Serial endosymbiotic theory SET

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Serial endosymbiotic theory (SET)

• Formalized by F.J.R.Taylor (1974)
• Modified by L. Margulis (1990)

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Hint of whats to come-

• Schimper in 1883
• - Proposed that chloroplasts are cyanobacteria
  living inside plant cells

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Evidence that these organelles have prokaryotic
traits

• Mitochondria and chloroplasts
• Circular DNA
• Synthesize proteins
• Divide by fission
• Mutate

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The Origin of Eukaryotic Cells

• Autogenous theory (Archezoa Hypothesis-
  Cavalier-Smith 1983)
• organelles evolved within the cell by progressive
  compartmentalization

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• endomembrane system evolved from inward folds of
  the plasma membrane of a prokaryotic cell

Plasma membrane
DNA
Cytoplasm
Ancestral prokaryote
Endoplasmic reticulum
Nuclear envelope
Nucleus
Cell with nucleus and endomembrane system
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Archezoa Hypothesis

• Several eukaryotic lineages evolved BEFORE
  mitochondrial endosymbiosis
• EVIDENCE AGAINST THE HYPOTHESIS
• Roger (1999)- all extant eukaryotes have
  mitochondrial genes in their nuclear DNA

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The Origin of Eukaryotic Cells

• Autogenous theory (Archezoa Hypothesis-
  Cavalier-Smith 1983)
• organelles evolved within the cell by progressive
  compartmentalization
• Serial endosymbiotic theory (SET (1974-1990))
  organelles are the result of successive
  engulfments

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• endosymbiosis generated mitochondria and
  chloroplasts

Anaerobic-lacking oxygen referring to an
organism, environment or cellular process that
lacks oxygen and may be poisoned by it
?-Proteobacteria
Aerobic heterotrophic prokaryote
Cyanobacteria
Aerobic cells use oxygen to release energy from
organic molecules by cellular respiration
Photosynthetic eukaryotic cell
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a larger prokaryote (or perhaps early eukaryote)
engulfed or surrounded a smaller prokaryote
(permanent resident) some 1.5 billion to 700
million years ago
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• The ancestors of mitochondria and chloroplasts
  were once free-living prokaryotes, but they
  became incorporated into a host cell, thus
  producing what we call today eukaryotes

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Insights from recent results

• Reduce the number of secondary losses of
  mitochondrial
• Group together many amitochondriate lineages
• Find evidence of very reduced mitochondria
• Mitochondrial monophyly?
• (discoid, flattened, and tubular cristae)
• Reduce the number of secondary endosymbioses
  (photosynthetic eukaryote being engulfed by
  another eukaryote)
• Group together branches with chloroplast
  surrounded by four membranes

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Giardia lamblia trophozoite
A modification of Woese from Brock et al. (1994)
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        Revised
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