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PLANT EVOLUTION By Feryal Kherissat


What is evolution? A gradual process in which something changes into a significantly different, especially more complex or more sophisticated, form. Biology: The theory that groups of organisms, as species, may change with passage of time so that descendants differ morphologically and physiologically from their ancestors. INTRODUCTION

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Age of earth: 4.5 billion. Oldest organic compounds: 3.85 billion YBP rock from Greenland. Oldest Fossils: Chain-like bacteria: Australian Black Chert 3.5 billion YBP. Evidence of photosynthetic organisms: 3.4 YBP. First bonified Blue-green algae (Stromatolites) Gunflint Chert of Canadian Shield 1.7 billion YBP. Eukaryotic cells : 1.7 billion YBP sediments in China. 2.1 billion YBP 0.5 meter long fossil called Grypania.

Grypania is an early, tube-shaped fossil from the Proterozoic . The organism could have been a giant bacterium or bacterial colony, but because of its size (over one centimeter) and consistent form, is more likely to have been a eukaryotic alga. : 

Grypania is an early, tube-shaped fossil from the Proterozoic . The organism could have been a giant bacterium or bacterial colony, but because of its size (over one centimeter) and consistent form, is more likely to have been a eukaryotic alga.

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What have been the important constraints and / or principles that have shaped the evolution of plants. Diversification. Form and function. Important particularities on evolution and speciation in plants R.A. Fisher (1958)→→Fundamental Theorem of Natural Selection “Rate of increase in the mean fitness of a population is proportional to the genetic variance in fitness”.

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Major ways genetic variation is introduced into populations: Mutation (UV, random error). Genetic recombination (meiosis) →→ including ‘crossing-over’. Immigration (into population).

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But plants ‘do’ two additional ‘tricks’ that enhance genetic variation: Polyploidy →→ an organism that has more than one complete set of the normal chromosome compliment. Most animals are diploids, many plants are polyploids. Occurs through processes such as chromosome duplication. Hybridization →→ crossing of closely related taxa (usually between species within a genus).

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Land plants first appeared in the Ordovician (~460 million years ago) but did not begin to resemble modern plants until the Late Silurian. By the close of the Devonian, about 360 million years ago, there were a wide variety of shapes and sizes of plants, including tiny creeping plants and tall forest trees. Today, with more than 250,000 species, they are second in size only to the insects.

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We now know that plants, like all living organisms, had aquatic ancestors. A specific group of freshwater green algae are the closest relatives to the land plants. Plant evolution is therefore inseparably linked with their progressive occupation of the land and their increasing independence from water for reproduction.

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Endosymbiotic Theory - evolution of plants. Lynn Margulis: Bacteria → → Mitochondria. Photosynthetic prokaryote (Prochloron) → → Chloroplast. Amoebae like with 9/2 flagella → → ingested other two.

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Evidence: Present day symbiotic associations occur. DNA of bacteria, mitochondria, chloroplasts all similar form. Ribosomes of all three are same size 70s vs. 80s in eukaryotic cells. All three divide by binary fission. Bleached Euglenas can't recover chloroplasts. Albino, wall-less, tobacco cells can take up chloroplasts and adopt them. The same antibiotics affect ribisomes of all three but not the ribosome of eukaryotic cells. Chloroplasts of red algae closely resemble blue-green algae (separate line of evolution).

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brown algae land plants green algae red algae cryptophytes red algae haptophytes euglenoids cyanobacteria Plastids How many endosymbiotic events does this suggest?

Multicellularity and plant evolution : 

Multicellularity and plant evolution Multicellularity evolved more than once! For plants, prokaryotic unicellular algae → → multicellular algae → → embryophytes. Plants began life in the seas and moved to land as competition for resources increased. The biggest problems a plant on land faces: Supporting the plant body. Absorbing and conserving water.

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Multicellularity has several interesting advantages: Cells can be specialized – division of labor (requires communication and transport). Organism can increase surface area for environmental exchange (access to more resources). Organism can increase in size – better buffering of environmental extremes → → live longer → → access to ‘additional’ resources.

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Some adaptations to life on land: Cuticle. Gametangia. Embryos. Pigments. Spore wall thickening. Mychorhizzae. Stomata. Aerenchyma.

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The first major period of plant evolution:from green algae to bryophyte.

What is Plant? : 

What is Plant?

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Plant Diversity

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Like the land plants, green algae contain two forms of chlorophyll (a and b). Green algae differ from plants in many ways. Because they live in the water,: They don't have a specialized transport or support systems. Their bodies are supported by the water. Almost all of the cells photosynthesize and have access to the nutrients present in the water. Therefore, transport of nutrients is not necessary. Lack cuticles and stomata.

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Green algae have two distinct lineages Chlorophytes – Gave rise to aquatic algae Streptophytes – Gave rise to land plants

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Charophytes (a group of green algae) appear to be the closest living relative of Embryophytes. These organisms now occupy the margins of ponds or marshes (meaning that the ‘jump’ to a terrestrial environment was in close proximity) Chara sp. (stonewort)

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Embryophytes invaded the terrestrial environment approximately 400–500 mya. Invading the land is more like ‘invading the air’, rather than soil: Water not as available and quickly lost from plant in the terrestrial environment. Gravity becomes very important. Dispersal of gametes is much more difficult outside of an aquatic environment

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The early land plants, represented today by the bryophytes, possessed two important features that allowed them to live on land: A waxy cuticle to protect against desiccation. Protection of gametes and embryos in a protective jacket made of cells. Mosses Liverworts Hornworts

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In bryophytes, the gametophytes are nutritionally independent of the sporophytes and the sporophytes are either completely or partially dependent on the gametophytes. Sperm are free swimming and require water to reach the egg. Liverworts have pores on their surface but lack stomata. Mosses and hornworts (and all other land plants) have stomata.

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Stomata on land plant leaf. Pore on surface of a liverwort. Stomata on a Horsetail fern

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Even with these adaptations: Early land plants were still closely tied to water → → → → They had swimming sperm and required water to complete their life cycle. Early land plants lacked true vascular tissue to carry water from the soil to the aerial parts of the plant → → → → Water moved over the plant body and distributed by relatively slow processes like diffusion.

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The second major period of plant evolution: The diversification of vascular plants.

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The evolution of efficient fluid-conducting systems, consisting of xylem and phloem, solved the problem of water and food transport throughout the plant body. The ability to synthesize lignin, which is incorporated into the cell wall of supporting and water conducting cells. Lignin adds rigidity to cell walls, making it possible for vascularized plants to reach great heights.

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We can distinguish between embryophytes that have tracheids (tracheophytes) and those that do not (non-tracheophytes). The first land plants either lacked vascular tissue or, like some mosses, had very simple conducting tissue that developed from dead cells. Tracheids

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Tracheophytes have two vascular tissue types: Xylem; transports water and dissolved minerals; cell types are tracheids and, in flowering plants, vessel elements. Phloem; transports sugars, products of photosynthesis; minerals too; cell types are sieve-tube members and companion cells.

From Green Algae to Plants : 

From Green Algae to Plants

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The shoot system of plants was well suited to the demands of life on land - namely, the acquisition of energy from the sun and carbon dioxide from the atmosphere. The gametophytic stage remained free-living, requiring water for fertilization, but over time, the gametophytic generation underwent a progressive reduction in size - the sporophyte phase became the dominant phase of the life cycle. The earliest vascular plants lacked seeds, a condition still represented by ferns.

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Therefore, the second major lineage of land plants to evolve is referred to as the seedless, vascular plants. Seedless vascular plants grew along side primitive seed plants but the seed plants were not dominant at that time. The seed plant rose to prominence only after the swamps began to dry up at the end of the Carboniferous. Seedless vascular plants dominated the landscape in shallow swamp like forests of the Carboniferous period about 300-350 mya.

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During the Devonian period club mosses (lycopods), horsetails, and ferns made the environment more hospitable to animals. Trees dominated during the Carboniferous period, resulting in forest that eventually become coal deposits. At the end of the Permian period, the 200-millionyear reign of the lycopod–fern forests came to an end as they were replaced by forests of seed plants. Horsetails

Club Mosses : 

Club Mosses Lycopodium obscurum

Horsetails : 

Horsetails Equisetum arvense Equisetum palustre

Fern : 

Fern Psilotum nudum Dryopteris intermedia

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The first tracheophytes were in the now-extinct phylum Rhyniophyta. Club mosses (Lycophyta), appeared in the Silurian period. Ferns, horsetails, and whisk ferns (Pteridophyta) appeared in the Devonian. These groups (Lycophyta and Pteridophyta) had true roots, true leaves, and a differentiation between two types of spores.

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Roots had their origins as branches, either as rhizomes or aboveground portion of stems.

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A leaf emerging laterally from a main axis or stem and possessing true vascular tissue. There are two leaf types: microphylls and megaphylls. The microphyll has a single vascular strand that has departed from the stem without disturbing the stem’s vascular structure → → club mosses . Microphylls may have evolved from sterile sporangia.

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Enation theory: outgrowth of cortex and epidermis → → Vascularization by one strand

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The megaphyll is larger, and more complex found in ferns and seed plants. May have arose from flattening of stems and development of overtopping (one branch differentiates from and extends beyond rest). Telome theory: Overtopping → → plantation → → Webbing → → fusion..

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Archaeopteris, a tree from the late-Devonian thought to have had the first true leaves. Archaeopteris fissilis; late Devonian (375-360 mya)

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The third major period of plant evolution: The origin of the seed.

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The seed, a specialized unit of reproduction, advanced the colonization of land by further providing a food source for the plant embryo and protecting it from harsh conditions. Seeds are also an important unit of dispersal (replacing the spore as the stage of the life cycle that disperses offspring). The seeds of gymnosperms are not enclosed in any special chambers.

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The gametophytes of seed plants became even more reduced than the gametophytes of ferns and other vascular plants. The reduction of the gametophyte set the stage for another major innovation - pollination. Pollination replaced swimming as the mechanism for delivering sperm to the egg. This became increasingly important because as the gymnosperms diversified, the climate was becoming drier.

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The fourth major period of plant evolution: The emergence of the flower.

Slide 51: 

The angiosperms arose during the early Cretaceous period about 130 mya. The main feature that led to their success was the evolution of flowers and fruits. The presence of the ovary is one of the major differences between angiosperms and the gymnosperms. The ovary develops into the fruit, which is an important structure for seed dispersal. Flowers also allow for specialized pollination by attracting and rewarding pollinators.

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Vascular tissue also became more refined during angiosperm evolution. Vessel elements, present in almost all angiosperms, are shorter and wider than trachieds (the xylem tissue in ferns and gymnosperms), and allow for more efficient water transport. Leaves (megaphylls) also became more diverse and specialized.

Slide 53: 

While these changes in the plant body were important, it was really the evolution of the flower that contributed most significantly to the success of the angiosperms. Today, the flowering plants are by far the most diverse and geographically widespread of all plants.

An indeterminate pentamerous fossil rosid flower (Celastrales, Rosidae) collected by Professor David L. Dilcher from the Lower Cretaceous Dakota Formation of North America. : 

An indeterminate pentamerous fossil rosid flower (Celastrales, Rosidae) collected by Professor David L. Dilcher from the Lower Cretaceous Dakota Formation of North America. 1 2 3 4 5

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The earliest fossil of a flowerin plant, Archaefructus liaoningensis, is dated about 125 million years old


FLOWER Evolved from modified leaves. Four evolutionary trends: Number of parts have become reduced. Parts fused. Radial → → bilateral symmetry. Position of ovary changed.

Evolution of the Carpel : 

Evolution of the Carpel Goethe, German writer, philosopher, and (in his spare time) noted botanist, proposed in 1790 that carpels evolved from leaves. Chambers in the pistil were probably formed from a sporophyll - a fertile leaf bearing ovules. Sporophyll had ovules (modified sporangia) on its outer edges. Edges of the leaf folded over and fused together to form a protective chamber - the carpel.

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Pistils probably formed by the fusion of several carpels along the midrib of the modified leaves. Goethe's "foliar theory of the carpel" is still the best hypothesis for explaining the evolution of the carpel. Margins rolled inward forming carpel

Evolutionary sequence of stamen : 

Evolutionary sequence of stamen

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Evolutionary trends among flowers

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Phylogenetic Analysis of the Green Plants Data Matrix-Solution

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Angiosperm 1.3 4.0 5.7 6.2 8.0 9.0 Green algae Liverwort Lycophyte Mosses Fern Gymnosperm

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Green algae Liverwort Mosses Fern Gymnosperm Angiosperm

The Evolution of Today's Plants : 

The Evolution of Today's Plants

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