Lecture 2 - Homeotic genes Flashcards
What is the process of lineage tracing/fate mapping/clonal analysis?
- X rays induce chromosome breaks to that homologous chromatids exchange parts in mitotic recombination
- in an embryo heterologous for a visible marker mutation like mwh after mitosis, this results in one daugher cell becoming homozygous recessive for the phenotype and expressing that phenotype
- all the progeny of this cell will also have this phenotype reuslting in a clone of genetically marked cells
- marking a cell early in development can trace its lineage
- however cannot test what the limits of the cell are
What was the result of the ‘minute technique’?
- defined compartments which corresponded to parasegments
- indicated that the parasegemnt is the developmentally important unit
What is the minute technique?
Minute is a gene that regulates the speed of the cell cycle
1. A fly heterozygous for mwh+/- and minute +/- was generated, with mwh conjugated to minute. As it is heterozygous for the minute mutation, cells divide slowly compared to WT
2. As drosophila are diploid (2 copies of each chromosome), if cell is hit with x rays during mitosis (when cells divide and chromosomes are copied) crossing over occurs and this results in mitotic recombination
3. Chromosomes segregate, resulting in two diploid daughter cells whose genotype is different from the parental cell
-one daughter cell is homozygous recessive for minute -/-, this is lethal and the cell dies
-one daughter cell is homozygous recessive for mwh -/- and is therefore marked. It is also WT at the minute locus so it divides at a wt rate. Therefore in a heterozygous embryo, this cell divides faster than other cells, generating a large clone of descendants marked with multiple wing hairs
Results show a clear boundary of cell lineage restriction
-cells in the parasegment stay in the parasegment
What is the hierarchy of genes involved in the control of the identity of the parasegments?
- Maternal genes control the gap genes and the primary pair rule genes
- Gap genes control the primary pair rule genes and the selector genes (homeotic genes)
- Primary pair rule genes control the secondary pair rule genes, the selector genes and the segment polarity genes
- Secondary pair rule genes control the segment polarity genes and the selector genes
What do mutations in the bithorax complex normally result in?
Homeobotic transformation
- where posterior identify is transformed into an anterior identity
e. g. - In the normal adult, the wing and the haltere are made up of anterior and posterior compartments
- In the bithorax mutant, the posterior compartment of T3 is transformed into into the anterior compartment of T2, so that it forms the anterior half of a wing
What experiments/phenotypes defined the parasegment as a compartment and the developmentally important unit?
- bithorax mutant phenotype
- minute technique to show cell lineage restriction within strict boundaries
What genes is the parasegment set out and patterned by?
- segmentation genes
- homeotic selector genes
What are the homeotic selector genes(Hom-C)?
The genes of the antennapaedia and bithorax complexes in drosophila
- set the future developmental pathway of each compartment
- clustered in the genome
- code for transcription factors, regulate the activity of other genes
- DNA binding domain (homeobox)
- required to maintain correct patterning of cells
What are the three main features of the homeotic genes?
- clustered (genetically linked)
- colinearity of expression
- conserved
What are the features of the colinearity of the homeotic genes?
- order of the genes along the chromosome is the same as the order of temportal and special expression along the AP axis of the embryo
- e.g. Ubx, which is more 3’ to AbdA is expressed more anteriorly and earlier than AbdA, which is more 3’ to AbdB and expressed more anteriorly and earlier than AbdB
How can the colinear expression of homeotic genes be viewed experimentally?
in situ hybridisation
What are the features of the expression of AbdB shown through insitu hybridisation?
- can have very resticted boundry of expression
- reflect what had previously been shown by clonal analysis, cells do not cross parasegment boundaries
What is the structure of patterning of the bithorax complex?
-compromises three genes, ubx, adbA and abdB
-expressed in a combinatorial manner (combined activity of these genes define the character of each parasegment)
-required for the correct patterning of parasegments 5-13
-initial patterning is set up by the gap genes and pair rule genes, also some cross talk between the genes:
AbdB represses Ubx (expression and/or activity)
=> repressive activity called the posterior prevelence or posterior dominance
What is posterior prevelence/dominance?
-where posteriorly expressed genes in the complex repress the activity and/or expression of the more anteriorly expressed genes
What do mutations in homeotic genes cause?
transformations in parasegment identity
-e.g. in the absence of AbdB the parasegmetns 10-13 are transformed into parasegment 9 (ie. all segments A3p/A4a)
-shows posterior prevelence
-in the absense of AbdA and AbdB, parasegments 7-13 are all transformed into parasegment 6 (ie. all segments T3p/A1a)
-expression again spreads posteriorly due to postierior prevelnce (AbdB normally represses Ubx)
-in the absence of the entire Bx-c, parasegments 5 -13 are all transformed into parasegment 4 (ie all semgnets pT1/aT2)
-under the control of the antennipedia complex
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