2.3 Eukaryotes & Protists PDF

2.3 The Evolution of Eukaryotic Cells & Protists Domain Eukarya Earliest fossils: bacteria ~3.5-3.8 billion years ago Possible fossils of early eukaryotes date to ~1.5 - 2 billion years ago All living eukaryotes are descendents of a single common ancestor Characteristics of Eukaryotic Common Ancesto...

2.3 The Evolution of Eukaryotic Cells & Protists Domain Eukarya Earliest fossils: bacteria ~3.5-3.8 billion years ago Possible fossils of early eukaryotes date to ~1.5 - 2 billion years ago All living eukaryotes are descendents of a single common ancestor Characteristics of Eukaryotic Common Ancestor 1. Cells with nuclei surrounded by a nuclear envelope 2. Mitochondria 3. Cytoskeleton 4. Flagella and cilia 5. Linear chromosomes organized by histones 6. Mitosis 7. Sexual reproduction and meiosis 8. Cell walls Evolution of Eukaryotes Endosymbiotic theory Evolution of Eukaryotes The first eukaryote may have originated from an ancestral prokaryote that had undergone membrane proliferation, compartmentalization of cellular function (into a nucleus, lysosomes, and an endoplasmic reticulum), and the establishment of endosymbiotic relationships with an aerobic prokaryote, and, in some cases, a photosynthetic prokaryote, to form mitochondria and chloroplasts, respectively. All eukaryotic cells have mitochondria while only some have chloroplasts, therefore photosynthetheic eukaryotes evolved secondarily. Evidence of Endosymbiosis Mitochondria and chloroplasts: Contain DNA, RNA, and ribosomes similar to bacteria ○ ○ ○ Their genome has circular DNA similar to prokaryotic genomes Some genes were transferred to nucleus of the cell → cannot survive outside of the cell DNA analysis shows close relationship between mitochondria and aerobic bacteria and between chloroplasts and cyanobacteria Similar shape, size, and membrane system as bacteria cells ○ Double membrane system probably leftover from original engulfment Divide independently of the cell by binary fission (like bacteria) Mitochondria are found in all eukaryotic cells = probably evolved first Similarity in photosynthetic pigments between chloroplasts and cyanobacteria Secondary Endosymbiosis In a primary endosymbiotic event, a heterotrophic eukaryote consumed a cyanobacterium. In a secondary endosymbiotic event, the cell resulting from primary endosymbiosis was consumed by a second cell. Some photosythetic protists have 3 or 4 membranes Primary & Secondary Endosymbiosis The origin and evolutionary tree of life that is based on small-subunit RNA. The branches that perform oxygenic photosynthesis are labeled with ‘O2’. The black arrows indicates the endosymbiotic events that resulted in the origin of eukaryotes from proteobacteria-like organisms (ultimately leading to mitochondria), and later eukaryotic photosynthesis from cyanobacteria-like organisms, which ultimately became chloroplasts in algae and later in plants. Evidence of Endosymbiosis Figure 23.4 Algae. (a) Red algae and (b) green algae (seen here by light microscopy) share similar DNA sequences with photosynthetic cyanobacteria. Scientists speculate that, in a process called endosymbiosis, an ancestral prokaryote engulfed a photosynthetic cyanobacterium that evolved into modern-day chloroplasts. Protists Evolution of Protists All of Domain Eukarya is divided into 6 supergroups with multiple kingdoms in each Protists are found in each of the 6 supergroups = paraphyletic ○ Eukaryotes that are not a plant, animal or fungi ○ Evolved multiple times ○ Often more closely related to another group (plants, animals, fungi) than to each other ○ Oldest eukaryotes Diversity of Protists Complexity and diversity of protists makes them difficult to classify Cannot be classified as plants ○ Gametes and zygotes are not protected from drying out Cannot be classified as fungi ○ Do not have chitin in their cell wall Cannot be classified as animals ○ Do not undergo embryonic development Characteristics of Protists Most are unicellular, not all Life cycles Most protists are free-living, some are parasitic Asexual reproduction common Sexual reproduction may occur when conditions deteriorate Some life cycles simple, many extremely complex Size Vary in size from microscopic algae and protozoans to kelp more than 200 m in length Protists Protists range from the microscopic, single-celled (a) Acanthocystis turfacea and the (b) ciliate Tetrahymena thermophila, both visualized here using light microscopy, to the enormous, multicellular (c) kelps (Chromalveolata) that extend for hundreds of feet in underwater “forests.” Metabolism of Protists Photoautotrophic forms: Have chloroplasts and produce oxygen Function as producers in both freshwater and saltwater ecosystems Diatoms, brown/ red/ green algae, dinoflagellates Major component of plankton ○ ○ Organisms that are suspended in the water Serve as food for heterotrophic protists and animals Metabolism of Protists Heterotrophic forms consume organic materials Amoebas ingest particles through phagocytosis Saprobes feed on dead organisms or their waste Many protists are symbionts Ranges from strict parasitism to mutualism Coral reefs greatly aided by symbiotic photoautotrophic protists in tissues of corals (animals) Mixotrophs can be both photoautotrophic and heterotrophic depending on availability of sunlight or organic molecules Transportation of Protists Protists use various methods for transportation. (a) Paramecium waves hair-like appendages called cilia to propel itself. (b) Amoeba uses lobe-like pseudopodia to anchor itself to a solid surface and pull itself forward. (c) Euglena uses a whip-like tail called a flagellum to propel itself. Reproduction of Protists Vegetative: fragmentation Asexual reproduction: binary fission, budding, or spores Sexual reproduction: meiosis and fertilization Many protists can switch from one to the other depending on environmental conditions Spirogyra Vegetative: fragmentation Asexual: spores Sexual: male and female cells fuse to form diploid organism Habitats Aquatic/ damp environments Important component of plankton Plant and animal parasites Dead organisms and waste Classification Genetic analyses have revealed new evolutionary relationships Protists are found in each of the 6 supergroups = paraphyletic ○ Evolved multiple times Protists that share common morphology may have evolved analogous structures → convergent evolution Figure 23.9 Eukaryotic supergroups. This diagram shows a proposed classification of the domain Eukarya. Currently, the domain Eukarya is divided into six supergroups. Within each supergroup are multiple kingdoms. Although each supergroup is believed to be monophyletic, the dotted lines suggest evolutionary relationships among the supergroups that continue to be debated. Archaeplastida Unicellular, large multinucleated cells, and multicellular organisms Glaucophytes: chloroplasts retain remnants of cell wall of ancestral cyanobacterial symbiont Red algae: primarily multicellular; range in size from microscopic to large “seaweed” Green algae: most abundant algae; similar to land plants in chloroplast and cell wall structure Figure 23.11 Volvox. Volvox aureus is a green alga in the supergroup Archaeplastida. This species exists as a colony, consisting of cells immersed in a gel-like matrix and intertwined with each other via hair-like cytoplasmic extensions. (credit: Dr. Ralf Wagner) Figure 23.12 A multinucleate alga. Caulerpa taxifolia is a chlorophyte consisting of a single cell containing potentially thousands of nuclei. (credit: NOAA). One of the largest singlecelled organisms. Algae, seaweed, and kelp Algae: aquatic, photosynthetic protists; not a taxonomic group Seaweed: term typically used for large marine algae; not a taxonomic group Kelp: phyla of brown algae Not plants – similarity in structures due to convergent evolution Amoebozoa Unicellular, large multinucleated cells, and multicellular organisms Most possess lobed pseudopodia Gymnamoeba: typical naked amoebas and shelled Slime molds: morphological similarities to fungi due to convergent evolution Opisthokonta Posterior flagellum pushes the organism Choanoflagellates include unicellular and colonial forms; resemble common ancestor of sponges and possibly all animals Figure 23.16 A Colonial Choanoflagellate. Rhizaria Possess thin pseudopodia Tests: armor-like structure composed of calcium carbonate that sink to the ocean floor upon death → storing carbon Major component of plankton Figure 23.17 Rhizaria. Ammonia tepida, a Rhizaria species viewed here using phase contrast light microscopy, exhibits many threadlike pseudopodia. It also has a chambered calcium carbonate shell or test. (credit: modification of work by Scott Fay, UC Berkeley; scale-bar data from Matt Russell) Chromalveolata Ancestor that engulfed a photosynthetic red algal cell → secondary symbiotic event Includes brown algae, Plasmodium, Paramecium, diatoms Large component of plankton Dinoflagellates can be bioluminescent and cause red tides Figure 23.21 Dinoflagellates Figure 23.24 Paramecium. Figure 23.27 Diatoms. Excavata Primarily single-celled organisms Giardia: intestinal parasite Euglena: chloroplast from secondary endosymbiosis of a green alga Trypanosoma: African sleeping sickness & Chagas disease Figure 23.30 Giardia. Protists in Ecosystems Food sources: zooplankton, symbiotic relationships with other species such as green algae and coral Primary producers for aquatic ecosystems 25% of the world’s photosynthesis is done by photosynthetic protists Human pathogens Figure 23.37 Potato blight. These ○ ○ Plasmodium falciparum: malaria Trypanosoma brucei: African sleeping sickness Plant pathogens ○ ○ Downy and powdery mildew Potato blight: Irish potato famine Decomposers return nutrients to the ecosystem from dead organisms unappetizing remnants result from an infection with P. infestans, the causative agent of potato late blight. (credit: USDA)