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1 The Endosymbiotic Theory The Tree of Life and its Main Branches At the highest level, life can be divided into three main groups, called Domains: the Archaea, the Eubacteria, and the Eukarya.! Characteristic Size Metabolism Bacteria small (0.2 - 10mm) all types of energy metabolism: some chemo-autotrophs, many fermenters, diverse photo- autotrophs, some respirers single loop no nucleus none Archaea small (0.2 - 10mm) all types of energy metabolism: many heterotrophs and chemo-autotrophs Membrane lipids Cell walls ester links ester links Stiff Stiff Reproduction Asexual - fission Asexual - fission Body plan Habitats unicellular/colonial anaerobic and aerobic habitats; very tolerant unicellular/colonial anaerobic and aerobic habitats; extremely tolerant of hot or acidic or salty habitats DNA structure Organelles? ! single loop no nucleus none Eukarya large (10 – 100mm) more limited types of metabolism: fermentation, oxygenic-producing photo-autotroph and aerobic respiration chromosomes in nucleus Mitochondria, plastids, flagella ester links Stiff (plants); flexile (animals ) Asexual - mitosis Sexual - meiosis chiefly multicellular most require oxygen and are intolerant of nasty environments 2 The Universal Tree of Life This phylogenetic tree shows the relationships of the major groups (Kingdoms) of organisms.! These Kingdoms are arranged within the three Domains of life, Archaea, Eukarya, and Eubacteria.! The tree is constructed from comparative analysis of the sequence of nitrogen-bearing bases on RNA molecules from organisms in each group.! Longer branches indicate greater genetic distance between organisms. (modified from J.I. Lunine 1999. Evolution of a Habitable World. Cambridge Univ. Press) The most primitive eukaryote - Giardia:no mitochondria or plastids, doesn't tolerate O2. More derived eukaryotes - a diversity of unicellular eukaryotes or Protists: all have mitochondria (e.g., amoebas, ciliates), some have plastids too (e.g., euglenids) Big Bang of Eukarya - Multicellularity arose many times within the Eukarya:! green algae, red algae, multicellular plants, multicellular animals, slime molds, and fungi: All these groups have mitochondria, plastids independently appeared in many different groups. 3 How did eukaryotes originate? The evolution of the compartmentalized nature of eukaryotic cells may have resulted from two processes Specialization of plasma membrane invaginations. Invaginations and subsequent specializations may have given rise to the nuclear envelope, ER, Golgi apparatus, and other components of the endomembrane system Endosymbiotic associations of prokaryotes may have resulted in the appearance of some organelles. Mitochondria, chloroplasts, and some other organelles evolved from prokaryotes living within other prokaryotic cells The Endosymbiotic Hypothesis The theory focuses mainly on the origins of chloroplasts and mitochondria It has been argued that mitochondria and plastids (and perhaps flagella) were once freeliving bacteria that took up residence inside the cell of another organism, probably an Archaean. More specifically, chloroplasts are believed to have descended from endosymbiotic photosynthesizng prokaryotes, such as cyanobacteria, living in larger cells Mitochondria are postulated to be descendents of prokaryotic areobic heterotrophs. 4 What observations support this hypothesis? They are of the appropriate size to be descendents of eubacteria They have inner membranes containing several enzymes and transport systems similar to those of prokaryotic plasma membranes The organelles are separated from the cytoplasm by complex membranes. Organelles have their own DNA (a simple loop); they reproduce by simple fission. Comparisons of the genetic contents of this DNA reveal that the organelles are more closely related to particular types of bacteria than they are to the nucleus of the cell in which they reside. Mitochondria are closely related to purple bacteria; chloroplasts are related to cyanobacteria. Chloroplasts have ribosomes more similar to prokaryotic ribosomes (with regards to size, biochemical characters, etc) than to eukaryotic ribosomes Mitochondrial ribosomes vary, but are also more similar to prokaryotic ribosomes The RNA of chloroplasts is more similar in basic sequence to RNA from certain photosynthetic eubacteria than to rRNA in eukaryotic cytoplasm Chlorplast rRNA is transcribed from genes in the chloropast while eukaryotic rRNA is transcribed from nuclear DNA Mitochondrial rRNA also has a base sequence which supports the eubacterial origin Environmental Implications? Because they have bigger cells, and because nearly all do aerobic respiration, Eukarya require higher levels of O2 to survive. Many Archaea and Eubacteria can’t tolerate O2. The rise in oxygen may have driven them into oxygen-poor zones where eukaryotes can’t go. What does the fossil record say about this transition? ! Sterols at 2.7 Bya. Sterol, which are present in the cell walls of the Eukarya, may just indicate that the "host" cell that was eventually invaded by Purple Bacteria and Cyanobacteria had evolved. They do not provide evidence that the key symbiotic events had taken place at 2.7 Bya. A better way to spot Eukarya in the fossil record is to look for big cells. Bacteria and Archaea typically form small cells.! If a cell is bigger than 60 microns, it was almost certainly was a eukaryote. 5 ~ 2.0 Bya: The Gunflint Chert These rocks contain the first abundant and diverse record of unicellular organisms.! The organisms are small, and all have morphologies very similar to living cyanobacteria. Why have cyanbacteria failed to evolve for 2.0 billion years? Asexual/Clonal reproduction - No sexual mixing of the genome. Genetic novelties can only arise by mutation. This limits the new genetic combinations upon which natural selection can work. 2.1 - 1.7 Bya: The first large fossil: Grypania It is not yet clear what Grypania is, but it is so large, it almost certainly is a eukaryote. Either large and unicellular with many nuclei or multicellular 1.7 to 1.2 Bya: non-diverse communities of unicellular Eukarya Fossils are much more common in rocks of this age.! Many contain unicellular organisms that are bigger than 60 microns, so we are probably dealing with eukaryotes 1.2 - 0.7 Bya:! Unicellular Eukarya diversify Acritarchs: a group of unicellular, eukaryotic fossils common at this time. Based on biochemical and morphologic evidence, it is thought that they are the resting cysts of protozoans that are common in the ocean today. Why the increase in diversity at 1.2 Bya? Invention of Sexual Reproduction Sexually reproducing organisms get copies of genes from each parent, then shuffle the genes from each parent before making gametes.! This allows for the perpetual generation of new genetic combinations.! Natural selection is presented with new genetic combinations every generation.! Much higher levels of variation allows much more rapid evolutionary change.