1. Primary and secondary endosymbiosis
Natalie Hurt and Kathleen Nelson

2. The endosymbiotic theory
What is it?
Concerns the origin of mitochondria and chloroplasts
Prokaryotes that came to live inside eukaryotes
“endo” = inside
“symbiosis” = cohabiting

3. Primary endosymbiosis
The engulfment of a bacterium by another free living organism
Unusually large host
Heterotroph
Loss of cell wall
Convolution of membrane
Inward folding of membrane
Emergence of skeletal elements
Primitive phagocyte
Developed symbiotic relationship

4. The hypothesis Primitive phagocyte was probably anaerobic
Acquired mitochondria
Evolved from an aerobic ancestor
Or acquired chloroplasts
Evolved from cyanobacteria
Both provided oxygen detoxification and other beneficial properties
Omm nom nom!

5. evidence Both: Have own genome similar to bacteria
Single, circular DNA
No histones associated
First amino acid is f-methionine
Have own protein synthesizing machinery
Double membrane
Transit sequence

6. The oxygen Holocaust Early atmosphere contained little to no oxygen
Appearance of cyanobacteria
Entrance of oxygen in appreciable amounts
2 billion years ago
Anaerobic bacteria fell victim
Survivors:
Found an oxygen-free niche
Hydrothermal vents
Developed protection against oxygen toxicity

7. Peroxisomes
Christian de Duve hypothesized that peroxisomes were engulfed before mitochondria or chloroplasts
Early oxygen detoxifiers
Mitochondria combust oxygen to form ATP (energy)
Peroxisomes release heat
Protected host cells while mitochondria developed more efficient processes

8. Peroxisomes as endosymbionts?
Acquire proteins like mitochondria and chloroplasts
Transit sequence - SKL
No remnants of genome
De Duve’s suggestion: the peroxisome lost its DNA to the nucleus
Older than mitochondria and chloroplasts

9. Refutes Peroxisomes can be made from the ER
Mitochondria and chloroplasts cannot
Few (if any) protein homologies with mitochondria and chloroplasts
Significant homologies with 6 ER proteins
Can show up in cells lacking peroxisomes if mutants are introduced to the wild type gene
If an endosymbiont, then it shouldn’t be made by the ER and have these homologies!

10. The plastid Originated from cyanobacteria
Algae with plastids that have more than 2 membranes
Algae that have plastids with remnants of a nucleus
How could this be???

11. Secondary endosymbiosis
The product of primary endosymbiosis is itself engulfed and retained by another free living eukaryote
Result = motile, autotrophic cell
Own nucleus
Own mitochondria
Own endoplasmic reticulum
Contains the primary endosymbiont

12. Eukaryotes acquired plastids
Heterotroph engulfs a red or green alga
Seen in photosynthetic eukaryotes
Algae
Cryptomonads
Chlorarachinophytes
Benefits from retaining algal plastid

13. Evidence Triple membrane Nucleomorph
Eukaryote with a double membrane (2)
Membrane from vesicle when engulfed (1)
Nucleomorph
Retained nucleus from eukaryotic endosymbiont
Genes for maintaining the plastid
Size varies from lineage to lineage
Others may have completely lost theirs to the host nucleus

15. Other Possibilities
Dinoflagellates seem to go through a series of secondary endosymbiosis
Tertiary endosymbiosis?
Debatable
Still only have 3 membranes
Maybe increasing number of membranes wasn’t favorable?

17. Works cited
Duve, Christian de. "The Birth of Complex Cells." Scientific American (1996):
McFadden, Geoffrey Ian. "Primary and Secondary Endosymbiosis and the Origin of Plastids." Journal of Phycology (2001):
BiologyPages/E/Endosymbiosis.html
_theory
07/12/secondary-endosymbiosis.html

18. Questions????