Endosymbiotic origin of
Eukaryotic Cells



1. Anaerobic prokaryote engulfed an aerobic prokaryote; stabilized as symbiont

- eventual transfer of genetic material to host cell genetic material, which becomes enclosed in membrane formed from invagination of cell membrane (nucleus)
- organelle = mitochondrion

- e.g., aerobic protist such as an amoeba

2. Aerobic eukaryotic cell engulfs a photosynthetic prokaryote such as a cyanobacterium; stabilized as symbiont

- eventual transfer of genetic material to host cell nucleus
- organalle = chloroplast

- e.g., photosynthetic protistans

3. Evidence for endosymbiotic origin

- similarity of mitochondria & chloroplasts to bacteria

- anatomy; ribosomes; responses to antibiotics; nucleotide sequences

- evidence for endosymbiosis in extant organisms (sea slugs with chloroplasts; unicellular algae with corals; etc.)


Evolution of multicellularity


1. Originated independently several times in Kingdom Protista

- multicellular red algae, brown algae, green algae have closest relatives in unicellar red, brown and green algae, respectively

2. Colonial forms of unicellular protists were the transitional stage

3. Next was the specialization of function and cell structure

- with this stage, communication between cells is essential (the differential expression of genes and signalling)


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Last updated January 25, 1999.