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Plant Diversity II: The Evolution of Seed Plants

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1 Plant Diversity II: The Evolution of Seed Plants
Chapter 30 Plant Diversity II: The Evolution of Seed Plants

2 Overview: Transforming the World
Seeds changed course of plant evolution, enabling their bearers to become dominant producers in most terrestrial ecosystems Seed consists of embryo and nutrients surrounded by protective coat Humans starting cultivating wheat, figs, maize, bananas, other wild seed plants ~13,000 years ago

3 In addition to seeds, following are common to all seed plants
Concept 30.1: Seeds and pollen grains are key adaptations for life on land In addition to seeds, following are common to all seed plants Reduced gametophytes Heterospory Ovules Pollen Allows plants to cope with terrestrial conditions (drought, exposure to UV radiation)

4 Advantages of Reduced Gametophytes
Gametophytes of seed plants develop within walls of spores that are retained within tissues of parent sporophyte Protects delicate female (egg-containing) gametophytes from environmental stresses (desiccation/UV) Dependent gametophytes obtain nutrients from sporophyte Free-living gametophytes of seedless plants fend for themselves

5 Fig. 30-2 PLANT GROUP Mosses and other nonvascular plants Ferns and other seedless vascular plants Seed plants (gymnosperms and angiosperms) Reduced, independent (photosynthetic/free-living) /visible to naked eye Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition Gametophyte Dominant Reduced, dependent on gametophyte for nutrition Sporophyte Dominant Dominant Gymnosperm Angiosperm Sporophyte (2n) Microscopic female gametophytes (n) inside ovulate cone Microscopic female gametophytes (n) inside these parts of flowers Sporophyte (2n) Gametophyte (n) Example Figure 30.2 Gametophyte/sporophyte relationships in different plant groups Microscopic male gametophytes (n) inside these parts of flowers Microscopic male gametophytes (n) inside pollen cone Sporophyte (2n) Sporophyte (2n) Gametophyte (n)

6 Heterospory: The Rule Among Seed Plants
Ancestors of seed plants were likely homosporous (produce one kind of spore, usually giving rise to bisexual gametophyte), while seed plants are heterosporous Megasporangia produce megaspores (each has single functioning one) that give rise to female gametophytes Microsporangia produce microspores (each produces vast numbers of) that give rise to male gametophytes

7 Ovules and Production of Eggs
Ovule consists of Megasporangium Megaspore One/more protective integuments (layer of sporophyte tissue that envelopes/ protects megasporangium) Gymnosperm megaspores have one integument Angiosperm megaspores usually have two

8 Pollen and Production of Sperm
Microspores develop into pollen grains, which contain gametophytes enclosed within pollen wall Pollination: transfer of pollen to part of seed plant containing ovules Pollen eliminates need for film of water and can be dispersed great distances by air or animals Flagella lost in most gymnosperms (except some, like ginkgos/cycads)/ all angiosperms If pollen grain germinates, it gives rise to pollen tube that discharges two sperm into female gametophyte within ovule

9 The Evolutionary Advantage of Seeds
Seed develops from whole ovule and includes sporophyte embryo/food supply packaged in protective coat Seeds provide some evolutionary advantages over spores Extra layers of seed coat provide extra protection to embryo over single-celled spores Seeds have stored food/spores don’t which enables seeds to remain dormant for days to years, until conditions are favorable for germination, and then give them critical support for growth May be transported long distances by wind or animals

10 Five Derived Traits of Seed Plants
Fig. 30-UN3 Five Derived Traits of Seed Plants Reduced gametophytes Microscopic male and female gametophytes (n) are nourished and protected by the sporophyte (2n) Male gametophyte Female gametophyte Heterospory Microspore (gives rise to a male gametophyte) Megaspore (gives rise to a female gametophyte) Ovules Integument (2n) Ovule (gymnosperm) Megaspore (2n) Megasporangium (2n) Pollen Pollen grains make water unnecessary for fertilization Seeds Seeds: survive better than unprotected spores, can be transported long distances Integument Food supply Embryo

11 Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones
Gymnosperms have “naked” seeds not enclosed by ovaries Seeds exposed on modified leaves (sporophylls) that usually form cones (stobili) Extant gymnosperms include Cycadophyta (cycads) Gingkophyta (one living species: Ginkgo biloba) Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia) Coniferophyta (conifers, such as pine, fir, and redwood)

12 Gymnosperm Evolution Fossil evidence reveals that by late Devonian period some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants (woody but did not bear seeds) First seed-bearing plants (gymnosperms) appear early in fossil record ~360 mya, more than million years before first angiosperm fossils Early seed plants/later lineages all become extinct but morphological/molecular evidence places surviving lineages of seed plants into two monophyletic sister clades, gymnosperms/angiosperms

13 Gymnosperms appear early in fossil record (~305 million years old) during Carboniferous ecosystems still dominated by lycophytes, horsetails, ferns, other seedless vascular plants Drier climatic conditions in Permian favored spread of gymnosperms which favored key terrestrial adaptations of seeds/pollen, along with thick cuticles, relatively small surface area of needle-shaped leaves Dominated Mesozoic terrestrial ecosystems Although angiosperms dominate most terrestrial ecosystems today, cone-bearing gymnosperms (conifers) dominate in northern latitudes

14 Phylum Cycadophyta (Cycads)
Individuals have large cones and palmlike leaves Thrived during Mesozoic, but relatively few species exist today sago palm

15 Phylum Ginkgophyta This phylum consists of single living species, Ginkgo biloba High tolerance to air pollution/popular ornamental tree, but only pollen-producing trees (fleshy seeds smell rancid as they decay)

16 Phylum Gnetophyta Phylum comprises three genera (Gnetum, Welwitschia, Ephedra) Species vary in appearance/some are tropical/others in deserts

17 Phylum Coniferophyta Phylum is by far largest of gymnosperm phyla
Most conifers are evergreens/carry out photosynthesis year round (1st live one discovered 1994) (lumber) (oldest living trees, >4600 years old) (“berries” are ovule-producing cones of fleshy sporophylls) (large/old, cousins (redwoods) grow to >100 m) (shed needles/cold tolerant)

18 The Life Cycle of a Pine: A Closer Look
Three key features of gymnosperm life cycle are Dominance of sporophyte generation Development of seeds from fertilized ovules Transfer of sperm to ovules by pollen Life cycle of pine provides example Pine tree is sporophyte/produces sporangia in male/female cones Small cones produce microspores (pollen grains), each of which contains male gametophyte Familiar larger cones contain ovules, which produce megaspores that develop into female gametophytes It takes nearly three years from cone production to mature seed

19 4. Pollination occurs when pollen Pollen grain Pollen grains (n)
Fig Most conifers have both ovulate/ pollen cones on each tree 3. Ovulate cone (each scale has two ovules each w/megasporangium) Key Haploid (n) Ovule Diploid (2n) Megasporocyte (2n) Integument Pollen cone Microsporocytes (2n) Mature sporophyte (2n) Megasporangium (2n) 4. Pollination occurs when pollen grain reaches ovule, pollen grain germinates, forms pollen tube that slowly digests its way through megasprangium Pollen grain Pollen grains (n) MEIOSIS MEIOSIS Microsporangia 2. Microsporangium (2n) divide by meiosis/produce haploid microspores which develop pollen grain (w/male gametophyte) Surviving megaspore (n) 5. Megasporocyte undergoes meiosis, producing four haploid cells/one survives as megaspore Seedling Archegonium Figure 30.6 The life cycle of a pine Seeds Female gametophyte Female gametophyte develops within megaspore, contains two or three archegonia, each of which will form egg 7. By time eggs mature, two sperm cells developed in pollen tube which extends to female gametophyte Fertilization occurs when sperm and egg nuclei unite, 8. usually one year after pollination. All eggs may be fertilized, but only one zygote develops into embryo Ovule becomes seed w/embryo, food supply, seed coat Food reserves (n) Sperm nucleus (n) Seed coat (2n) Pollen tube Embryo (2n) FERTILIZATION Egg nucleus (n)

20 They are most widespread and diverse of all plants
Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits Characteristics of Angiosperms Angiosperms Angiosperms are seed plants with reproductive structures called flowers and fruits (ripened matured ovaries that contain seeds) They are most widespread and diverse of all plants All angiosperms are classified in single phylum, Anthophyta Name comes from Greek anthos, flower

21 Flowers Flower: angiosperm structure specialized for sexual reproduction Pollinated by insects/animals (direct contact) or wind (dense populations) Specialized shoot with up to four whorls (rings) of modified leaves (floral organs)

22 Carpel produce megaspores Stamen Anther where pollen is Style
Fig. 30-7 Stigma sticky to receive pollen Carpel produce megaspores and their products, gametophytes (ovules) Stamen produce microspores that develop into pollen grains Anther where pollen is produced Style leads from stigma to ovary Filament stalk Ovary At base of carpal Contains one or more ovules If fertilized, develop into seed Figure 30.7 The structure of an idealized flower Petal Sterile Brightly colored Attract pollinators Sepal Sterile Enclose flower Ovule

23 Fruits Fruit typically consists of mature ovary (ovary walls thicken) but can also include other flower parts Protect dormant seeds/aid in their dispersal Mature fruits can be either fleshy where ovary walls become soft during ripening (oranges, plums, grapes) or dry (beans, nuts, grains) Various fruit adaptations help disperse seeds Carried by wind (parachutes/propellers), water, or animals (burrs) to new locations When eaten, most pass through digestive tract unharmed/deposited with feces

24 The Angiosperm Life Cycle
Flower of sporophyte composed of both male and female structures Male gametophytes contained within pollen grains produced by microsporangia of anthers Each has two haploid cells (generative cell that divides, forming two sperm/tube cell that produces pollen tube Female gametophyte (embryo sac) develops within ovule contained within ovary at base of stigma Contains only few cells, one is egg

25 Gnetophyta’s double fertilization leads to two embryos only
Most flowers have mechanisms to ensure cross-pollination between flowers from different plants of same species (enhances genetic variability) Pollen adheres to stigma  pollen tube  micropyle (pore in integuments of ovule)  two sperm cells discharged into embryo sac  double fertilization  one fertilizes egg forming diploid zygote  sporophyte embryo w/rudimentary root and one or two seed leaves (cotyledon) other fuses w/two nuclei in large central cell of female gametophyte producing triploid cell (unique to angiosperms)  divides repeatedly  endosperm (tissue rich in starch/other food reserves for embryo) Double fertilization may prevent flowering plants from squandering nutrients on infertile ovules (synchronizes development of food storage w/development of embryo) Gnetophyta’s double fertilization leads to two embryos only

26 1. On anther, each microsporangium contains
Fig Key 1. On anther, each microsporangium contains microsporocytes that divide by meiosis, producing microspores Anther Haploid (n) Diploid (2n) Microsporangium Microspore develops into pollen grain. Generative cell of gametophyte divides, forming two sperm. Tube cell produces pollen tube Mature flower on sporophyte plant (2n) 7. Seed germinated, embryo develops into mature sporophyte Microsporocytes (2n) MEIOSIS Generative cell Microspore (n) Ovule (2n) Tube cell Male gametophyte (in pollen grain) (n) Ovary Pollen grains 3. In megasporangium of each ovule, megasporocyte divides by meiosis, producing 4 megaspores. One survives and forms female gametophyte 4. After pollination, two sperm cells discharged in each ovule Germinating seed 6. Zygote develops into embryo packaged w/food into seed (fruit tissues surround seed) MEIOSIS Stigma Megasporangium (2n) Pollen tube Embryo (2n) Endosperm (3n) Seed coat (2n) Sperm Seed Megaspore (n) Style Antipodal cells Central cell Synergids Egg (n) Figure The life cycle of an angiosperm Female gametophyte (embryo sac) Pollen tube Nucleus of developing endosperm (3n) 5. Double fertilization occurs. One sperm fertilizes egg, forming zygote. Other fertilizes central cell, forming endosperm (food supply, 3n) Sperm (n) FERTILIZATION Zygote (2n) Egg nucleus (n) Discharged sperm nuclei (n)

27 Angiosperm Evolution Clarifying origin/diversification of angiosperms poses fascinating challenges to evolutionary biologists Angiosperms originated at least 140 mya During late Mesozoic, major branches of clade diverged from their common ancestor By mid-Cretacous (100 mya), adaptive radiation allowed angiosperms began to dominate many terrestrial ecoosystems

28 Fossil Angiosperms Primitive fossils of 125-million-year-old angiosperms display derived and primitive traits Fossils, phylogenetic analyses, developmental studies offer insights into origin of flowers Archaefructus sinensis has anther/seeds but lacks petals/sepals May belong to earliest-diverging group of angiosperms known Suggests that ancestors of flowering plants were herbaceous rather than woody May have originated as aquatic plants (fossils found w/fish fossils/had bulbous structures) Much debate/need transitional fossil

29 Archaefructus sinensis, a 125-million-year-old fossil
Fig A primitive flowering plant? Carpel Stamen 5 cm (a) Archaefructus sinensis, a 125-million-year-old fossil (may represent sister group to all other angiosperms) Figure 30.11 (b) Artist’s reconstruction of Archaefructus sinensis

30 Angiosperm Phylogeny Which seed plants, including fossil species, are most closely related to angiosperms? Molecular/morphological evidence suggest that living gymnosperms are monophyletic group whose earliest lineages diverged from ancestors of angiosperms and gymnosperms ~305 mya Angiosperms may be closely related to Bennettitales, extinct seed plants with flowerlike structures Also need to work out order in which angiosperm clades diverged from one another Amborella/water lilies are likely descended from two of most ancient angiosperm lineages

31 Microsporangia (contain microspores)
Fig Angiosperm evolutionary history Living gymnosperms Microsporangia (contain microspores) Bennettitales Amborella Water lilies Most recent common ancestor of all living angiosperms Star anise and relatives Monocots Magnoliids Eudicots Figure 30.12 Ovules Millions of years ago (a) A possible ancestor of the angiosperms? Bennettitales, extinct group of seed plants might be more closely related to angiosperms than to gymnosperms (b) Angiosperm phylogeny One current hypothesis of angiosperm evolutionary Relationships, based on morphological/molecular evidence Angiosperms originated at least 140 mya Dotted line represents uncertain position of Bennettitales, possible sister group to angiosperms

32 Developmental Patterns in Angiosperms
Egg formation in angiosperm Amborella resembles that of gymnosperms (possible common ancestor) Scientists are curious how second integument of angiosperms originated (only one in gymnosperms) Researchers are currently studying expression of flower development genes in gymnosperm and angiosperm species

33 Angiosperm Diversity Two main groups of angiosperms are monocots (one cotyledon) and eu’dicots (“true” dicots) Clade eudicot includes some groups formerly assigned to paraphyletic dicot (two cotyledons) group Basal angiosperms are less derived and include flowering plants belonging to oldest lineages Comprised of 3 small lineages (Amborella trichopoda, oldest lineage which diverged into  water lilies/star anise Magnoliids share some traits with basal angiosperms but are more closely related to monocots and eudicots (evolved later) Include magnolias/laurels/black pepper plants

34 >¼ are monocots >2/3 are eudicots

35 Evolutionary Links Between Angiosperms and Animals
Animals/plants influenced evolution of each other To keep herbivores from eating roots/leaves/seeds, plants evolved defenses Pollination of flowers/transport of seeds by animals are two important relationships in terrestrial ecosystems Clades with bilaterally symmetrical flowers (insect pollinator obtains nectar only when approaching flower form certain direction) have more species than those with radially symmetrical flowers Likely bilateral symmetry affects movement of pollinators (have to move from flower to flower) and reduces gene flow in diverging populations Speciation should occur more rapidly (there are more clades w/bilaterally symmetrical flower than radially symmetrical flowers)

36 Mean difference in number of species Bilateral symmetry (N = 15)
Fig EXPERIMENT Time since divergence from common ancestor “Bilateral” clade Compare numbers of species Common ancestor “Radial” clade RESULTS: flower shape affects rate at which new species formed 3,000 2,000 Mean difference in number of species Figure Can flower shape influence speciation rate? 1,000 Bilateral symmetry (N = 15) Radial symmetry (N = 4)

37 Concept 30.4: Human welfare depends greatly on seed plants
No group of plants is more important to human survival than seed plants Key sources of food, fuel, wood products, and medicine Our reliance on seed plants makes preservation of plant diversity critical

38 Products from Seed Plants
Most of our food comes from angiosperms Six crops (wheat, rice, maize, potatoes, cassava, and sweet potatoes) yield 80% calories consumed by humans Angiosperms feed livestock (takes 5-7 kg of grain to produce 1 kg of grain-fed beef) Modern crops are products of relatively recent genetic change resulting from artificial selection Other edible products (tea, coffee, cacao, spices) Many seed plants provide wood (fuel, paper, construction) Secondary compounds of seed plants are used in medicines (25% US Rx drugs contain one or more active ingredients extracted or derived from plants)

39 Table 30-1a Table 30.1

40 Threats to Plant Diversity
Destruction of habitat is causing extinction of many plant species Greatest problem in tropics, where >½ lives and where population growing fastest 35 million acres of tropical rain forest (size of Iowa) cleared (slash-and-burn) each year/completely gone in years Loss of plant habitat is often accompanied by loss of animal species that plants support At current rate of habitat loss, 50% of Earth’s species will become extinct within next 100–200 years Explored potential uses of tiny fraction of >290,000 known plant species Our food is based on cultivation of ~24 species of seed plants <5000 plant species studied for medicinal purposes

41 3/25/2017 Division Common name Dominant generation Fluid transport
Sperm transport Dispersal unit Bryophyta Mosses Haploid Gametophyte Nonvascu-lar Flagellated sperm Spores Lycophyta Club mosses Sporophyte Vascular Sphenophyta Horsetails Pterophyta Ferns Diploid Coniferophyta Conifers Wind-dispersed pollen Naked Seeds Anthophyta Flowering plants Wind-, or animal-dispersed pollen Seeds in flowers 3/25/2017

42 Charophyte green algae
Fig. 30-UN4 a. flowers b. embryos c. seeds d. vascular tissues Charophyte green algae Mosses Ferns Gymnosperms Angiosperms

43 You should now be able to:
Explain why pollen grains were an important adaptation for successful reproduction on land List and distinguish among the four phyla of gymnosperms Describe the life history of a pine; indicate which structures are part of the gametophyte generation and which are part of the sporophyte generation Identify and describe the function of the following floral structures: sepals, petals, stamens, carpels, filament, anther, stigma, style, ovary, and ovule Explain how fruits may be adapted to disperse seeds Diagram the generalized life cycle of an angiosperm; indicate which structures are part of the gametophyte generation and which are part of the sporophyte generation Explain the significance of Archaefructus and Amborella Describe the current threat to plant diversity caused by human population growth

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