What are the principles of pattern formation?
- Must establish positional information (spatial information) within the developing embryo
- Direct simple patterns early and use to elaborate more complex patterns
- Use a cascade of determination events to coordinate timing
- Use master regulatory proteins each of which will be expressed in specific groups of cells marking them for a particular fate
What are the steps of drosophila development?
- oocyte cell is fertilized to generate the zygote
- following fertilization, the zygotic nucleus undergo 13 rounds of synchronous mitotic divisions without any cytokinesis - forming a syncytial blastoderm
- By the 9th or 10th round of divisions the nuclei migrate to the periphery and the pole cells (germ line precursors) are formed
- At the end of the 13th round of division (about 6000 nucleii) cellularization occurs producing — the cellular blastoderm
- Cell division, and migration occurs during gastrulation producing a segmented larva with shape and form
What are the two major axis?
- anterior to posterior
- dorsal to ventral
Are developmental fates of blastoderm cells fixed?
yes
Is segmentation fixed at the cellular blastoderm stage?
yes
How do we know broad decision of cell fate occur during blastoderm?
- laser ablation experiment
- two posterior ends even with cytoplasm “transplants” from the same organism but from the opposite end
How does one identify morphological determinants?
- biochemistry
- genetics
How is genetics used to determine morphological determinants?
- isolate mutants that are defective in morphogenesis
- use the mutant phenotype to clone the gene required for morphogenesis. Use the cloned gene to deduce the function of the product and time and place of expression
What are the two types of embryo lethal patterning mutants?
- zygotic embryo lethal patterning mutants
- maternal effect embryo lethal patterning mutants
What is a morphogen?
a chemical reagent able to cause or determine morphogenesis
What are the four maternal-effect patterning genes and where are the located?
- bicoid (anterior Ab) (TF)
- hunchback (anterior Ab) (TF)
- nanos (Posterior H, Th) (RNA binding protein)
- caudal (Posterior H, Th) (TF)
What does transplanting cytoplasm from the anterior of a bicoid+ embryo to a bicoid- embryo do?
Partially rescue the bicoid- phenotype
Therefore, bicoid- is missing cytoplasmic determinants
What happens when a bicoid+ cytoplasm is delivered to the centre of a bicoid- embryo?
the head/thoracic segments are induced in both directions from the site of injection, suggesting a morphogen like effect
What do morphogens do?
- are molecules that can induce the acquisition of different cell fates (aka morphogenesis) on the basis of the concentration of molecule to which a cell is exposed
- a gradient of morphogen across an embryo or developing tissue provides positional information for the cells
- transcription factors could act as morphogens by activating target genes in different nuclei in a concentration-dependent manner
How are the protein gradients established?
- bcd+ and nos+ protein gradients are established through diffusion from site of translation
- Establishment of the hb+ and cad+ protein gradients are more complex
What special aspect of drosophila embryogenesis allows this to happen?
- syncytial blastoderm many nuclear divisions without cytokinesis
- protein translated from the mRNA is able to diffuse through the blastoderm
How does bcd+ and nos+ mRNA get localized to the anterior/posterior poles of the zygote?
- at the stage of oocyte, there is already polarity
- the polarity of the oocyte is set up by the surrounding maternal environment and is used to direct the orientation of the spindle
- the orientation of the spindle is used to direct bcd+ and nos+ mRNA to one end or the other
- 3’ UTR of bcd+ and nos+ mRNAs have cis-acting sequences
- these sequences within the 3’ UTR are recognized by specific RNA binding proteins that determine microtubule attachment and movement
