Primary and secondary endosymbiosis Natalie Hurt and Kathleen Nelson

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

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

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!

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

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

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

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

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!

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???

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

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

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

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?

Works cited Duve, Christian de. "The Birth of Complex Cells." Scientific American (1996): 50-57. McFadden, Geoffrey Ian. "Primary and Secondary Endosymbiosis and the Origin of Plastids." Journal of Phycology (2001): 951- 959. http://users.rcn.com/jkimball.ma.ultranet/ BiologyPages/E/Endosymbiosis.html http://en.wikipedia.org/wiki/Endosymbiotic _theory http://serialendosymbiosis.blogspot.com/20 07/12/secondary-endosymbiosis.html

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