Download Lab 08-Flower development

LAB 08: FLOWER PATTERNING AND SYMMETRY
I. FLOWER PATTERNING: Homeotic mutants (Organ identity)
1. Observe and describe wild type (normal, as opposed to mutant) flowers of the model
plant species Arabidopsis thaliana (thale cress), starting with the outermost organs and
following the floral diagram as you dissect a flower. In most Eudicots, floral organs are
organized in whorls (indicated by the numbers 1-4). Other arrangements, such as spiral,
are common in early-diverging angiosperms.
Label each whorl as you proceed. (This will be your “control” when observing mutants.)
1:
2:
3:
4:
Modified from Howell 1998
2. Use the basic ABC model below to reason the gene combinations (A, B and C class) that
are responsible for organ identity in each whorl (1-4). You will be using modifications of
this basic diagram later to describe the different mutants.
Modified from Howell 1998
3. The “extended ABC model” below includes the additional gene class E (ovule identity used to
be assigned to class D, but now it is considered part of carpel identity or C class). A and C
class are mutually exclusive, in the absence of one, the other takes over.
Full E class mutants have leaves in all floral whorls.
What do the E mutant phenotype and the model above suggest about the role of
class E genes?
_______________________________
4. We have grown four types of Arabidopsis floral homeotic mutants (floral organs at one
position in the flower are replaced by organs appropriate to another position). Three of them
correspond to single mutations in each of the three organ identity gene classes: A, B and C (E is
not available). The fourth is a mutant for the floral meristem identity gene LEAFY, that acts
upstream (earlier in the developmental pathway) of the organ identity genes, turning the
meristem into a flower and turning on the flower organ identity genes of the ABC model. If no
flower organ identity is established, what type of organ do you think will develop as the default
developmental program? ______________________________
Fill in the table below: Observe 2-3 flowers under the dissecting scope, then dissect and draw a
flower diagram for each of the 3 organ identity mutants by modifying the simplified wild type
floral diagram below, then draw a modified ABC model (refer to (2)). Based on this data,
determine the class of gene that is mutated in each case (A, B, C) or LFY.
Sepal
Petal
Stamens
`
Gynoecium (2 carpels)
Flower diagram
1
2
Modified ABC model
Type of mutant
3
4
Depending on availability, we may also have double and triple mutants:
Complete the table below and answer the questions for one double and a triple mutant:
Flower diagram
Modified ABC model
Type of mutant
Double
Triple
What would you expect a triple “abc” mutant to look like? ______________________________
What does this suggest about the evolutionary origin of floral organs? ____________________
_____________________________________________________________________________
Describe the double bc mutant: ____________________________________________________
What other mutant class would result in a similar phenotype? ___________________________
5. Based on the ABC model, propose hypotheses to explain the floral phenotypes on display.
Again, modify the basic ABC model diagram in (2). Gene classes can be missing completely
or their domains of expression (the region where the gene is expressed and the protein
performs its function) may have contracted or expanded.
a. “Double flower” horticultural varieties (roses, camellias).
Can you match this phenotype to any of the Arabidopsis mutants?
Hint: for the next two, consider the “sliding boundary” idea discussed in lecture.
b. Flowers with a petaloid calyx (e.g. tulips and lilies).
c. Flowers with a sepaloid corolla (e.g. the sorrel Rumex).
II-SYMMETRY
In this part of the lab you will:
1. Study a snapdragon cyc dich double mutant.
2. Learn about a Linaria epigenetic mutant.
3. Discuss two short papers by Dr. Enrico Coen on ABC model and flower symmetry.
PELORIC MUTANTS
Antirrhimum majus (snapdragon) is another model system in flower development that
contributed to the elucidation of the ABC model of flower development. Unlike Arabidopsis
thaliana, these flowers have bilateral symmetry (or monosymmetric), and were also used to
elucidate the establishment of the adaxial/abaxial axis of the flower. Mutations that transform
bilaterally symmetric flowers into flowers with radial symmetry (polysymmetric) are called
peloric.
1. Observe and dissect wildtype (wt) flowers of snapdragon, use the diagrams below as
a guide to identify all floral parts, including the different types of petals.
From Leyser & Day 2003
Recall from lecture that the genes CYCHLOIDEA and DICHOTOMA (CYC and DICH) are
the main players in floral symmetry in this species. We will have plants that are double
mutants for both of these genes (cyc dich).
2. Illustrate a flower of the double mutant in front view. Would you guess this is a
snapdragon? How does it compare to a wildtype flower?
What kind of symmetry does it have? __________________________________________.
Make your own dissections of wildtype and mutant petals and compare them below.
From Leyser & Day 2003
Mutant
Petals
(insert your
diagram here)
Can you still observe a differentiation between Ad, L and Ab petals? _________________
Are the petals of the double mutant similar to adaxial, lateral or abaxial wild type petals?
______
What can you conclude about the role of CYC and DICH in floral symmetry in snapdragon?
________________________________________________________________________
Linaria epigenetic peloric mutant (wildtype plants on display)
The peloric mutant of the plant displayed has very similar characteristics to the snapdragon cyc
dich double mutants. But rather than being due to changes in the genes themselves, the
radialization of Linaria it is due to modifications of the DNA by addition of methyl groups
(methylation) that inactivate the gene, which is otherwise normal. This is called an epigenetic
mutation. The phenotype is the same as in the genetic mutant, but it can change from flower to
flower, so that a plant is a chimera of mutant and wild type flowers. Because mutant flowers
arise spontaneously and are rare, we are showing you a picture.
Dissect and draw the petals of a Linaria wild type flower below.
Observe the photograph of the peloric mutant: which of the petals is repeated in the mutant? ___
Is this mutant adaxialized or abaxialized?