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25 The Origin and Diversification of Eukaryotes
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Eukaryotic Cell Eukaryotes defined as:
Having a nucleus & membrane-bound organelles Dynamic endomembranes Facilitate movement & feeding Dynamic cytoskeletons Network of proteins that allow them to change shape, move, and transfer substances Eukaryotes have an internal scaffolding of proteins made of actin filaments and microtubules that allows it to change shape and remodel itself quickly. These dynamic proteins make up the cell’s cytoskeleton. Eukaryotes also have dynamic internal membranes that make up the endomembrane system. The endomembrane system includes the nuclear envelope the endoplasmic reticulum the Golgi apparatus a plasma (cell) membrane All membranes of the endomembrane system are interconnected directly or by the movement of vesicles. Vesicles transport particles (such as nutrients, waste, or other cells) into or out of the cell, and through the cytoplasm. Molecules can move through the cell much faster than by diffusion alone, which allows eukaryotic cells to become much larger than most bacteria. Many biologists say that the membranes of eukaryotic cells are in dynamic continuity.
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Concept 25. 1: Eukaryotes arose by endosymbiosis more than 1
Concept 25.1: Eukaryotes arose by endosymbiosis more than 1.8 billion years ago Early eukaryotes were unicellular Then simple multicellular eukaryotes evolved In the form of filaments, hollow balls, or sheets of undifferentiated cells Eukaryotes are fairly limited in the ways in which they obtain carbon and energy when compared to prokaryotes. And the ways in which energy and carbon are obtained take place in specific organelles: mitochondria (aerobic respiration) chloroplasts (photosynthesis) Single-celled eukaryotic heterotrophs can engulf particulate food. The cells engulf food particles and package them inside a membrane vesicle and transport them to the cytoplasm in a process called phagocytosis. Within the cytoplasm, enzymes break down the particles into molecules that can be processed by the mitochondria. The dynamics of the cytoskeleton and membrane system, as well as the compartmentalization of metabolism, help explain why eukaryotic cells have so many shapes and prokaryotic cells have so few.
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Evidence for the presence of eukaryotes dates back to 2.7 bya
Figure 25.2 Evidence for the presence of eukaryotes dates back to 2.7 bya The earliest fossils of eukaryotic cells are 1.8 billion years old 4.5 bya 3.5 2.5 1.5 500 mya 535 1.8 bya 1.5 bya 1.3 bya 1.2 bya 750 mya 635 mya 600 mya 550 mya mya 0.5 cm 1 cm 20 mm 20 mm 20 mm (e) Bonneia, a vase-shaped eukaryote (a) A 1.8-billion- year-old fossil eukaryote (b) Tappania, a 1.5-billion-year-old fossil that may represent an early alga or fungus (f) An early member of the Ediacaran biota (g) Spriggina floundersi, an early animal with many body segments Figure 25.2 Exploring the early evolution of eukaryotes 25 mm 25 mm (c) Bangiomorpha, an ancient red alga (d) Proterocladus, classified as a green alga Appearance of novel features, including complex multicellularity, sexual life cycles, and eukaryotic photosynthesis, arose 1.3 billion to 635 million years ago © 2016 Pearson Education, Inc.
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Diversity in Eukaryotes
Membrane dynamics Compartmentalized metabolism Genome organization/complex gene regulation Genetic diversity by means of sexual reproduction Life cycles Diversity in eukaryotes is achieved in several different ways: Membrane dynamics Compartmentalization of metabolic pathways Genome organization enabling complex gene regulation Genetic diversity from independent assortment and fertilization Their life cycles
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Eukaryotic success What makes eukaryotes so successful?
Their ability to form cells of diverse size and shape Their ability to produce multicellular structures in which multiple cell types act together
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Eukaryotes are “combination” organisms; by endosymbiosis
Table 25.1 Eukaryotes are “combination” organisms; by endosymbiosis Table 25.1 Inferred origins of key eukaryotic features © 2016 Pearson Education, Inc.
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Symbiosis and Origin of Chloroplasts
27.2 The Endosymbiotic Hypothesis proposes that chloroplasts and mitochondria engulfed by eukaryotic cells were originally free-living bacteria. Chloroplasts resemble cyanobacteria Mitochondria resemble proteobacteria Chloroplasts resemble photosynthetic bacteria, specifically cyanobacteria. Over a century ago, a Russian botanist (Konstantin Sergeevich Merezhkovsky) recognized this similarity and suggested that chloroplasts were once free-living cyanobacteria that became incorporated into a host. His radical hypothesis suggested that the symbiont (cyanobacteria) lived within another cell (a plant cell), making an endosymbiotic relationship between the two. In 1967, American biologist Lynn Margulis resurrected this hypothesis and used transmission electron microscopy to show the structural similarities in the photosynthetic membranes of cyanobacteria, algae, and chloroplasts. The chloroplasts were found to have two membranes, which makes sense when considering that one was once the cyanobacteria’s membrane and the other was the membrane of the cell that engulfed it. The two also show similar biochemistry during photosynthesis.
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Origin of Mitochondria and Plastids
The endosymbiont theory proposes that mitochondria and plastids were formerly small prokaryotes that began living within larger cells An endosymbiont is a cell that lives within a host cell Prokaryote ancestors to mitochondria and plastids probably entered the host cell as undigested prey or internal parasites © 2016 Pearson Education, Inc. 9
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The relationship between endosymbiont and host cells could have become mutually beneficial
For example, anaerobic host cells may have benefited from aerobic endosymbionts as oxygen increased in the atmosphere Over time, the host and endosymbionts would have become a single organism © 2016 Pearson Education, Inc. 10
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Cytoplasm DNA Ancestral prokaryote Engulfed aerobic bacterium Plasma
Figure 25.3 Cytoplasm DNA Ancestral prokaryote Engulfed aerobic bacterium Plasma membrane Engulfed photo- synthetic bacterium Endoplasmic reticulum Nucleus Nuclear envelope Mitochondrion Mito- chondrion Figure 25.3 A hypothesis for the origin of eukaryotes through endosymbiosis Ancestral heterotrophic eukaryote Plastid Ancestral photosynthetic eukaryote © 2016 Pearson Education, Inc.
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Concept 25.2: Multicellularity has originated several times in eukaryotes
The evolution of eukaryotic cells allowed for a greater range of unicellular forms A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals The first multicellular forms were colonies, collections of connected cells with little or no differentiation Multicellularity arose from different unicellular organisms on 6 separate occasions in history: for red, green, and brown algae, plants, fungi, and animals arose independently from different single-celled ancestors © 2016 Pearson Education, Inc. 12
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Figure 25.5 Figure 25.5 Pediastrum © 2016 Pearson Education, Inc.
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Multicellular Eukaryotes
Simple multicellularity: Adjacent cells stick together, but few specialized cells Most cells have full range of functions Every cell is in direct contact with external environment Complex multicellular organisms: Adhesion molecules Specialized structures for cell communication Tissue and organ differentiation……...including reproduction Only some cells in contact with external environment Helps organisms avoid predators Cell or tissue loss can be fatal for organism © 2016 Pearson Education, Inc.
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Flagellum Cytoplasm Chlamydomonas Cell wall Gonium One Gonium cell
Figure 25.6 Flagellum Cytoplasm Chlamydomonas Cell wall Gonium One Gonium cell Extracellular matrix Pandorina Figure 25.6 Morphological change in the Volvox lineage Cells Cells Volvox Extracellular matrix © 2016 Pearson Education, Inc.
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Concept 25.3: Four “supergroups” of eukaryotes have been proposed based on morphological and molecular data Eukaryote diversity is influenced by their hybrid origins and the independent origins of complex multicellularity All eukaryotes are now divided into 4 supergroups Excavata SAR Archaeplastida Unikonta Animals fall into superkingdom Unikonta Plants fall into superkingdom Archaeplastida © 2016 Pearson Education, Inc. 16
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Stramen- opiles lates Alveo- rians Rhiza- algae Green Amoebo- zoans
Figure Diplomonads Excavata Parabasalids Euglenozoans Stramen- opiles Diatoms Brown algae Dinoflagellates SAR lates Alveo- Apicomplexans Ciliates Forams rians Rhiza- Cercozoans Red algae Archaeplastida algae Green Chlorophytes Charophytes Figure Exploring eukaryotic diversity (part 1: phylogenetic hypothesis of eukaryotic supergroups) Plants Amoebo- zoans Tubulinids Slime molds Unikonta Opisthokonts Nucleariids Fungi Choanoflagellates Animals © 2016 Pearson Education, Inc.
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Excavata SAR Archaeplastida Unikonta 5 mm 50 mm 20 mm 25 mm 100 mm
Figure Excavata 5 mm SAR 50 mm Archaeplastida 20 mm 25 mm Unikonta 100 mm Figure Exploring eukaryotic diversity (part 2: micrographs) 100 mm © 2016 Pearson Education, Inc.
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Excavates The supergroup Excavata is composed of protists characterized by their feeding groove named excavata Monophyletic group (equivalent to a clade): a common ancestor and all of its descendents Diplomonads: often parasites; e.g.Giardia intestinalis “swimming pool protist” Parabasalids : Trichomonas vaginalis, a human STD Euglenozoans: Euglena, Trypanosoma, which causes sleeping sickness in humans © 2016 Pearson Education, Inc. 19
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SAR: Stramenopiles, Alveolates, and Rhizarians
SAR is a diverse monophyletic supergroup of protists named for the first letters of its 3 diverse member clades: Stramenopiles: some of the most important PS organisms; e.g. diatoms, brown algae Alveolates: photosynthetic species, as well as pathogens; e.g. dinoflagellates, paramecium, Plasmodium Rhizarians: forams © 2016 Pearson Education, Inc. 20
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Excavata Diatoms Stramenopiles Brown algae Dinoflagellates SAR
Figure 25.UN03 Excavata Diatoms Stramenopiles Brown algae Dinoflagellates SAR Apicomplexans Alveolates Ciliates Forams Rhizarians Figure 25.UN03 In-text figure, the SAR clade, p. 509 Cercozoans Archaeplastida Unikonta © 2016 Pearson Education, Inc.
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Stramenopiles Diatoms: highly diverse, unicellular algae with a unique 2 part, glass-like wall of silicon dioxide Most important PS organisms in aquatic environments Fish food Make up diatomaceous earth: porous rock as filtering agents in swimming pools, medical absorbants, etc. © 2016 Pearson Education, Inc. 22
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Brown algae: the largest and most complex algae
Brown Algae (Kelp) Stramenopila Brown algae: the largest and most complex algae All are multicellular, and most are marine Giant seaweeds (kelp) that form forests on ocean floor Often used as fertilizer/mulch, sources of iodine, used to wrap sushi Algin: the gel-forming substances in their cell walls used to thicken many processed foods, such as pudding, ice cream, salad dressing, etc. © 2016 Pearson Education, Inc. 23
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Blade Stipe Holdfast Figure 25.14
Figure Seaweeds: adapted to life at the ocean’s margins Holdfast © 2016 Pearson Education, Inc.
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Alveolates The alveolates have membrane-enclosed sacs (alveoli) just under the plasma membrane E.g. Dinoflagellates, Paramesium, and Plasmodium alveoli flagella © 2016 Pearson Education, Inc. 25
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Paramesium Alveolate Contractile vacuole Oral groove Cell mouth Cilia
Figure 25.17 Paramesium Alveolate Contractile vacuole Oral groove Cilia Cell mouth 50 mm Micronucleus Figure Structure and function in the ciliate Paramecium caudatum Food vacuoles Macronucleus © 2016 Pearson Education, Inc.
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Unicellular algae with flagella; abundant in plankton
Figure 25.16 Dinoflagellates Alveolate Unicellular algae with flagella; abundant in plankton They are a diverse group of phototrophs, heterotrophs, and mixotrophs, organisms that function as both heterotrophs and phototrophs Toxic “red tides” are caused by dinoflagellate blooms Symbionts with coral reefs Figure Pfiesteria shumwayae, a dinoflagellate Flagella 3 mm © 2016 Pearson Education, Inc.
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Plasmodium All parasitic Causes malaria
Alveolate All parasitic Causes malaria Plasmodium requires both Anopheles mosquitos and humans to complete its life cycle Destroys liver and red blood cells Approximately 900,000 people die each year from malaria Efforts are ongoing to develop vaccines that target this pathogen © 2016 Pearson Education, Inc.
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Inside mosquito Inside human Merozoite Sporozoites (n) Liver Liver
Figure 25.26 Inside mosquito Inside human Merozoite Sporozoites (n) Liver Liver cell Oocyst Apex MEIOSIS Merozoite (n) Red blood cell 0.5 mm Red blood cells Zygote (2n) Figure The two-host life cycle of Plasmodium, the apicomplexan that causes malaria FERTILIZATION Gametes Gametocytes (n) Haploid (n) Diploid (2n) © 2016 Pearson Education, Inc.
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Rhizarians Many species of rhizarians are amoebas, but not all amoebas are rhizarians Amoebas move and feed by pseudopodia, extensions that bulge from the cell surface Most rhizarian amoebas have threadlike pseudopodia E.g. Forams © 2016 Pearson Education, Inc. 30
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Tests are made of hard calcium carbonate
Forams Rhizarian Foraminiferans or forams, are named for their porous shells, called tests Threadlike pseudopodia extend through the pores in the test and are used for swimming, feeding, and test formation Tests are made of hard calcium carbonate Most attach to rocks and algae © 2016 Pearson Education, Inc. 31
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Archaeplastids Archaeplastida: include key PS species that form the base of the food web in many aquatic The photosynthetic descendants of this ancient protist evolved into red algae and green algae Land plants are descended from the green algae E.g. red algae, green algae (chlorophyta and charophyta) and plants © 2016 Pearson Education, Inc. 32
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Excavata SAR Archaeplastida Red algae Chlorophytes Green algae
Figure 25.UN04 Excavata SAR Archaeplastida Red algae Chlorophytes Green algae Charophytes Plants Figure 25.UN04 In-text figure, Archaeplastida, p. 511 Unikonta © 2016 Pearson Education, Inc.
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Red Algae Archaeplastida Red algae are reddish in color due to an accessory pigment called phycoerythrin Multicellular seaweeds Abnchor to rocks below tide A derivative is called agar; used in bacterial petri dishes Japanese “nori” used to wrap sushi Common uses: toothpaste, ice-cream, etc © 2016 Pearson Education, Inc. 34
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Red Algae Bonnemaisonia hamifera Nori Figure 25.19
Figure Red algae © 2016 Pearson Education, Inc.
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Green Algae Archaeplastida Green algae have many molecular and cellular similarities with land plants Some advocate including green algae in an expanded “plant” kingdom, “Viridiplantae” Green algae are divided into charophytes and chlorophytes Charophytes are most closely related to land plants © 2016 Pearson Education, Inc. 36
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Figure 25.20 (a) Ulva, or sea lettuce 2 cm Some unicellular chlorophytes are free-living, while others live symbiotically within other eukaryotes Larger size and greater complexity are found in multicellular species including Volvox and Ulva Figure Multicellular chlorophytes (b) Caulerpa, an intertidal chloro- phyte © 2016 Pearson Education, Inc.
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Unikonts Includes animals, fungi, and some protists
This group includes 2 major clades: Amoebozoans Opisthokonts (choanoflagellate protists, fungi, and animals) © 2016 Pearson Education, Inc. 38
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Excavata SAR Archaeplastida Tubulinids Amoeboezoans Slime molds
Figure 25.UN05 Excavata SAR Archaeplastida Tubulinids Amoeboezoans Slime molds Unikonta Nucleariids Fungi Figure 25.UN05 In-text figure, Unikonta, p. 512 Choanoflagellates Animals © 2016 Pearson Education, Inc.
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Amoebozoans Amoebozoans: amoebas that have lobe- or tube-shaped, rather than threadlike, pseudopodia Most are heterotrophs that feed on bacteria and other protists; some also feed on detritus (nonliving organic matter) Are soil predators to other organisms and can cause human disease E.g. Amoeba proteus; E.g. Entamoeba histolytica: Anaerobic protists that causes amoebic dysentery in humans; spread by contaminated drinking water, food, or eating utensils © 2016 Pearson Education, Inc. 40
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Opisthokonts Opisthokonts: a diverse group, including
Choanoflagellate protists: collared unicellular protist; closest living relatives of animals Fungi (Chapter 26) Animals (Chapter 27) © 2016 Pearson Education, Inc. 41
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Choanoflagellates: closest living relatives of animals
Figure 25.7 Choanoflagellates: closest living relatives of animals Individual choanoflagellate Choano- flagellates OTHER EUKARY- OTES Sponges Animals Figure 25.7 Three lines of evidence that choanoflagellates are closely related to animals Collar cell (choanocyte) Other animals © 2016 Pearson Education, Inc.
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QUICK CHECK Do plants and animals have a common multicellular ancestor? © 2016 Pearson Education, Inc.
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ANSWER No Plants form 1 branch of the green algal tree, whose early branches are all unicellular forms The branch of animals has choanoflagellates and other unicellular forms in its lower branches Thus, plants and animals evolved multicellularity independently of each other © 2016 Pearson Education, Inc.
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Results Common ancestor of all eukaryotes DHFR-TS gene fusion
Figure 25.21 Results Choanoflagellates Animals Unikonta Fungi Common ancestor of all eukaryotes Amoebozoans Diplomonads Excavata Euglenozoans Stramenopiles Alveolates SAR Figure Inquiry: What is the root of the eukaryotic tree? DHFR-TS gene fusion Rhizarians Red algae Green algae Archaeplastida Plants © 2016 Pearson Education, Inc.
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Concept 25.4: Single-celled eukaryotes play key roles in ecological communities and affect human health The majority of eukaryotic lineages are protists Protist: informal name for mainly unicellular eukaryotes Exhibit a wide range of structural and functional diversity Can be very complex since all biological functions must be carried out within an individual cell Most protists are aquatic, but they are found in diverse environments, including moist terrestrial habitats Protists show a wide range of nutritional diversity Photoautotrophy, heterotrophy, and mixotrophy have arisen independently in protists many times © 2016 Pearson Education, Inc. 46
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Prokaryotic producers Protistan producers
Figure 25.23 Producers for Food Webs Other consumers Herbivorous plankton Carnivorous plankton Figure Protists: key producers in aquatic communities Prokaryotic producers Protistan producers In aquatic communities, photosynthetic protists and prokaryotes are the main producers, organisms that convert CO2 to organic compounds using light energy © 2016 Pearson Education, Inc.
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