Exam 5: Unit 9 : Angiosperm Evolution

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Introduction to Anthrophyta

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1. most diverse & widespread group of plants with 300,000 species known (and there are many more yet to be discovered)

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4 Basic types of Angiosperms

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1. monocot
2. eudicot
3. basel angiosperms
4. magnolids

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Monocot characteristics

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1. one cotyledon
2. leaf veins parallel
3. vascular bundles scattered in stem
4. fibrous root system
5. flower parts in 3's
6. e.g. Grasses (wheats), Palm, Lily

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Eudicot characteristics

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1. 2 cots
2. veins net-like in leaves
3. vascular bundles have rings arrangement in stem
4. tap root
5. flower parts in 4 or 5's
6. e.g. Roses, avocados, peas, oaks

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Advantages / Disadvantages of the flower

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a. Monoecious (both female&male) flowers have the male androecium and the female gynoecium in the same flower in addition to the sterile parts of the flower (calyx and corolla (petals make up this)
i. androecium: stamens (filament+anther)
ii. gynoecium: carpels (megasporophylls =
ovary + style + stigma)
b.therefore fertilization is virtually assured

c. but self-fertilization is not always desirable, so monecious plants are often self-incompatable, meaning they can not self.

d. on other hand, plants can be dioecious (the stamine flower and carpellate flower are on different plants i.e. male plants and female plants). This assures out-crossing

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Advantages / Disadvantages of the flower continued

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e. perfection in flowers
i. perfect flower is staminate & carpellate
ii. imperfect flower is staminate or carpellate

f. Animal Pollination: gives delivery of male gamete to female gamete (vs. wind for gymnosperms)
i.This direct delivery system extremely efficient
and a huge advantage over wind pollination
ii.Animal pollination is so important that there are
many flowers that have co-evolved with their
animal pollinators to such an extent that only a
single animal can pollinate that particular flower

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Fruits have TWO mature ovaries
&
there are TWO types

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1. Dry fruits: (a) protective, (peas in a pod or pomegranate), (b) help in seed dispersal (maple seed = wings , cocklebur = hooks)

2. Fleshy fruits: (a) pericarp develops from ovary walls (b) form a "bait" that animals will eat and carry away and help in seed dispersal (apple,tomato,banana)

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Three types of fruit

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1. Simple fruit: develep from 1 carpel or a few carpels that make a single frui (e.g. grape, apple)
know: carpel wall
2. Aggregate fruit: many indivual carpels of a single flower, (sometimes fused, often not) aggregate to form a fruit (e.g. strawberry & rasberry)

3. Multiple fruit: many separately fertilized gynoecia fuse to form the fruit (e.g. pineapple)

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Simple Fruit Types

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1. berry: one to many carpels, each with many seeds, carpel wall is fleshy (e.g. grape &tomato)

2. Drupe: one to many carples, each with 1 seed, inner carpel wall is stony and adheres to the fleshy portion of the enlarged carpell wall (e.g. cherry & peach)

3. Pome: (all are from the rose family) compound inferior ovary, where the parianth enlarges and becomes fleshy, 1 seed per locule (e.g. pear & apple)

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Accessory fruits

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If a fruit contains parts other than just the enlarged pericarp, then the fruit is known as accessory fruit
(a).strawberry fruit is an enlarged receptacle with
the seed on the outside. It is an aggregate fruit
(individual fruits are fused)& an accessory fruit
(b)Pineapple fruit is many separately fertilized
gynoecia fusd to form the fruit with other flower
parts squeezed between. A pineapple is a
multiple & accessory fruit

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Coconut

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One of the oddest fruit of all
(a). drupe: one to several carpel,each with 1 seed, inner carpel wall (endocarp) is stony and normally adheres to the outer fleshy portion of the enlarged carpel wall
(b) Coconut has 3 carpels. the fused outer carpel walls are a fibrous husk. The flesh is solid endosperm of the seed and the coconut milk is a liquid endosperm. The embryos float in the liquid endosperms

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Life Cycle in Anthophyta

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1. The angiosperm life cycle takes advantage of flowers and fruit characteristics to make the angiosperm life cycle the most efficient in the plant kingdom
2. Angiosperms are the ultimate in heterospory
a. Microgametophy is a 3 celled pollen grain
b. Megagametophy is a 7 or 8 cell embryo sac

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Microsporogenesis

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1.) Cells in the anther develop into diploid
microsporocytes (microspore mother cells)
2.) Microspore mother cells undergoes meiosis to form 4 haploid pollen cells
3.) Each pollen cell undergoes mitosis twice to form 4 haploid cells of which 2 or 3 survive and develop into a mature pollen grain or microgametophyte
4.) usually 2 of the 3 cells of the microgametophyte are sperm cells

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Megasporegenesis

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1.) In the early ovule there is the nucellus (the megasporangium) and a single diploid megasporocyte (megaspore mother cell)
2.) Megaspore mother cell undergoes meiosis to form 4 haploid megaspores
3.) 3 of the 4 megaspores disintegrate and the remaining cell forms the megagametophyte
4.) the nucleus of the megagametophyte undergoes mitosis 3 times to form 8 haploid nuclei
5.) The 8 nuclei undergoes development into 7 different cells, one of which has 2 nuclei (the central cell). But only limited cell division occurs. This structure is the embryo sac and is the mature megagametophyte
6.) important cells of the embryo sac: egg cell & polar nuclei of the central cell

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Fertilization

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1. Pollen lands on the stigma and the tube germinates from the pollen grain (pollination)
2. Pollen tube contains the tube nucleus and 2 sperm cells. Tube grows down the stigma to the ovule and enters through the micropyle
3. Upon entering the embryo sac, the pollen tube disgorges the tube nucleus and 2 sperm cells
4. One sperm fused with the egg cell to make the zygote & the other fuses with the 2 polar nulcei of the central cell = double fertilization

result: diploid embryo & triploid endosperm

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Development of the Seed

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1. In the embryo sac, the triploid central cell expands to form the endosperm

2. Integuuments become the seed coat

3. Zygote develops into an embryo

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Why is the Angiosperm life cycle so effective?

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1. Gametophyte generation is much reduced, sporophyte dominates
2. Vulnerable gametophytes held and protected in the robust sporophyte
3. Direct, efficient delivery of gametophytes to each other, usually aided by animals
4. Resulting embryo well protected & provided with food by seed and sporophyte
5. Several mechanisms to ensure out-crossing including (some of these seen in gymnorsperms too)
(a). some flowers contain only one of two sets of
sex organs (dioecious) & so there are
staminate & carpellate plants
(b).different times for ripping of male & female
flowers
(c). Self-incompatability

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Introductuon to Angiosperm Evolution

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A) Angiosperms evolution timeline
i. Silurian Period (~440 mya) 1st fossil vasc. plant
ii.Devonian Period (~408 mya) 1st fossil vasc. plant
disappear from fossil record, diversification of later lycophytes, pteridophytes & seed ferns
ii. Permian Period (~290 mya) 1st fossil gymnosperms, later diversification of gymnosperms, lycophyte trees, seed ferns eventually disappear
iii. Cretaceous Period: (~146 mya): 1st fossil angiosperms; by 90 mya diverse and dominant

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Darwin's Question: How cant it be that angiosperms appear late in fossil record, but then very rapidly become the dominant and most diverse plant group?

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The answer was not forthcoming in Darwin's time. However with new fossils, a better understanding of plant anatomy / morphology and new molecular techniques, science has a somewhat better understanding of angiosperm evolution than it did 15 years ago

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Latest Idea in Angiosperm Evolution

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a. Recent morphological and molecular anaylsis point to the Bennettitales or one of the extinct Gnetophytes as the most likely candidates for angiosperms ancestors.

b. Bennettitales and Gnetophytes 1st appear in the fossil record 225 mya. If they're the ancenstors of the angiosperms, then agiosperms are less than 225 myo. Some believe that that is not enought time to evovle the diversity seeen

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Evolution Continued: 2 possible origins for the Angiosperms have been hypothesized

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1. Family Bennettitales (a reconstruction), an extinct gymnosperms
2. Class Gnetales, a gymnosperm clade
i. flower like reproductive structures are found in the Bennettutales and Gnetophytes

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Latest Ideas in Angiosperm Evolution Continued

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I. Characters unique to Angiosperms (but not necessarily found in all angiosperms, especially basal angiosperms
(a). flowers (a few basal angio's' don't have)
(b). closed carpels (few basal angio's have open)
(c). double fertilization with triploid endosperm
(d). 3 nuclei in microgametophytes
(e). 8 nuclei in megagametophytes
(f). stamens with 2 pairs of pollen sacs
(g). phloem with sieve tube member & companion cells
(h) Triapperturate pollen (single being the ancestral state, triapperturate is derived, found in eudicots)

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Latest Ideas in Angiosperms Evolution Continued

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II. A single ancestral plant to the Angiosperms is not know
(a).this many, highly specific characteristics occurring in angiosperms is strong evidence that angiosperms are monophyletic
(b).therefore there should be a single ancestral plant
(c).no fossils have been found among the angiosperms that lack these characteristics: in particular scientists are looking for fossil angiosperms that lack flowers, fruits have open carpels and single aperture pollen
(d). earliest Angiosperm fossil plant parts are 130 myo

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Latest Ideas in Angiosperms Evolution Continued

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III. Earliest complete fossil Angiosperm known is Archaefructus (125 mya)
(a). had closed carpels but lacked modern flowers
(b). scientists are still looking for earlier forms

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Are the Basal Angiosperms clues to Angiosperm Evolution?

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1. What are the Basal Angiosperms?
(a). make up only 3% of living Angiosperms
(b). neither monocots or eudicots
(c). morphological evidence indicates that many of the basal Angiosperms split from the monocots and eudicot line before the monocots and eudicots split

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Are the Basal Angiosperms clues to Angiosperm Evolution Continued?

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2. What are some ancestral characteristics found in various basal angiosperms?
(a). single aperture pollen
(b.) sepals and petals undifferentiated
(c). 4 whorls seperate, not fused (colyx, corolla, gynoecium, androecium)
(d). Most archaic: flowe parts in spiral not whorls
(e).flowers with radial symmetry
(f). stamens very diverse, no set strucutre
(g). carpels open, unspecialized for pollinators
(h). ovary superior
(i). flower part numbers indeterminate

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Some examples of Basal Angiosperms

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Magnolia are not Basal Angiosperms but have many of their characters:
(a). numerous stamens
(b). many carpels on an elongate receptacle

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So Are the Basal Angiosperms clues to Angiosperm Evolution?

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The answer is YES, the basal angiosperms give clue to the origin of Angiosperms in that some of the ancestral states are seen in the basal Angiosperms

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Are the Basal Angiosperms clues to Angiosperm Evolution : Some characteristics that you may have not noticed

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1. They have tepals (no sepalrs or petals)
2. Tepals are in spiral (no whorls present)
3. Dioecious with imperfect flowers
4. Overall the flower are small, simple and not "showy"
5. Found only in New caledonia
6. Amborella is the only genus in its family
7. Therefore clearly is a remnant family from earlier epochs
8. Amborella mito were recently found to contain the comple genomes of 3
9. Recent genetic studies by Soltis & Janson agree that Amborella is the earliest extant angiosperm

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Evolution of the Flower

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1. Basic Evolutionary trends in flower
(1a). Ancestral Flowers: many parts, indefinite in #
(2a). Modern Flowers: few parts, definite # of parts
(1b). Ancestral Flowers: flower parts in spirals or 4 separate whorls of floral parts
(2b). Modern Flowers: whorls fused/reduced to 1,2,or 3
(1c) Ancestral Flowers: ovary superior
(2c) Modern Flowers: ovary inferior
(1d) Ancestral Flowers: radial symmetry
(2d). Modern Flowers: bilaretal symmetry

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Origin of Flower Petals

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Most extant flower petals evolved from sterilized stamens, not leaves. Evidence:
(a). petal vascular pattern matches that of stamens and not leaves
(b). Archaic stamens are often of various shapes, colors, may even be scented or broad and fleshy
(c). molecular evidence confrims this view

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Co-evolution of flowers and animal pollinators

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1. The driving force in evolution is making sure that the genetic material is passed to the next generation.
(a). therefore any change that aids in passing the genetic material to next generation is vert likely to be retained through natural selection
(b). since animal pollination is a very efficient method of passing pollen to the gynoecium, structures that aid in that process are strongly selected for.
(c). the same applied to animal pollinators, as the flower are animals food source

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Examples of Co-evolution of Flowers and Pollinators: hummingbird

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1. Hummingbirds/Monkey flower
(a). flower has long, tubular corolla. Exclued bees&beetles, long beak of hummingbird can easily acess nectar
(b). lots of nectar, necessary to attract a bird
(c). stamen positioned so that anther "dusts" head of hummingbird with pollen as it feeds

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Examples of Co-evolution of Flowers and Pollinators: Bees

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Bees: tongue is a tube for sucking nectar, have pollen collecting combs on legs, have pollen baskets on 3rd pair of legs and collect pollen from the combs
Rosemary: (a).stamens, stigmas curve upward to touch a visiting bee: exchange of pollen assured (b). lower petal is specialized landing platform that correctly positions bee to exchange pollen with the flower (c) other adaptations

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Family Asteraceae: examples of a highly derived flowers

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1.Radial disk flwoers + bilareal ray flowers: composite
2. Disk flowers (many flowers on a head)
i. 5 fused stamens
ii. 5 fused petals, also fused to ovary
iii. inferior ovary (epigynous)
iv. sepals often absent
3. bilaretal ray flowers (sometimes sterile)