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Gene Expression > Homeotic genes and floral identity > Flashcards

Homeotic genes and floral identity Flashcards

1
Q

What is homeosis (defined by Bateson)?

A

Assumption by one member of a meristic series, of the form or characters proper to other members of the series.

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2
Q

What is a meristic series?

A

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.

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3
Q

What is the result of a homeotic mutation?

A

One member of a repeating series is replaced by another member (e.g. stamens replace petals).

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4
Q

What do homeotic genes control?

A

Differences between repeated units. Can be more extreme in some species compared to others e.g. fly vs centipede.

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5
Q

Are homeotic genes conserved?

A

Yes.

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6
Q

What is a whorl?

A

A ring of plant organs.

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7
Q

How are whorls arranged?

A

Concentrically - one inside the other.

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8
Q

What is the function of sepals?

A

Enclose and protect the flower and bud.

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9
Q

What is the function of petals?

A

Attract insects by producing nectar and scents.

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10
Q

What is the function of the stamens?

A

Produce pollen with the male gametes.

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11
Q

What is the function of the carpels?

A

Produce fruit after fertilization and contain the ovules (female gametes).

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12
Q

What are the numbers of each organ in arabidopsis?

A

4 sepals, 4 petals, 6 stamen, 2 carpels (fused).

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13
Q

Which whorls do class A mutants affect?

A

1 (sepals) and 2 (petals).

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14
Q

Which whorls do class B mutants affect?

A

2 (petals) and 3 (stamens).

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15
Q

Which whorls do class C mutants affect?

A

3 (stamens) and 4 (carpels).

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16
Q

What is the phenotype of a class A null mutant?

A

(Partial) carpels, stamens, stamens, carpels.

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17
Q

What is the phenotype of a class B mutant?

A

Sepals, sepals, carpels, carpels.

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18
Q

What is the phenotype of a class C mutant?

A

(sepals, petals, petals)n
4th whorl is a repeat of the same pattern and so on.
Flower is indeterminate (indefinite number of whorls)

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19
Q

What are examples of class A mutants?

A

Arabidopsis apetala 1 and 2 (ap1/2).

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20
Q

What happens with partial homeotic mutants?

A

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.

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21
Q

What are examples of class B mutants?

A
  • Arabidopsis apetala 3 (ap3) and pistillata (pi)
  • Antirrhinum deficiens (def) and globosa (glo)
22
Q

What is an example of a class C mutant?

A
  • Arabidopsis agamous (ag)
  • Antirrhinum plena (ple)
23
Q

What are the functions of gene C?

A
  • Floral organ identity
  • Stopping of flower (determinacy)
24
Q

How do class A and class C genes interact?

A

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.

25
Q

How would you work out the phenotype of double mutants?

A

Visualise the ABC model aligned with whorls (Lecture 5, slide 15) but knock out whichever are mutants. Remember no C = loss of determinacy!

26
Q

What is the phenotype of an abc triple mutant?

A

Flower partly converted into a shoot. Organs are leaf-like (but smaller than leaves). They have branched trichomes, like leaves.

27
Q

What idea did Goethe correctly propose in the 18th century?

A

Flowers are modified shoots; floral organs are modified leaves, and growth has been limited. Based on observations of homeotic mutants.

28
Q

What type of proteins do homeotic genes encode?

A

Transcription factors.

29
Q

Which domain do a, b and c TFs all have in plants?

A

MADS box (DNA binding).

30
Q

Which domain do homeotic proteins in animals have?

A

Homeobox (DNA binding).

31
Q

How do you test when / where homeotic genes are active?

A

Make sequences that hybridise to and allow visualisation of the mRNA. Use these at different developmental stages.

32
Q

Where are the different classes of homeotic genes usually expressed?

A

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.

33
Q

What happens to A and C homeotic gene expression in opposing mutants?

A

It spreads throughout the flower - as hypothesised.

34
Q

How was the abc model tested?

A

Made mutants that were hyperactive for each gene. Not present in nature.

35
Q

What is the phenotype when B genes are active throughout the flower (hyperactive)?

A

(Petals, petals, stamens, stamens)n.

Only C alone specifies determinacy.

36
Q

How is a B class gain of function mutant engineered?

A

AP3 and PI genes overexpressed using viral enhancers.
Must both be overexpressed as turns out they heterodimerise to function.

37
Q

What happens when class A or C genes are hyperactive (gain of function)?

A

They repress the opposite one.

38
Q

Can we do the opposite of a triple mutant and turn a leaf into a flower by overexpressing abc genes?

A

No. In this experiment leaves could not be transformed.

39
Q

Why is floral organ identity changed by homeotic gene overexpression, but leaf identity is not?

A

There must be flower-specific cofactors that allow homeotic protein function.

40
Q

What are the sepallata (SEP) genes?

A

3 genes closely related (similar sequence) to agamous that are expressed in flowers and not leaves.

41
Q

What is the effect of sep gene single mutants?

A

No change in the flower.

42
Q

What is the effect of sep1 / sep2 / sep3 triple mutant?

A

All floral organs are sepals. A is active, but not B and C.

43
Q

What are the sepallata genes (1/2/3) required for?

A

B and C class TF activity.

44
Q

What is the sep4 gene required for?

A

A class TF activity.

45
Q

How do SEP proteins activate A / B / C TFs?

A

They are cofactors. They do not promote A / B / C expression as there is still transcription of them in SEP mutants.

46
Q

What do SEP proteins allow B and C class TFs to do?

A

Interact with each other.

47
Q

How can we turn a leaf into a petal?

A

Overexpressing the abc genes and ALSO the sep3 gene.

48
Q

Which protein domains do abc genes all encode?

A
  • MADS box
  • Intervening domain
  • Keratin like domain (K domain)
49
Q

How do the abc proteins form tetramers with sepallata proteins?

A

Interactions of their K domains.

50
Q

If abc proteins are similar, how do they have different effects?

A

There are specific domains with the shared domains.

51
Q

What expression is required for correct homeotic gene function?

A

Expression beginning in early development and persisting throughout development as cells divide. If disrupted can lead to cancer.

Gene Expression (5 decks)

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