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.
How are the A-P (&D-V) axes formed?
Collaborative effort between mother and embryo. Gradients are formed by: Bicoid (maternal effect gene) HB-M (hunchback-maternal) Nanos (maternal effect gene) Caudal (also maternal effect gene) These proteins are all transcription factors. Gradients are set up by mother in oocyte.
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.
What determines hunchback-maternal gene gradient?
HB-M protein gradient depends on Nanos protein. Nanos blocks translation of HB-M mRNA. Resulting differential gradients of Bicoid and HB-M determine the axis.
Which morphogenic gene/s regulate structures on anterior end?
Bicoid and HB-M
Which morphogenic gene/s regulate structures on posterior end?
Nanos
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
How are posterior structures initiated?
They are initiated AFTER fertilisation by a maternal effect gene called caudal. Caudal mRNA is is evenly distributed but the A-P gradient is formed by bicoid which inhibits caudal synthesis so bicoid does not act as a TF in this case.
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.
