Biodiversity – Sept. 24/12
- How did the eukaryotic cell evolve from the prokaryotic cell?: The endosymbiotic theory of
eukaryote evolution, or the serial endosymbiosis theory (SET) is the most widely accepted theory.
This theory was proposed in a landmark paper by Lynn Margulis in 1967, called The Origin of
Mitosing Eukaryotic Cells. Her work was rejected for year, until independent experimental evidence
supported her hypothesis.
- Eukaryotic cells have a nucleus with an organized genome and they contain mitochondria (many),
which are the powerhouse of the cell and they have an independent genome. Mitochondrial DNA
shows substantial similarity to bacterial DNA. Photosynthetic eukaryotic cells also contain
chloroplasts. Chloroplasts have an independent genome with similarities to the cyanobacterial
genome. Based on these things, SET was proposed.
- Primary endosymbiosis: 1. One prokaryotic organism engulfed another and over time evolved into a
eukaryotic cell with a mitochondrion. These eukaryotic cells multiplied and diversified (prokaryote +
prokaryote = eukaryote). 2. Later in time, one of these eukaryotic cells engulfed a cyanobacterium
and over time evolved into a eukaryotic cell with a chloroplast. These photosynthetic eukaryotic cells
multiplied and diversified and gave rise to the plant lineage (eukaryote + prokaryote = eukaryote).
- In step 1 of primary endosymbiosis, the prokaryote that was engulfed became the mitochondria. In
step 2, the cyanobacterium (is photosynthetic) that was engulfed became the chloroplast. Primary
endosymbiosis gave rise to photosynthetic plants.
- Secondary endosymbiosis: A eukaryotic cell engulfed a plant cell (probably a green alga or red alga).
This occurred many times (eukaryote + eukaryote = eukaryote). This is the origin of photosynthetic
taxa in non-plant lineages.
- Underlying theme of SET: Interdependent, cooperative existence of prokaryotes. One prokaryote
became an endosymbiot of another. Symbiosis means living in cooperation.
- Phylogeny of the eukaryote: Some taxa were formerly classified in the kingdom protista, but protists
have no evolutionary or phylogenetic meaning. Rhodophyta (red alga) arose from primary
endosymbiosis. Two branches that we will be focusing on are the chromalveolates and the excavates.
- Two synapomorphies of the erukaryote: Primary genome of cell consisting of chromosomes within a
membrane-bound nucleus. Mitochondria – organelles with diverse functions, including aerobic
respiration, independent genome.
- Definitions: Parasite – Relationship between members of different species. Parasite benefits at
expense of host. Algae – Aquatic photosynthetic organisms. Algae is not a phylogenetic term because
it doesn’t represent a taxon. Protozoan – Old terminology referring to heterotrophic unicellular
eukaryotes. A heterotroph is an organism that eats another organism.
- General characteristics of chromalveolata and excavate: Mostly unicellular and microscopic (less
than 1mm). Some are colonial. Some are multicellular and a few are enormous (up to 50m). They are
diverse with possibly 30 phyla. They are mostly aerobic. Most have cilia or flagella at some stage.
They are all aquatic – freshwater, marine, in tissues of other, soil. Many are photosynthetic. Some are
predaceous. Many are parasitic. And some are disease-causing (ex. dysentery, chagas, malaria, grape
mildew, potato blight). They are essential for ecosystem functioning (ex. primary producers, carbon
- Cilia and flagella: Both cilia (singular cilium) and flagella (singular flagellum) are complex organs
consisting of 9 pairs of microtubules in a ring surrounding a central pair of microtubules, all
surrounded by an extension of the cell membrane. A bacterial flagellum is much simpler – a single,