LAB 07: ANGIOSPERM SYNAPOMORPHIES
Introduction:
Phylogenetic relationships in the
flowering plants (Cronk 2009, Fig. 1.8)
Diversity in flower morphology:
A complete flower consists of sepals,
petals, stamens and pistil (of the
gynoecium, with one or more carpels).
In many species, flowers are
incomplete, lacking one or more of the
four types of floral organs. Virtually all
wind-pollinated flowers, for example,
are unisexual and commonly lack
perianth parts (petals and sepals).
Incomplete flowers. A,
Pistillate flower of Salix
(willow) containing only a
pistil subtended by a bract,
staminate flower consists only
of stamens; B, Saururus
cernuus (lizard’s tail) lacking
both petals and sepals;
C, Caltha palustris (marsh
marigold) lacking petals.
The diversity of floral form often correlates tightly with pollination vectors. The suite of
characters that constitute adaptations to a particular pollinator are known as pollination
syndromes.
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Examples of diversity in flower form and pollinating vectors. A, Salvia, pollinated by bumblebees;
B, Lilium with hawkmoth pollinator; C, Arum, an inflorescence pollinated by carrion flies; D, trumpet-vine with
hummingbird; E, Kigelia with bat; F, Vallisneria, a rare case of water pollination in which water conveys the entire
male flowers to the female flowers.
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Laboratory Exercise: Angiosperm synapomorphies
I. The flower as a synapomorphy and a key innovation:
Dissect at least 4 flowers, one from each lineage listed. Work on damp paper towel, using the
dissecting scope and making cross/long sections of pistils. Fill in the Table below.
Lineage
Name of Flower
Are all flower parts present?
(complete or incomplete)
Basal
(ANITA clade)
Or Magnolid
Monocot
Basal Eudicot
Core Eudicot
(Caryophyllid
Asterid or Rosid)
Symmetry:
bilateral or radial
.
Calyx: color, number of sepals,
fused or not?
Corolla: color, number of petals,
fused or not?
Stamens: color, number,
fusion of filaments or anthers?
fusion to petals?
Pistils: simple or compound.
If compound, number of carpels
(by externally visible
compartmentalization of ovary;
number of styles or stigmas;
number of internal compartments
seen in cross-section)
Ovary position
inferior or superior
Ovules: approximate number
per pistil *In ovary long
section/prepared slides.
Nectaries: absent or present,
location and shape.
(Eudicots: often a yellow disk at
base of ovary; Monocots: within
ovary, with surface exit pores)
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II. The flower in evolutionary context:
Observe the flowers provided, the material is organized evolutionarily, following the current
understanding of angiosperm relationships as depicted in the phylogenetic tree below. Write in
an example and summarize floral features on the phylogeny below for each lineage or grade.
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1.) Summarize 3-4 floral characteristics that you have observed in your dissections and in lab
demos that are ancestral, as observed in the ANITA grade or Magnolid dicots.
2.) Summarize 3-4 floral characteristics that you have observed in your dissections and in lab
demos that are derived, as observed in the core eudicots.
III. The flower as a reproductive unit: Pollination syndromes
1-Observe plants demonstrating different types of floral specializations to pollinators.
Although most gymnosperms are wind-pollinated (some Gnetophytes and Cycads being
exceptions), wind-pollination in angiosperms is a derived condition. The oldest known
angiosperm pollen is of the insect type. But where ecological circumstances have favored
wind-pollination, such as high wind velocity and a paucity of animal pollinators, a number of
angiosperm taxa have shifted from animal to wind pollination. Animal pollination is by far the
most common and clearly represents an enormous increase in efficiency over the wind
pollination that predominates in gymnosperms.
Angiosperms are pollinated by many kinds of animals (bees, butterflies, moths, flies, birds,
bats, rodents, marsupials, etc.). The wide variety of shapes, colors and “rewards” (nectar,
pollen, wax, oils, scents) are part of the “pollination syndrome”, a suite of adaptations to
attract a particular kind of animal to transfer pollen among different flowers. Specialists in
pollination biology can usually tell what kind of animal pollinates a particular flower from its
morphology. For example, big sturdy red/orange flowers with little odor are often birdpollinated; blue/yellow flowers with nectar guides (stripes or dots on the petals, often visible
only in UV light) are usually bee pollinated; blue, yellow or red flowers massed in a flattopped inflorescence (a “landing platform”) are often butterfly pollinated; nectaries at the base
of a long corolla tube are often associated with long-tongued insects such as hawkmoths or
butterflies, or if the flowers are red - by hummingbirds; night-blooming flowers of white/dull
color with a strong odor may be moth or bat pollinated; flowers that have foul or spicy odors,
often with mottled red colors are usually beetle or carrion fly-pollinated (they mimic rotten
flesh). Analyze the Table below and fill in an example for each syndrome in the last column,
among the flowers available in the lab.
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FLORAL CHARACTERISTICS
Pollinator
Color
Scent
Time of
Flowering
Corolla
Reward
Bee
Blue, yellow,
purple
Fresh, strong
Day
Bilateral landing
platform
Nectar and /or
pollen
Butterfly
Bright;
often red
Fresh, weak
Day
Landing platform;
sometimes nectar
spurs
Nectar only
Moth
White or
pale
Sweet, strong
Night or
dusk
Dissected;
sometimes nectar
spurs
Nectar only
Fly (reward)
Light
Faint
Day
Radial, shallow
Nectar and /or
pollen
Fly (carrion)
Brownish,
purplish
Rotten, strong
Day or night
Enclosed or open
None
Beetle
Often green
or white
Various, strong
Day or night
Enclosed or open
Nectar and /or
pollen
None
Day
Tubular or pendant;
ovary often inferior
Nectar only
Birds
Bright;
often red
Bat
Whitish
Musky, strong
Night
Showy flower or
inflorescence
Nectar and /or
pollen
Non-flying
mammals
Dull-colored
Unscented to
variously strong
Night
Robust ,exerted
styles and stamens
Copius nectar
and/or pollen
Example
Modified from Judd et al 2002
2-Match flower scents in the scent demonstration cups to pollination syndromes in the
table. Not all scents will be obvious (nor pleasant!), have fun!
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ANGIOSPERM REPRODUCTION
Background
In the majority of angiosperm species, the ovule has two integuments. In certain derived families
where only one integument is present (e.g. Asteraceae, Orchidaceae), the single integument is
interpreted as a loss of one integument or fusion of two integuments. Within the ovule, the
nucellus (megasporangium) of angiosperms may be considerably reduced in comparison to
gymnosperms. In many taxa it consists of little more than a cell layer over the megaspore mother
cell. In other taxa, there is a more substantial nucellus, but even in that case it is largely
destroyed as the female gametophyte develops.
Female gametophyte. In most angiosperms and gymnosperms, meiosis produces a linear array
of potential megaspores. The megaspore wall of angiosperms is unique among seed plants in
being thin and lacking sporopollenin. The sequence of nuclear divisions, abortions and
migrations that produce the mature female gametophyte can follow over ten different
developmental pathways. About 80% of the angiosperm female gametophytes follow a particular
sequence of events known as the Polygonum type (a derived type, see lecture notes for ancestral
gametophyte development, e.g. in basal angiosperms). The megaspore nucleus undergoes mitosis
and the two nuclei migrate to opposite ends of the cell. Two subsequent mitoses produce a total
of four nuclei at each end of the cell. Two nuclei, one from each group of four move toward the
center of the cell and are not involved in subsequent cellularization. The other six nuclei, three at
each end of the cell, develop walls. The trio of cells at the micropylar end consists of an egg
flanked by two synergids. There is no hint of archegonia. The three cells at the chalazal end
(near the ovule stalk) are known as antipodals and have no known function. The two nuclei that
are not walled off are called polar nuclei. They lie near each other in the original megaspore
cytoplasm forming the central cell. Thus, the mature female gametophyte, or embryo sac,
consists of 7 cells and 8 nuclei. The female gametophyte develops very quickly and is complete
before pollination occurs.
Development of a Polygonum type
angiosperm ovule beginning with the
formation of the integuments and single
megasporocyte (A,B), continuing through the
formation of megaspores (C-E), and
concluding with the successive stages in
development of the embryo sac (F-J). (From
Gifford and Foster, 1998)
chalazal
end
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Male gametophyte. In 70% of angiosperm species, when pollen is released from the anther it
consists of just 2 cells, the generative cell and the tube cell. When the generative cell divides,
the products differentiate directly as sperm. In 30% of angiosperm species, the sperm are already
present when the pollen is released, 3-celled pollen represents the most derived condition. The
pollen wall of angiosperms has unique structural features that make it recognizable in the fossil
record. In almost all species the outermost wall (exine) consists of columns of fused
sporopollenin granules. Many gymnosperms are monosulcate. A sulcus is a slit or furrow in the
exine at the distal pole of the pollen grain (the one furthest from the point of contact between
microspores in a tetrad). The oldest angiosperm pollen is monosulcate also. In more derived
angiosperms, pollen grains are characterized by multiple apertures (e.g. three in Eudicots) with a
variety of shapes and positions.
Pollen is in a dessicated condition when shed from anthers. It hydrates after being deposited on a
compatible stigma. About 75% of angiosperm species are self-incompatible, due to interactions
between proteins in the pollen wall and in the stigmatic cells. In self-incompatible species pollen
grains fail to germinate or stop growing shortly after germination. Self-incompatibility may have
evolved in angiosperms following the evolution of bisexual flowers. Bisexuality involves the risk
of self-pollination leading to low genetic variability and sometimes to inbreeding depression.
Self-pollination is favored in some species - such as those that are weedy or long distance
migrators.
Fertilization. In a compatible reaction, pollen grains germinate on the stigma and the tubes grow
down through the style to the ovule. Angiosperm pollen tubes grow about two millimeters per
hour, a rate about 2,000 times faster than that seen in most gymnosperms. In flowering plants
only a few hours elapse between pollination and fertilization.
Pollen tubes grow into the ovary and find their way to the ovules guided by signals from the
synergid cells. Before the tubes reach the embryo sac (female gametophyte) one of the two
synergids flanking the egg degenerates. It has been shown in several species that pollen tubes
penetrate the embryo sac via the degenerating synergid. The two sperm within a pollen tube are
discharged into the synergid. Although the egg and synergid are mostly walled cells, they lack
walls at their chalazal ends. It is assumed that the sperm can move through the cell membrane at
that end. One moves into the egg and fuses with the egg nucleus and the other moves into the
central cell and there fuses with the two polar nuclei to produce the endosperm, a triploid tissue
that nourishes the developing embryo. Double fertilization in angiosperms should be contrasted
with that of the two Gnetophytes Ephedra and Gnetum. In both genera, one fertilization event
produces supernumerary embryos that ultimately die. Nourishment of the surviving embryo is
carried out by cells of the female gametophyte. In angiosperms, the female gametophyte consists
of just seven cells (8 nuclei). Nourishment of the embryo in most flowering plant species is
carried out by the endosperm. If fertilization fails in an angiosperm ovule, no food supply for the
embryo is produced - an obvious efficiency. In Cycads and Ginkgo, the cells of the female
gametophyte are fully stocked with food whether or not fertilization occurs - an obvious
inefficiency. In conifers, an intermediate situation prevails. The cellular female gametophyte
develops, but does not accumulate a full complement of reserve material if fertilization does not
occur.
Fruits. It is possible that a shift from wind to animal-pollination constituted the selection
pressure that led to carpel evolution. Many seeds are good to eat and there may have been strong
selection to shield ovules from visiting pollinators. As the seeds within it mature, so does the
ovary wall, creating a fruit- a structure unique to the flowering plants. Fruits may function as
intact dispersal units: fleshy ones often being eaten by animals that defecate the seeds at some
distance, dry fruits being wind-dispersed; or fruits may split open and the seeds themselves
dispersed by a variety of means.
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Laboratory Exercise: angiosperm reproduction
I. Male Gametophyte (Pollen) and Pollen Tube
1. Prepare Tradescantia pollen for in vivo germination. Place pollen in a drop of pollen
germination medium on a slide, add a coverslip and place in a humid chamber (petri dish
lined with damp kimwipe) to avoid evaporation. Observe after about 1 hour.
2. Microsporogenesis- Observe slides of sequential development of pollen in Lilium anthers,
beginning with the sporogenous tissue and ending with mature pollen (early anthers, pollen
mother cells, first and second division, mature anthers and pollen tetrad).
Lilium - Microsporogenesis:
A. Microsporocyte,
B. End of Meiosis I.
C. Tetrad of Microspores.
3. Observe slides of germinating Lilium pollen on stigma.
II. Female Gametophyte (Embryo Sac)
There are 11 recognized types of female gametophyte development! They can be grouped into
three major categories, depending on the number of megaspores that participate in the formation
of the embryo sac: monosporic, bisporic and tetrasporic. Review the typical Polygonum type in
the introductory notes (monosporic, follow the diagrams), then observe slides of the Lilium type
(tetrasporic). Study the following sequence in the slides provided. Draw and label the stages in
the space provided:
a) megasporocyte, Identify integuments and nucellus.
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b) mature embryo sac: three antipodal cells, two synergid cells, one egg nucleus and
one central cell,
c) fertilization, look for evidence of pollen tube intrusion into embryo sac,
d) young embryo with endosperm. What is the ploidy level of the endosperm?________
e) mature seed.
III. The Fruit: Use the attached key to identify different types of fruits available in lab.
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Key To Fruit Types
1a. Fruit derived from several ovaries of one or more flowers
2a. Fruit arising from the several ovaries of as many flowers
(examples: pineapple, mulberry)
MULTIPLE FRUIT
2b. Fruit arising from the coalescence of several ripened ovaries
of one flower (example: raspberry, blackberry)
AGGREGATE FRUIT
1b. Fruit derived from a single ovary (simple or compound)
3a. Fruit fleshy or juicy when ripe
4a. Ovary wall of fruit (or pericarp) entirely or in part fleshy
5a. Fruit indehiscent
6a. Ovary wall entirely fleshy (examples: tomato, cranberry,
grape, currant, banana, melon [pepo], and citrus fruit
[hesperidium])
BERRY
6b. Ovary wall of three distinct layers, the inner one bony
(endocarp), the middle fleshy (mesocarp), and the outer "skinlike" (exocarp) (examples: peach, plum, cherry)
DRUPE
5b. Fruit dehiscent
7a. Fruit derived from one carpel
FOLLICLE
7b. Fruit derived from a compound gynoecium
CAPSULE
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4b. Ovary wall (e.g., the outer layer of an apple 'core') of fruit papery,
surrounded by a fleshy material that represents the coalescent parts of
the stamens, petals, sepals, and (some believe) receptacle (examples:
apple, pear, quince)
POME
3b. Fruit typically dry and usually hardened when ripe
8a. Fruit indehiscent (does not open or dehisce when mature), generally
with one seed
9a. Ovary wall of varying thickness, usually not bony
10a. Fruit not winged (examples: buttercup, 'seeds' of strawberry,
sunflower family, sedges, grasses [ovary wall adherent to and
surrounding seed, may be called
caryopsis or grain])
ACHENE
10b. Fruit winged (examples: elm, tulip tree)
SAMARA
9b. Ovary wall hardened and bony
11a. Fruit usually > 5mm long (examples: oak, chestnut,
hazelnut)
NUT
11b. Fruit small, usually < 5mm long (examples: borage and mint
families [Boraginaceae and Lamiaceae]
NUTLET
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8b. Fruit dehiscent (opens or dehisces when mature, usually along certain definite
lines or sutures), with one or more seeds
12a. Fruit derived from a single carpel
13a. Fruit dehiscing along one side (examples: columbine, larkspur, magnolia,
milkweed)
FOLLICLE
13b. Fruit dehiscing along two sides or breaking crosswise into onesegments
seeded
14a. Fruit dehiscing along two sides (example: only
the legume family [Fabaceae or Leguminosae])
LEGUMES
14b. Fruit breaking into oneseeded segments
(example: only the legume family [Fabaceae
or Leguminosae]
LOMENT
12b. Fruit derived from a compound gynoecium of two or more carpels (types
of capsules)
15a. Fruit always 2-carpellate, two-celled, and with parietal placentation
16a. Fruit > 2-3 times longer than wide (example: only the mustard family
[Brassicaceae or Cruciferae])
SILIQUE
16b.
Fruit <2-3 times longer than wide (example: only the mustard family
[Brassicaceae or Cruciferae])
SILICLE
15b. Fruit 2 or more carpellate, one or more celled, and with various types of
placentation.
CAPSULE
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17a. Fruit dehiscing by pores (poricidal dehiscence; example: poppy)
PORICIDAL CAPSULE
17b. Fruit dehiscing along the septa or into the locules or by a lid.
18a. Fruit dehiscing by a lid (examples: Portulacaceae and some
Caryophyllaceae)
CIRCUMSCISSILE CAPSULE
18b. Fruit dehiscing directly into the locules or along the septa.
19a.
Fruit dehiscing directly into the locules (examples: iris, phlox,
pyrola, violet, waterleaf)
LOCULICIDAL CAPSULE
19b. Fruit dehiscing along the septa
20a. Fruit dehiscing to form 1-seeded segments called mericarps
(examples: carrot, maple, spurge)
SCHIZOCARPOUS CAPSULE
or SCHIZOCARP
20b. Fruit dehiscing to form several-seeded segments (examples:
peppers, figwort, rhododendron))
SEPTICIDAL CAPSULE
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