is responsible for creating asymmetries and chemical gradients that the zygote and the embryo uses.
Where are gene products of maternal effect genes located?
They are produced by mother and the gene products are in the ova.
The proteins are already present at fertilisation.
Phenotype of offspring depends on genotype of the mother because source is mother’s genes.
When does the body plan start to take action?
At the stage of the egg. The egg with maternally deposited mRNA has 2 poles. An anterior pole and a posterior pole. The anterior pole has the maternal proteins and this diffuses to the posterior end of the cell creating a gradient of concentration of maternal effect proteins. This is influenced by maternal effect genes.
What is bicoid gene?
Maternal effect gene that is concentrated in anterior end. All progeny of the same mother have the same phenotype but may have different genotypes. Knocking this gene out results in lack of polarity and death.
What is the role of the cytoskeleton in development?
Microfilaments, intermediate filaments, and microtubules have distinct ‘+’and ‘-‘ends which serve as highway systems for intracellular transport.
What do dynein and kinesin do?
Dynein pulls to anterior -ve pole and kinesin to posterior +ve pole.
How does mRNA localisation assist development?
Organisation of cytoskeleton permits localisation of mRNA encoding TFs that will provide positional information.
This is by creating high concentration of TFs on the mRNA pole and the diffusion creates a gradient that decreases in concentration in the direction of the posterior pole.
What happens to germline cells during development?
In animals, germline cells are set aside from soma in early development.
What is a morphogen?
A diffusable molecules that determines cell fate in a concentration-dependent manner.
What model explains how cell fate depends on morphogen concentration?
The French Flag Model.
Each cell has potential to develop in to blue white or red.
Position of each cell is defined by the concentration of morphogen.
Positional value is interpreted by the cells which differentiate to form a pattern.
Do morphogens get expressed on all poles of the cell?
Yes, some start anteriorly, others posteriorly, others dorsally, and others ventrally. This sets up gradients that broadly defines areas creating basic body plan map.
*You can imagine the complexity this creates when multiple morphogens can act on exactly the same cells in different ways.
Logic of pattern formation:
Cytoskeleton imposes asymmetry on egg
Germline cells develop
Gradients of morphogens are laid down on both A-P axis and D-V axis.
Broad domains created Within broad bands more complex gene interactions subdivide further and these identities are remembered.
How is body plan created along A-P axis?
Maternal effect genes then zygotic genes initiate the polarity. Gap genes divide embryo into broad regions. Pair-rule genes divide embryo into stripes, defining segment border (on-off, on-off, etc) Segmentation polarity genes divide segments into anterior and posterior halves. Homeotic selector genes specify the identity of each segment.
Where is bicoid tethered?
Maternal bcd mRNA is packaged into head of developing embryo. Bcd (italics) mRNA tethered to (-) ends of microtubules of the cytoskeleton via 3’ UTR by dinein at the anterior pole.
Where is nanos tethered?
Maternal nanos mRNA is packaged into tail of developing embryo. Nanos (italics) mRNA tethered to (+) ends of microtubules of the cytoskeleton via 3’ UTRy kinesin to the posterior pole.
Which morphogenic gene/s regulate structures on anterior end?
Bicoid and HB-M
How is HB-M mRNA translation controlled by Nanos?
When no nanos is bound to nanos response element the HB-M protein is adenylated which activates translation and promotes production of anterior structures. When nanos is bound to nanos response element there is no adenylation and so no translation of HB-M protein and so abdominal formation can take place. *NO LEARNING OUTCOME HERE
Summary of maternal effect genes IN OOCYTE:
Gradient formed A-P direction. 2 maternal effect gene products are constant throughout the cell from A - P; caudal and HB-M. 2 maternal effect gene products form a gradient from anterior to posterior; bicoid is highest anteriorly and drops off more posteriorly and nanos is highest posteriorly and drops off more anteriorly.
Summary f EARLY EMBRYONIC proteins:
Bicoid and nanos gradients extend further in their respective directions however, they still drop off quickly. HB-M is constant until abdomen and then begins to drop off. Caudal drops off towards the anterior side but is highest posteriorly. Remember high nanos = low HB-M and high bicoid = low caudal
What happens if maternal bicoid dosage is increased?
It “pushes” anterior segments further down the embryo. ( the cephalic brow is pushed further down)
How does a fertilised egg, with a single nucleus, develop into a properly patterned adult organism with specialised cells and tissues in the right place?
The egg is special:
Large stores of ribosomes & nutritive proteins; poised for rapid initiation of cell division.
Unequal distribution of mRNAs & proteins provides basis for cell polarity, setting up for development (maternal morphogenetic factors).
Protective e.g. UV filters, DNA repair enzymes.
What is the developmental strategy?
- Mother deposits material that creates asymmetries in egg
- These set up gradients that broadly define areas (basic body plan map).
- Gene interaction subdivides these areas (& cells differentiate).
- These identities are remembered.
Cells can be influenced by their environment.
Cells can have varying levels of commitment.
How do morphogen gradients contribute to the determination of cell fate?
Recurring theme in developmental decision-making = creation of morphogen gradients & threshold-dependent determination of cell fate.
Small group of cells secrete morphogen: gradient formed, cells in different positions perceive different morphogen concentrations, levels of activated TF vary, → → different sets of genes expressed.
How do the concerntration of morphogens impact gene expression?
• morphogen concentration
→ number of receptors occupied → level of activated TF in nucleus → pattern of genes turned on/off
• Decisions within soma require choreographed set of steps that lead to all
cell types.
• First set up overall body plan (axes), then groups of cells set aside to become various structures.
What were the three experiments that showed that bicoid contributes to A-P positional information.
- bcd null mutants develop without a head or thorax, instead have 2 tails
- Over expression of bcd in mother (transgenic) “pushes” fate of anterior segments further down embryo
- Transfer of anterior cytoplasm from wild-type embryo or bcd mRNA to bicoid deficient embryo restores A-P patterning
