Lecture 2 - Zygote Formation and How Acquisition of Cell Polarity Underpins Subsequent Developmental Fate Flashcards
what is the fucus zygote?
an excellent system to study the acquisition of polarity
The zygote is initially apolar - Polarity determines cell fate
what is the Fucus zygote polarity is influenced by environmental factors?
Rhizoid develops on shaded side
Takes 12 h to appear
Polarity not fixed until > 10 h
In addition polarity is determined by:
- fertilization (rhizoid at entry point)
- heat (rhizoid to warm side)
- pH and salt (rhizoid to alkaline pH and
salt)
- Electrical gradient (rhizoid at -ve pole)
how does Polarizing zygotes generate an electrical voltage?
Implies that development of polarity is accompanied by production of ionic gradients
how is calcium involved in establishing Polarity?
Disruption of calcium gradient prevents development of polarity
Localisation of Ca2+ channels is observed after 5-6 h of illumination
what is the polarity in drosophila?
anterior-posterior (head to tail) polarity of the embryo, larva and adult derives from polarity established in the egg
How does Bicoid mRNA become localised at the anterior pole?
Bicoid mRNA is synthesised in the nurse cells of the maternal ovaries and transferred to the oocyte where it becomes localised at the anterior end
what are the Additional Maternal Effect Genes are involved in determining anterior-posterior polarity in Drosophila?
Bicoid and Hunchback regulate production of anterior structures
Nanos and Caudal regulate production of posterior structures
Bicoid, Hunchback and Caudal are transcription factor proteins that regulate other genes controlling later steps in development
what is a maternal effect gene?
Bicoid encodes a localised cytoplasmic determinant required for normal anterior development in Drosophila. Its localisation is established during oocyte development in the maternal ovary
what does the polar distribution of components in the egg cell and hence zygote will give rise to ?
differences in daughter cell composition and developmental fate following the first division
Concept of ‘localised cytoplasmic determinants’
where are eggs produced?
The eggs are produced in the maternal ovaries. The precursor to the egg is the oocyte. Nurse cells in the ovary synthesise macromolecules that are transported to the oocyte as it develops via cytoplasmic bridges. Some of these macromolecules (e.g. particular mRNAs) are assymetrically distributed
what do some regions of the fertilise egg do?
give rise to different tissues during embryogenesis, i.e. have localised cytoplasmic determinants
what does the transplantation of cytoplasmic regions do?
Transplantation of cytoplasmic regions into different parts of the egg give rise to altered cell fate.
E.g. movement of myoplasm - which gives rise to muscle cells - into cells which normally give rise to epidermal cells, results in production of muscle cells
what does the asymmetry of the zygote reflect?
The assymetry of the zygote reflects a polar distribution of molecules and organelles in the egg cell gamete
is the zygote initialy apolar?
However, in many plant and animal systems the zygote is not initially apolar but already has an assymetric distribution of components
what is the Drosophila bicoid mutant ?
The Drosophila bicoid mutant has altered embryo development
The embryo lacks the head and thorax at the anterior end and instead has a second set of posterior structures
Therefore the wild-type Bicoid gene product is required for normal anterior development of the embryo
what is The bicoid mutant phenotype?
The bicoid mutant phenotype can be rescued by injecting wild-type Bicoid mRNA into the anterior end of the embryo
Shows that the spatial localisation of Bicoid mRNA is important
what is the polar distribution in amphibian oocytes?
The polar distribution of components is sometimes visible, e.g. in amphibian oocytes, but to show that a component has an effect on development need to demonstrate its action by e.g. removal or translocation
Albino mutants lacking pigmentation of the oocyte are not altered in development, so pigment is not controlling cell fate
where is the polar distribution of bicoid and nanos established?
The polar distribution of Bicoid and Nanos mRNAs is established in the ovary
The Bicoid protein inhibits the translation of Caudal mRNA at the anterior pole, resulting in the accumulation of Caudal protein towards the posterior
Similarly, Nanos inhibits translation of Hunchback mRNA at the posterior
The spatial distribution of these proteins regulates subsequent steps in development
what can we say about ascidians?
The polar distribution of components and their role in determining cell fate has been studied in ascidians
The ascidian egg has regions of cytoplasm with different coloured inclusions. These show reproducible patterns of distribution during early embryogenesis and are linked to the fate of cells
