Lecture 31 - Developmental genetics I Flashcards
Time for drosophila fertilized egg to adult fly
10 days
3 stages of drosophila development and time for each
Embryonic stage (1 day), larval stage (4-5 days), Pupal stage (4-5 days)
Embryonic stage description
1 cell becomes alive animal with muscle, nervous system, trachea, …
Larval stage description
Growth stage, eats a lot and increases size and weight
Pupal stage description
Metamorphosis (transition from growth stage to reproductive stage). Reorganisation of tissues. Controlled by hormones
3 main features of embryonic stage
No zygotic transcription until 2h into embryogenesis
No plasma membrane (syncytium) in early embryogenesis
Maternal factors regulate early embryogenesis
4 steps of the 2 first hours of embryogenesis (and time)
Single-celled diploid zygote Multinucleated syncytium (70 min) Syncytial blastoderm (120 min - 2h) Cellular blastoderm (180 min - 3h)
Multinucleated syncytium description
9 nuclear divisons but still 1 cell in 1 membrane -> multinucleated syncytium
Syncytial blastoderm description
Nuclei migrate to the periphery of the embryo and divide 4 more times. Pole nuclei on one end
Cellular blastoderm description
Cell membrane grows around each nucleus. Pole nuclei become pole cells (primordial germ cells) on one end
Description of the 2 hour embryo (syncytial blastoderm)
4 segments (anterior/posterior + dorsal/ventral) established
Description of the 10 hour embryo
Number and orientation of segments (within segments) established. Distinguish head, thoracic segments and abdominal segments
Observing drosophila embryogenesis video : 3 things to note
13 synchronous divisions - 8000 nuclei. Back and forth pulling and stretching coordinated movement. Separation of segments and orientation
Heidelberg screen what + goal
Mutagenesis screen (forward genetics) to ID genes required for organizing the drosophila embryo
2 screens in the Heidelberg screen
- For genes required in the mother for normal development of the embryo
- For genes required in the embryo’s genome
Name of genes in the first screen done and idea behind it (why do it)
Maternal-effect genes. Early embryo: maternal mRNA deposited into the egg and required for early dev. before zygotic transcription
What determines if embryos of a mother will have normal or mutant phenotype associated with early dev. (before zygotic transcription)
Mother has to carry one copy of maternal-effect gene and all offspring will be normal
Maternal-effect gene, offspring phenotypes in these crosses. m/+ father x m/+ mother. m/m father x m/+ mother. anything father x m/m mother.
Normal. Normal. Mutant.
Maternal effect gene : m/+ father x m/+ mother -> 1 +/+ , 2 m/+ , 1 m/m -> phenotypes of these offsprings
All normal, even the m/m offspring (simply because mother is m/+ and carries a WT copy of the maternal-effect gene)
Name of genes in second screen and inheritance pattern followed
Zygotic genes. Mendelian inheritance pattern.
m/+ father x m/+ mother for zygotic genes. offspring pheno ratio
3 normal (+/.) 1 mutant (m/m)
Common characteristic the identified genes were found to have
Most are transcription factors
5 categories they put the genes from the screen in + maternal-effect or zygotic. + based on what
Based on ressemblant mutant phenotype. Egg-polarity genes (maternal). Gap genes, pair-rule genes, segment polarity genes, segment identity genes (zygotic)
Maternal effect genes general function
Establish anterior-posterior (AP) axis
2 maternal-effect genes + function and type
Bicoid (TF) : anterior structures (head, thoracic segments)
Nanos (translational repressor protein): posterior segments (abdominal)
Regions affected in bicoid and nanos mutant
Bicoid mutant: Anterior embryo affected
Nanos mutant: Posterior embryo missing/affected
Observation when labelled bicoid protein in white
Localizes more to anterior region and less and less to posterior region
General conclusion about the observation on bicoid gradient
Factors (mRNA) from the mother localize to specific regions. Embryo’s nuclei are all exposed to different amount of each protein.
Bicoid mRNA localization and where it is translated
Localized in anterior region only. Translated in anterior region into bicoid protein
Bicoid protein localization and how is this important
Diffuses because still multinucleated syncyntium. Gradient from A to P. More in A and less and less towards P. Gives information on distance from anterior
How would bicoid protein/bicoid mRNA localize if there was a plasma membrane separation in the multinucleated syncytium’s nuclei
Bicoid protein would only localize to anterior, just like bicoid mRNA (couldn’t diffuse)
2 experiments done to show that bicoid mRNA is sufficient to form anterior structures
- Synthesized bicoid mRNA, injected in bicoid mutant, and rescued pheno (developped anterior part)
- Injected mRNA in middle of embryo and head/anterior-like structure developped there
Nanos mRNA and protein, where they localize
Nanos mRNA : localizes to posterior region. Nanos protein gradient from P to A (more towards P less towards A) bc can diffuse
Nanos protein type and mode of action
Translational repressor. Binds RNA and inhibits translation
Maternal hunchback mRNA and protein distribution
HB mat mRNA uniformly distributed. HB mat protein slightly more towards A and less and less towards P
Maternal hunchback type of protein
transcription factor
Nanos, what it does and what it explains
Translational repressor of maternal Hunchback. It explains the HB mat protein gradient, opposite to nanos gradient.
4 maternally loaded factors (3 already seen) and their A-P distribution
Bicoid (BCD) and maternal hunchback (HB mat): more in A and less and less towards P
Caudal (CAD) and Nanos (NOS): more in P and less and less towards P
Appearance of offspring of nos-/nos- male and bcd+/bcd- female and why
100% WT appearance. female has a WT copy of bcd and nanos
Gap genes: number IDed and what they do
9 IDed. Translate maternal-effect genes A-P gradient into BROAD subdomains (not individual segments yet)
Gap genes type of protein and maternal-effect or zygotic
Gap genes = ALL transcription factors. Zygotic genes
How gap genes discovered
Mutants found missing several consecutive segments and were mutant in zygotic genes
Gap genes some examples
zygotic hunchback, kruppel, knirps, giant
Gap genes: how they translate maternal-effect genes A-P gradient into broad subdomains
Gap genes are transcriptionally activated or repressed over certain regions by maternal-effect genes
Which maternal-effect gene regulates HB-z expression
Bicoid protein
HB-z mRNA localization/gradient (mRNA means transcription occured)
Anterior region and is not a gradient but rather sharp border between region where present and region where absent
How Bcd regulates HB-z expression and advantage of that
Hb-z has 3 enhancers (binding sites) for Bcd protein. Allows more sensitive detection of GRADIENT of bcd protein
What 2 factors can influence activation of a gene (like the HB-z gene for example).
Concentration of transcriptional activator (ex. Bcd) and number of enhancer (binding) sites
Experiment done to test the effect of multiple binding site for Bcd on the Hb-z gene and results
Introduced Hb-z constructs with 0,1,2 binding sites for Bcd and noticed that more binding sites = larger anterior region (Hb-z expressed further from anterior)
2 extreme situations of TF concentration and number of binding sites (important concept for embryonic development)
Genes with only one binding site for their TF will only be expressed where TF concentration is high
Genes with the most binding sites for their TF are the only ones expressed when the TF concentration is low
What controls the pattern of Gap genes expression (2)
The previously established gradients of maternal-effect proteins (bcd, hb mat, nanos, cad)
Other gap genes gradient (they feedback on each other too)
Pair-rule genes: Number identified. Type of protein. Pattern of expression
8 identified. ALL TFs. Expressed in alternating segments.
What happens in pair-rule genes mutant
Alternating segments are missing but ones in between are present
Some pair-rule genes examples
even-skipped, fushi tarazu, odd-skipped, paired, runt
What regulates Eve (even-skipped) transcription in stripe 2 (and pair-rule genes transcription in stripes in general)
The high Hb-z concentration, medium Bcd concentration, low Gt and low Kr concentration.
In general, enhancers tailored to act within specific concentrations of diff TFs.
What is responsible for the regularity in the stripes of expression of pair-rule gene
Independent enhancers in each stripe.
Some enhancers (activators’ sites) and silencers (repressors’ sites) for the eve gene
Enhancers: Bcd-1 to Bcd-5, Hb-z 3
Silencers: Kr3 to Kr5, Gt1 to Gt3
Other name for maternal-effect genes
Egg-polarity genes
