Plant Diversity II: The Evolution of Seed Plants

Transcription

1 Plant Diversity II: The Evolution of Seed Plants

2 Overview: Feeding the World Seeds changed the course of plant evolution Enabling their bearers to become the dominant producers in most terrestrial ecosystems Figure 30.1

3 Concept 30.1: The reduced gametophytes of seed plants are protected in ovules and pollen grains In addition to seeds, the following are common to all seed plants Reduced gametophytes Heterospory Ovules Pollen

4 Advantages of Reduced Gametophytes The gametophytes of seed plants DO NOT fend for themselves Develop within the walls of spores retained within tissues of the parent sporophyte Protected from UV radiation and protect against drying out Obtain nutrients from parent

5 Gametophyte/sporophyte relationships Sporophyte (2n) Sporophyte (2n) Gametophyte (n) Gametophyte (n) (a) Sporophyte dependent on gametophyte (mosses and other bryophytes). Microscopic female gametophytes (n) in ovulate cones (dependent) (b) Large sporophyte and small, independent gametophyte (ferns and other seedless vascular plants). Microscopic male gametophytes (n) inside these parts of flowers (dependent) Microscopic female gametophytes (n) inside these parts of flowers (dependent) Microscopic male gametophytes (n) in pollen cones (dependent) Sporophyte (2n) (independent) Sporophyte (2n), the flowering plant (independent) Figure 30.2a c (c) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms).

6 Heterospory (Two): The Rule Among Seed Plants Seed plants evolved from plants that had megasporangia Which produce megaspores that give rise to female gametophytes Seed plants evolved from plants that had microsporangia Which produce microspores that give rise to male gametophytes

7 Ovules and Production of Eggs An ovule consists of A megasporangium, megaspore, and protective integuments Integument Spore wall Megasporangium (2n) Megaspore (n) Figure 30.3a (a) Unfertilized ovule. In this sectional view through the ovule of a pine (a gymnosperm), a fleshy megasporangium is surrounded by a protective layer of tissue called an integument. (Angiosperms have two integuments.)

8 Pollen and Production of Sperm Microspores develop into pollen grains Which contain the male gametophytes of plants Pollination Is the transfer of pollen to the part of a seed plant containing the ovules

9 If a pollen grain germinates It gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule Female gametophyte (n) Spore wall Egg nucleus (n) Male gametophyte (within germinating pollen grain) (n) Discharged sperm nucleus (n) Micropyle Pollen grain (n) Figure 30.3b (b) Fertilized ovule. A megaspore develops into a multicellular female gametophyte. The micropyle, the only opening through the integument, allows entry of a pollen grain. The pollen grain contains a male gametophyte, which develops a pollen tube that discharges sperm.

10 Pollen, which can be dispersed by air or animals Eliminated the water requirement for fertilization

11 The Evolutionary Advantage of Seeds A seed Develops from the whole ovule Is a sporophyte embryo, along with its food supply, packaged in a protective coat Seed coat (derived from Integument) Food supply (female gametophyte tissue) (n) Embryo (2n) (new sporophyte) Figure 30.3c (c) Gymnosperm seed. Fertilization initiates the transformation of the ovule into a seed, which consists of a sporophyte embryo, a food supply, and a protective seed coat derived from the integument.

12 Concept 30.2: Gymnosperms bear naked seeds, typically on cones Among the gymnosperms are many well-known conifers Or cone-bearing trees, including pine, fir, and redwood

13 The gymnosperms include four plant phyla Cycadophyta Gingkophyta Gnetophyta Coniferophyta

14 Exploring Gymnosperm Diversity PHYLUM CYCADOPHYTA PHYLUM GINKGOPHYTA Cycas revoluta PHYLUM GNETOPHYTA Gnetum Welwitschia Ovulate cones Ephedra Figure 30.4

15 Exploring Gymnosperm Diversity PHYLUM CYCADOPHYTA Douglas fir Common juniper Pacific yew Wollemia pine Bristlecone pine Sequoia Figure 30.4

16 Gymnosperm Evolution Fossil evidence reveals that by the late Devonian Some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants Figure 30.5

17 Gymnosperms appear early in the fossil record And dominated the Mesozoic terrestrial ecosystems Living seed plants Can be divided into two groups: gymnosperms and angiosperms

18 A Closer Look at the Life Cycle of a Pine Key features of the gymnosperm life cycle include Dominance of the sporophyte generation, the pine tree The development of seeds from fertilized ovules The role of pollen in transferring sperm to ovules

19 The life cycle of a pine Figure In most conifer species, each tree has both ovulate and pollen cones. Mature sporophyte (2n) Seedling 8 Fertilization usually occurs more than a year after pollination. All eggs may be fertilized, but usually only one zygote develops into an embryo. The ovule becomes a seed, consisting of an embryo, food supply, and seed coat. Embryo (new sporophyte) (2n) Ovulate cone Pollen cone Longitudinal section of ovulate cone Microsporocytes (2n) MEIOSIS Longitudinal section of Sporophyll pollen cone Microsporangium A pollen cone contains many microsporangia 3 held in sporophylls. Each microsporangium contains microsporocytes (microspore mother cells). These undergo meiosis, giving rise to haploid microspores that develop into pollen grains. Seeds on surface of ovulate scale Food reserves (gametophyte tissue) (n) Seed coat (derived from parent sporophyte) (2n) FERTILIZATION Micropyle Ovule Integument Megasporocyte (2n) Megasporangium Germinating Pollen pollen grain grains (n) MEIOSIS (containing male gametophytes) Surviving megaspore (n) Germinating pollen grain Female gametophyte Archegonium Egg (n) Integument Germinating pollen grain (n) Discharged sperm nucleus (n) Pollen tube Egg nucleus (n) 2 An ovulate cone scale has two ovules, each containing a megasporangium. Only one ovule is shown. 4 A pollen grain enters through the micropyle and germinates, forming a pollen tube that slowly digests through the megasporangium. 5 While the pollen tube develops, the megasporocyte (megaspore mother cell) undergoes meiosis, producing four haploid cells. One survives as a megaspore. 6 The female gametophyte develops within the megaspore and contains two or three archegonia, each with an egg. Key Haploid (n) Diploid (2n) 7 By the time the eggs are mature, two sperm cells have developed in the pollen tube, which extends to the female gametophyte. Fertilization occurs when sperm and egg nuclei unite.

20 Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits Angiosperms Are commonly known as flowering plants Are seed plants that produce the reproductive structures called flowers and fruits Are the most widespread and diverse of all plants

21 Characteristics of Angiosperms The key adaptations in the evolution of angiosperms Are flowers and fruits

22 Flowers The flower Is an angiosperm structure specialized for sexual reproduction

23 A flower is a specialized shoot with modified leaves Sepals, which enclose the flower Petals, which are brightly colored and attract pollinators Stamens, which produce pollen Carpels, which produce ovules Stamen Anther Stigma Style Carpel Filament Ovary Petal Sepal Figure 30.7 Receptacle Ovule

24 Fruits Fruits Typically consist of a mature ovary (a) Tomato, a fleshy fruit with soft outer and inner layers of pericarp (b) Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp (c) Nectarine, a fleshy fruit with a soft outer layer and hard inner layer (pit) of pericarp Figure 30.8a e (d) Milkweed, a dry fruit that splits open at maturity (e) Walnut, a dry fruit that remains closed at maturity

25 Can be carried by wind, water, or animals to new locations, enhancing seed dispersal (a) Wings enable maple fruits to be easily carried by the wind. (b) Seeds within berries and other edible fruits are often dispersed in animal feces. Figure 30.9a c (c) The barbs of cockleburs facilitate seed dispersal by allowing the fruits to hitchhike on animals.

26 The Angiosperm Life Cycle In the angiosperm life cycle Double fertilization occurs when a pollen tube discharges two sperm into the female gametophyte within an ovule One sperm fertilizes the egg, while the other combines with two nuclei in the center cell of the female gametophyte and initiates development of food-storing endosperm The endosperm Nourishes the developing embryo

27 The life cycle of an angiosperm Key Haploid (n) Diploid (2n) Mature flower on sporophyte plant (2n) 1 Anthers contain microsporangia. Each microsporangium contains microsporocytes (microspore mother cells) that divide by meiosis, producing microspores. Anther Microsporangium Microsporocytes (2n) MEIOSIS 2 Microspores form pollen grains (containing male gametophytes). The generative cell will divide to form two sperm. The tube cell will produce the pollen tube. 7 When a seed germinates, the embryo develops into a mature sporophyte. Microspore (n) Ovule with megasporangium (2n) Male gametophyte (in pollen grain) Generative cell Tube cell 6 The zygote develops into an embryo that is packaged along with food into a seed. (The fruit tissues surrounding the seed are not shown). Germinating Seed Embryo (2n) Endosperm (food Supply) (3n) Seed coat (2n) Female gametophyte (embryo sac) Ovary 3 In the megasporangium of each ovule, the megasporocyte divides by meiosis and produces four megaspores. The surviving megaspore in each ovule forms a female gametophyte Seed (embryo sac). Antipodal cells Polar nuclei Synergids Egg (n) MEIOSIS Megasporangium (n) Surviving megaspore (n) Pollen tube Stigma Pollen tube Sperm Pollen tube Style Pollen grains Zygote (2n) Figure Nucleus of developing endosperm (3n) 5 Double fertilization occurs. One sperm fertilizes the egg, forming a zygote. The other sperm combines with the two polar nuclei to form the nucleus of the endosperm, which is triploid in this example. FERTILIZATION Egg Nucleus (n) Discharged sperm nuclei (n) Sperm (n) 4 After pollination, eventually two sperm nuclei are discharged in each ovule.

28 Angiosperm Evolution Clarifying the origin and diversification of angiosperms Poses fascinating challenges to evolutionary biologists Angiosperms originated at least 140 million years ago And during the late Mesozoic, the major branches of the clade (group) diverged from their common ancestor

29 Fossil Angiosperms Primitive fossils of 125-million-year-old angiosperms Display both derived and primitive traits Carpel Stamen 5 cm (a) Archaefructus sinensis, a 125-million-yearold fossil. (b) Artist s reconstruction of Archaefructus sinensis Figure 30.11a, b

30 An Evo-Devo Hypothesis of Flower Origins In hypothesizing how pollen-producing and ovuleproducing structures were combined into a single flower Scientist Michael Frohlich proposed that the ancestor of angiosperms had separate pollen-producing and ovuleproducing structures Evo-Devo is evolution and development

31 Angiosperm Diversity (and Genes) The two main groups of angiosperms Are monocots and eudicots ( true dicots) Other dicots exist and are grouped as: Basal angiosperms (first line of dicots) Are less derived and include the flowering plants belonging to the oldest lineages Magnoliids (next line of dicots) Share some traits with basal angiosperms but are more closely related to monocots and eudicots

32 Exploring Angiosperm Diversity BASAL ANGIOSPERMS Amborella trichopoda Water lily (Nymphaea Rene Gerard ) HYPOTHETICAL TREE OF FLOWERING PLANTS Star anise (Illicium floridanum) Amborella Water lilies Star anise and relatives Magnoliids Monocots Eudicots MAGNOLIIDS Figure Southern magnolia (Magnolia grandiflora)

33 Exploring Angiosperm Diversity Orchid (Lemboglossum fossii) MONOCOTS Monocot Characteristics Embryos Eudicot Characteristics EUDICOTS California poppy (Eschscholzia californica) One cotyledon Two cotyledons Veins usually parallel Leaf venation Veins usually netlike Pyrenean oak (Quercus pyrenaica) Lily (Lilium Enchantment ) Pygmy date palm (Phoenix roebelenii) Vascular tissue scattered Stems Vascular tissue usually arranged in ring Roots Barley (Hordeum vulgare), a grass Root system Usually fibrous (no main root) Pollen grain with one opening Pollen Taproot (main root) usually present Pollen grain with three openings Dog rose (Rosa canina), a wild rose Pea (Lathyrus nervosus, Lord Anson s blue pea), a legume Flowers Figure Anther Filament Stigma Ovary Floral organs usually in multiples of three Floral organs usually in multiples of four or five Zucchini (Cucurbita Pepo), female (left) and male flowers

34 Evolutionary Links Between Angiosperms and Animals Pollination of flowers by animals and transport of seeds by animals Are two important relationships in terrestrial ecosystems (a) A flower pollinated by honeybees. This honeybee is harvesting pollen and nectar (a sugary solution secreted by flower glands) from a Scottish broom flower. The flower has a tripping mechanism that arches the stamens over the bee and dusts it with pollen, some of which will rub off onto the stigma of the next flower the bee visits. Figure 30.13a c (b) A flower pollinated by hummingbirds. The long, thin beak and tongue of this rufous hummingbird enable the animal to probe flowers that secrete nectar deep within floral tubes. Before the hummer leaves, anthers will dust its beak and head feathers with pollen. Many flowers that are pollinated by birds are red or pink, colors to which bird eyes are especially sensitive. (c) A flower pollinated by nocturnal animals. Some angiosperms, such as this cactus, depend mainly on nocturnal pollinators, including bats. Common adaptations of such plants include large, light-colored, highly fragrant flowers that nighttime pollinators can locate.

35 Concept 30.4: Human welfare depends greatly on seed plants No group is more important to human survival than seed plants

36 Products from Seed Plants Humans depend on seed plants for Food Wood Many medicines Table 30.1

37 Threats to Plant Diversity Destruction of habitat Is causing extinction of many plant species and the animal species they support