Topic 11 - The Evolution of Plants PDF

Topic 11 The evolution of plants Learning Outcomes • • • • • • • • • • Identify the main plant groups based on key characteristics Place the main plant groups on a phylogenetic tree List physical, chemical and biological problems faced by plants during land colonization; and their solutions Expla...

Topic 11 The evolution of plants Learning Outcomes • • • • • • • • • • Identify the main plant groups based on key characteristics Place the main plant groups on a phylogenetic tree List physical, chemical and biological problems faced by plants during land colonization; and their solutions Explain the expression “reduction of the gametophyte” Associate a plant structure with its ploidy level Provide arguments for the importance of vascularization in plants Explain the advantages of heterospory in plants Justify the evolutionary importance of five key innovations in plants Contrast key characteristics of gymnosperms and angiosperms Explain the process of double fertilization and its biological importance 2 https://www.wooclap.com/BIO1130 3 Topic 11 The evolution of plants 11.1 – Key innovations of plants Plants Plants (= embryophytes): 470Mya • Eukaryotes • Multicellular • Embryo • Photoautotrophs • Cell walls made of cellulose • Chloroplasts with chlorophylls, beta-carotenes, xantophylls • Sexual reproduction but asexual reproduction is common • Development of an embryo that is dependent on its parent • 1st photosynthetic organisms that lived permanently on land 5 Plants ~320,000 species of plants were described Reminder: fungi are not plants! Are responsible for a large amount of atmospheric O2 First direct evidence of plant (fossil): Cooksonia à à à à no leaves, no roots, no flowers had vascular tissues to conduct water was liberating spores had cell specialized in gas exchanges 6 Colonization on land Problems of living above the water line: • Dry environment • Strong effect of gravity • No nutrients in the atmosphere • Rapid changes in temperatures Advantages of living above the water line: • Brighter sunlight, unfiltered by water and phytoplankton • More CO2 in the atmosphere than in the water • Abundance of nutrients on the shoreline Many adaptations allowed plants to colonize land: … the protection of the spores, the gametes, the zygote and the embryo … maximizing photosynthesis … growth to compensate the lack of movement towards resources 7 Key innovations Alternation of generations (haplodiplontic life cycle): • multicellular 2n individual (sporophyte) • multicellular n individual (gametophyte) (2n) (n) Embryo retained in the maternal gametophyte tissues à protection and nutrition (sugars, amino acids, etc.) (n) • The spores are protected by a wall made of a highly resistant polymer (sporopollenin) (2n) • Spore dispersion occurs in the air (independently from water) (n) 8 Key innovations (n) • The egg is non-motile but the sperm cell can often swim in water (n) Apical meristem: region of stem cell division at the tip of roots and shoots. Plants don’t move and these undifferentiated cells allow indeterminate growth à maximized exposure to resources: • Nutrients and water (roots) • Sunlight and CO2 (shoot: stems, branches, leaves, flowers) Presence of cuticle: protects against desiccation Presence of stomata: pore surrounded by guard cells in the epidermis of leaves and stems allows gas exchange (CO2 and O2) and water loss regulation. 9 Key innovations 10 Topic 11 The evolution of plants 11.2 – Non-vascular plants ts lan p r ula ytes c as ph n-v bryo o N = Bryophytes: paraphyletic group of all non-vascular plants. They do not produce seeds or flowers. Absence of specialized tissues that conduct water and nutrients 12 Plants ts lan p r ula ytes c as ph n-v bryo o N = Marchantia polymorpha Cups contain gemmae (cell buds) that can propagate and (asexual reproduction) and grow into a new individual. 13 Life cycle of a moss (bryophyte) • Bryophytes have life cycles that are dominated by gametophytes. • Spore germination and sperm cell swimming depend on water. • No vascular system (stem/roots) Rhizoids: filament that attaches to the substrate. à no absorption of water/minerals Bryum argenteum Bryum argenteum 14 Topic 11 The evolution of plants 11.3 – Vascularization ts lan es p yt lar cu eoph s Va ach r =t Tracheophytes: monophyletic group of all vascular plants à life-cycle dominated by the sporophyte (larger and more complex). à The gametophyte is reduced in size! 16 Vascularization Vascularization: presence of lignified tissues that transport water, nutrients andsugars through the plant. On land, sunlight, CO2, nutrients and H2O are not in the same location. à need a transport system to complete photosynthesis Two specialized transport tissues: • Xylem: water and minerals to the leaves • Phloem: sugars, the products of photosynthesis Lignin: polymer in cell walls (impermeable to water and structural support for gravity) à Plants now have a the support to grow tall, disperse farther and compete for light First forests (Devonian ~380Mya) 17 Production of spores Leaves can be specialized to produce spores… … only 1 type (homosporous) producing a bisexual gametophyte: à most seedless vascular plants Fern … or 2 types (heterosporous) producing either a ♀ gametophyte or a ♂ gametophyte: à all seed plants Microsporangium Megasporangium Advantages of heterospory: • for each spore: specific selection with specific functions • a separate ♀ gametophyte can better nourish the embryo (no energy spent producing ♂ gametes) • ♂ and ♀ gametophytes can mature at different times (no self-fertilization = higher genetic diversity) 18 Topic 11 The evolution of plants 11.4 – Seed and flower plants Spermatophytes: seed plants ts lan hyte p ed top Se rma pe =s Seed: embryo surrounded by nutritive substances and a protective coat 20 Five key innovations of seed plants 1. An extremely reduced gametophyte (often microscopic) is protected from environmental stresses, from UV and from desiccation and is directly nourished from the sporophyte. = Pollen grain 21 Five key innovations of seed plants 2. Ovule: structure containing the megaspore à Fertilization without requiring water from the environment 3. Seed plants are heterosporous: • Microspore à ♂ gametophyte (n) which can disperse farther. • Megaspore à ♀ gametophyte (n) and nourishes of the developing embryo. 4. Pollen grain: ♂ gametophyte (n) enclosed within a pollen wall. à Can disperse very far (wind, animals, etc.). 22 Five key innovations of seed plants 5. Production of a seed: • ↑ survival of plants during reproduction. • embryo is nourished and can resist drought or low temperatures à Seed germination during favourable conditions. à Adaptations to many new environments. E.g., Jack pine: germination only after periodic fires Pinus banksiana (Jack pine) 23 Svalbard global seed vault (Norway) Long-term storage of duplicates of seeds collected around the world. à Loss of seeds due to mismanagement, accident, equipment failures, funding cuts, and natural disasters. Indigenous communities have deposited seeds duplicates: • Parque de la Papa in Peru deposited 750 samples of potatoes (2015) • The Cherokee Nation became the first US tribe to deposit 9 samples of heirloom food crops which predate European colonization. -18°C, low oxygen Cherokee Nation Principal Chief Chuck Hoskin Jr. and Secretary of Natural Resources Chad Harsha As of 2021… 1,081,026 distinct crop samples (>13,000 years of agricultural history) 24 Angiosperms g rin e low nts n-f pla o N eed s Gymnosperms produce seeds (not enclosed in chambers) but no flowers 25 Gymnosperms Gymnosperms (gymno: naked, sperm: seed) à the seed is exposed on sporophylls and can survive years before germination. Seed after fertilization by pollen Pollen and seeds = key terrestrial adaptations. Evolved as the climate became much dryer (outcompeted many vascular seedless plants) Most sperm cells from gymnosperms are not flagellated (exception: Ginkgo) Sequoia sempervirens Sequoia Pinus halepensis Aleppo pine 26 Angiosperms r we o l F nts pla Angiosperms flowering plants, produce seeds (enclosed in chambers: ovaries) à 90% of all living plant species 27 Flower plants Angiosperms (angio: receptacle, sperm: seed) ~250,000 species à seed enclosed in a chamber (carpel) that matures into a fruit Flower: modified leaves (sporophylls) specialized in reproduction Flower à carpel (megasporophyll) produces the ♀ gametophyte à stamen (microsporophyll) produces the ♂ gametophyte Seed after fertilization by pollen Coevolution with animal species who participate in pollination Fruit: mature ovary of a flower, that helps seed dispersal • Wind (anemochory) • Animals (zoochory) 28 Flower plants The transfer of pollen (pollination) to the female egg is independent of water (non-motile sperm). Cross-pollination (between individual plants) Double fertilization: • one sperm cell fertilizes the egg à zygote • the other sperm cell fuses with two nuclei of the central cell à endosperm (tissue that nourishes the developing embryo) The ovary matures into a fruit The ovules mature into seeds 29