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. 80 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. 81 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 Basal Monocot Basal Eudicot Core Eudicot (ANITA clade) (Caryophyllid Name of Flower Or Magnolid Asterid or Rosid) Are all flower parts present? (complete or incomplete) 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) 82 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. 83 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 bird- pollinated; 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 flat- topped 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. 84 FLORAL CHARACTERISTICS Time of Pollinator Color Scent Corolla Reward Example Flowering Blue, yellow, Bilateral landing Nectar and /or Bee Fresh, strong Day purple platform pollen Landing platform; Bright; Butterfly Fresh, weak Day sometimes nectar Nectar only often red spurs Dissected; White or Night or Moth Sweet, strong sometimes nectar Nectar only pale dusk spurs Nectar and /or Fly (reward) Light Faint Day Radial, shallow pollen Brownish, Fly (carrion) Rotten, strong Day or night Enclosed or open None purplish Often green Nectar and /or Beetle Various, strong Day or night Enclosed or open or white pollen Bright; Tubular or pendant; Birds None Day Nectar only often red ovary often inferior Showy flower or Nectar and /or Bat Whitish Musky, strong Night inflorescence pollen Non-flying Unscented to Robust ,exerted Copius nectar Dull-colored Night mammals variously strong styles and stamens and/or pollen 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! 85 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 86 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.