Homeotic genes and floral identity Flashcards
What is homeosis (defined by Bateson)?
Assumption by one member of a meristic series, of the form or characters proper to other members of the series.
What is a meristic series?
A series of repeated, homologous units. Units don’t have to be exactly the same but are recognisably similar. E.g. insect segments, floral whorls, teeth.
What is the result of a homeotic mutation?
One member of a repeating series is replaced by another member (e.g. stamens replace petals).
What do homeotic genes control?
Differences between repeated units. Can be more extreme in some species compared to others e.g. fly vs centipede.
Are homeotic genes conserved?
Yes.
What is a whorl?
A ring of plant organs.
How are whorls arranged?
Concentrically - one inside the other.
What is the function of sepals?
Enclose and protect the flower and bud.
What is the function of petals?
Attract insects by producing nectar and scents.
What is the function of the stamens?
Produce pollen with the male gametes.
What is the function of the carpels?
Produce fruit after fertilization and contain the ovules (female gametes).
What are the numbers of each organ in arabidopsis?
4 sepals, 4 petals, 6 stamen, 2 carpels (fused).
Which whorls do class A mutants affect?
1 (sepals) and 2 (petals).
Which whorls do class B mutants affect?
2 (petals) and 3 (stamens).
Which whorls do class C mutants affect?
3 (stamens) and 4 (carpels).
What is the phenotype of a class A null mutant?
(Partial) carpels, stamens, stamens, carpels.
What is the phenotype of a class B mutant?
Sepals, sepals, carpels, carpels.
What is the phenotype of a class C mutant?
(sepals, petals, petals)n
4th whorl is a repeat of the same pattern and so on.
Flower is indeterminate (indefinite number of whorls)
What are examples of class A mutants?
Arabidopsis apetala 1 and 2 (ap1/2).
What happens with partial homeotic mutants?
The mutated gene is partially functional and elements so there is not complete conversion to a different organ. Parts of the original organ(s) can still be present, nothing could be present in that place, the organ may be leaf-like or there may be an organ that resembles the replacement organ.
What are examples of class B mutants?
- Arabidopsis apetala 3 (ap3) and pistillata (pi)
- Antirrhinum deficiens (def) and globosa (glo)
What is an example of a class C mutant?
- Arabidopsis agamous (ag)
- Antirrhinum plena (ple)
What are the functions of gene C?
- Floral organ identity
- Stopping of flower (determinacy)
How do class A and class C genes interact?
They are antagonistic. In class A mutant, C function expands into all 4 whorls. In class C mutant, A function expands into all 4 whorls.
How would you work out the phenotype of double mutants?
Visualise the ABC model aligned with whorls (Lecture 5, slide 15) but knock out whichever are mutants. Remember no C = loss of determinacy!
What is the phenotype of an abc triple mutant?
Flower partly converted into a shoot. Organs are leaf-like (but smaller than leaves). They have branched trichomes, like leaves.
What idea did Goethe correctly propose in the 18th century?
Flowers are modified shoots; floral organs are modified leaves, and growth has been limited. Based on observations of homeotic mutants.
What type of proteins do homeotic genes encode?
Transcription factors.
Which domain do a, b and c TFs all have in plants?
MADS box (DNA binding).
Which domain do homeotic proteins in animals have?
Homeobox (DNA binding).
How do you test when / where homeotic genes are active?
Make sequences that hybridise to and allow visualisation of the mRNA. Use these at different developmental stages.
Where are the different classes of homeotic genes usually expressed?
Throughout development in the whorls that are affected when they are mutated.
Exception of class A genes which are expressed throughout the flower early on, but later does become whorl 1 and 2 specific.
What happens to A and C homeotic gene expression in opposing mutants?
It spreads throughout the flower - as hypothesised.
How was the abc model tested?
Made mutants that were hyperactive for each gene. Not present in nature.
What is the phenotype when B genes are active throughout the flower (hyperactive)?
(Petals, petals, stamens, stamens)n.
Only C alone specifies determinacy.
How is a B class gain of function mutant engineered?
AP3 and PI genes overexpressed using viral enhancers.
Must both be overexpressed as turns out they heterodimerise to function.
What happens when class A or C genes are hyperactive (gain of function)?
They repress the opposite one.
Can we do the opposite of a triple mutant and turn a leaf into a flower by overexpressing abc genes?
No. In this experiment leaves could not be transformed.
Why is floral organ identity changed by homeotic gene overexpression, but leaf identity is not?
There must be flower-specific cofactors that allow homeotic protein function.
What are the sepallata (SEP) genes?
3 genes closely related (similar sequence) to agamous that are expressed in flowers and not leaves.
What is the effect of sep gene single mutants?
No change in the flower.
What is the effect of sep1 / sep2 / sep3 triple mutant?
All floral organs are sepals. A is active, but not B and C.
What are the sepallata genes (1/2/3) required for?
B and C class TF activity.
What is the sep4 gene required for?
A class TF activity.
How do SEP proteins activate A / B / C TFs?
They are cofactors. They do not promote A / B / C expression as there is still transcription of them in SEP mutants.
What do SEP proteins allow B and C class TFs to do?
Interact with each other.
How can we turn a leaf into a petal?
Overexpressing the abc genes and ALSO the sep3 gene.
Which protein domains do abc genes all encode?
- MADS box
- Intervening domain
- Keratin like domain (K domain)
How do the abc proteins form tetramers with sepallata proteins?
Interactions of their K domains.
If abc proteins are similar, how do they have different effects?
There are specific domains with the shared domains.
What expression is required for correct homeotic gene function?
Expression beginning in early development and persisting throughout development as cells divide. If disrupted can lead to cancer.
