Systematics and Origin of Life

Systematics

• Polyploid refers to a cell or organism with more than two sets of chromosomes
• Allopolyploidy refers to when original parents are different species
• Autopolyploidy refers to when there is only one parent source
• Between 50-70% of plant species are polyploid
• Systematics is the study of diversity and its evolutionary history
• The overall goal of systematists is to discover all branches of the phylogenetic tree
• Carl Linnaeus established a method of naming plants in 1753 and animals in 1758
• Topology refers to branching order, where all living species are considered equally evolved
• Relatedness refers to the recency of common ancestry

Origin of Life

• Visible fossils resembling bacteria have been detected back to 3.5 Ga (1 billion years ago)
• Isotopic evidence of life dates back to 3.7 Ga
• Conditions of the Earth were uninhabitable prior to 4.4 Ga
• Life is estimated to have evolved 3.7-4.4 billion years ago
• At 4 Ga, the Earth was hot and at high pressure
• The atmosphere contained reducing gases such as CO2, CO, H2, ammonia (NH3), and hydrogen sulfide (H2S)
• There was no oxygen gas (O2)
• Similar conditions exist today in deep-sea hydrothermal vents

Modern Prokaryotes

• Modern prokaryotes that are hyperthermophiles include:
• Thermotoga (bacteria): Optimal temperature = 80°C
• Aquifex (bacteria): Optimal temperature = 85°C
• Methanopyrus (archaea): Optimal temperature = 98°C
• Pyrodictium (archaea): Optimal temperature = 105°C
• L.U.C.A. (last universal common ancestor) was most likely a prokaryote
• Parsimony suggests that, given a set of explanations, the simplest is most likely correct
• Hyperthermophily is living in very hot environments
• Chemoautotrophy is using chemical energy to fix carbon

Bacteria

• Bacteria exhibit diverse metabolism
• Bacteria are essential for nutrient cycling (carbon, nitrogen, sulfur, phosphorous, etc.)
• Bacteria form many symbiotic associations, including mutualism and parasitism

Cyanobacteria

• Cyanobacteria are ecologically important
• They are primary producers
• They are nitrogen fixers, located in heterocysts
• Oxygenic photosynthesis
• 6CO2 + 12 H2O → C6H12O6 + 6O2 + 6H2O
• Water is the electron donor, creating O2, which takes lots of energy
• Requires two photosystems, each evolved individually
• Origin of oxygenic photosynthesis involves lateral gene transfer, bringing together two distinct photosystems
• This event is one of the most important in the evolution of life
• Oxygen accumulated in the atmosphere only after iron was oxidized
• Ozone (O3) layer developed
• Permitted the origin of aerobic respiration

Archaea

• Many, but not all, Archaea can live in extreme environments
• Some are mutualists, but none are parasites
• They are metabolically diverse and very important in nutrient cycling
• Archaea have similarities to eukaryotes

Cell Type Differences

• Prokaryotes
• Lack internal vesicles (membrane-bound structures)
• Eukaryotes
• Have lots of membrane-bound structures
• Include mitochondria, plastids (e.g., chloroplasts), energy, and a nucleus
• Nucleus is the home of DNA
• Include endoplasmic reticulum for the processing of proteins for secretion

Cell Type Characteristics

• Bacteria

• Cell type: Prokaryotic

• Nuclear envelope: Absent

• Number of chromosomes: 1

• Chromosome configuration: Circular

• Organelles (mitochondria & plastids): Absent

• Cytoskeleton: Absent

• Chlorophyll-based photosynthesis: Absent

• Archaea

• Cell type: Prokaryotic

• Nuclear envelope: Absent

• Number of chromosomes: 1

• Chromosome configuration: Circular

• Organelles (mitochondria & plastids): Absent

• Cytoskeleton: Absent

• Chlorophyll-based photosynthesis: Absent

• Eukarya

• Cell type: Eukaryotic

• Nuclear envelope: Present

• Number of chromosomes: More than 1

• Chromosome configuration: Linear

• Organelles (mitochondria & plastids): Present

• Cytoskeleton: Present

• Chlorophyll-based photosynthesis: Present

Origin of Eukaryotic Life

• An endosymbiotic origin of mitochondria & plastids is firmly established
• Evidence for endosymbiosis includes:
• Double membrane - one original from bacteria, one from cell that engulfed it
• Never arise from scratch (always by division)
• Similarities to bacteria
• Circular DNA genome, bacterial ribosomes, similar biochemistry
• Phylogenetic analysis of organellar genes
• Inside-out theory: A theory that proposes that the cytoplasm is a new compartment that evolved by fusion of extracellular blebs from an archaeal cell around surface bacteria

Eukaryote Supergroups

• Opisthokonta: animal / fungal clade
• Heterotrophs
• Glycogen storage
• Chitin production
• Single, posterior, smooth flagella
• Fungi:

• 150,000 species named

• ~5-6 million or more
• Heterotrophs by absorption
• The fungal body is a mycelium of filaments called hyphae
• Chitinous cell wall
• Septum
• They provide many uses to humans
• Decomposers (nutrient cycling)
• Mutualisms
• Mycorrhizae: fungal-root mutualism
• The fungus benefits from a steady supply of sugar from the host plant
• The host plant benefits because the fungus increases the surface area for water uptake and mineral absorption
• ~90% of plant species that have been examined make mycorrhizal associations
• Mycorrhizae are more efficient at nutrient uptake than plants and can protect plant roots from various pathogens
• Two main types of mycorrhizae:
  • Ectomycorrhizae
  • Does not penetrate deep into the plants' cortical cells
  • Found on mostly woody trees
  • Arbuscular or endomycorrhizae
  • Penetrates into the plants' cortical cells
  • ~80% of plants have this type of association
• Lichens:
• 13,000 species
• 5 independent origins

Algae

• Protists: Eukaryotes that are not animals, plants, or fungi
• Algae: Photosynthetic eukaryotes (except land plants)
• Euglenoids:
• Anterior flagella (one or more)
• Mostly in fresh water
• Plastid with 3 membranes
• Heterokonts/brown algae:
• Diatoms
• Plastid with 4 membranes
• Uni- to multicellular
• Tinsel and smooth flagella
• Green algae:
• Includes land plants
• Two anterior flagella
• 2 plastid membranes
• Red algae (rhodophyta):
• Oldest multicellular fossils (~1 Ga)
• Important in marine systems (especially deep water)
• 2 plastid membranes
• No flagella
• Dinoflagellates:
• 2 flagella
• Plastids with 4 membranes
• Symbionts of corals
• Important plankton
• Apicomplexans:
• Non-photosynthetic parasites of animals
• Have plastids (4 membranes)
• No flagella
• Serial endosymbiosis:
• There was a single primary endosymbiotic event in which plastids were engulfed
• Green and red algae have primary plastids
• Other Eukaryotes acquired Eukaryotic plastids by secondary endosymbiosis