Endosymbiotic Theory

Universal Tree of Life

Or…...• According to this tree the earliest eukaryotic cells were Archaezoa that are amitochondriate organisms that were adapted to an anaerobic environment such as the extant Diplomonads (Giardia), Parabasalia (trichomonads) and the Microsporidia. Mitochondria then were acquired at a later stage from a bacterial endosymbiont belonging to the group of the alpha-proteobacteria.

Key Differences among the Major domains of Life

Giardiaeuglenids

Single-celled Eukaryotes

Multi-cellular Eukaryotes

Green algae (Ulva)

Red algae

Multi-cellular Eukaryotes cont.

Multi-cellular plants (ferns)

Multi-cellular animals

Multi-cellular Eukaryotes cont.

Slime molds

Fungi

Endosymbiotic Theory

The evolution of the compartmentalized nature of eukaryotic cells may have resulted from two processes:

Specialization of plasma membrane invaginations

Endosymbiotic associations of prokaryotes may have resulted in the appearance of some organelles

Endosymbiotic Theory cont.• The theory was developed extensively by Lynn Margulis• It focuses mainly on the origins of chloroplasts and mitochondria• Chloroplasts are believed to have descended from endosymbiotic photosynthesizng prokaryotes, such as cyanobacteria, living in larger cells• Mitochondria are postulated to be descendents of prokaryotic areobic heterotrophs. • Perhaps they gained entry as parasites or undigested prey of larger prokaryotes. Then the association progressed from parasitism or predation to mutualism.

Observations supporting the hypothesis

The organelles are of the appropriate size to be descendents of eubacteria They have inner membranes containing several enzymes and transport systems similar to those of prokaryotic plasma membranes The organelles are separated from the cytoplasm by complex membranes. The organelles have their own DNA and they reproduce by simple fission. DNA of the organelles are more closely related to particular types of bacteria than they are to the nucleus of the cell in which they reside. Chloroplasts have ribosomes more similar to prokaryotic ribosomes (with regards to size, biochemical characters, etc) than to eukaryotic ribosomes• Molecular systematic evidence: RNA of chloroplasts is more similar in basic sequence to RNA from certain photosynthetic eubacteria than to rRNA in eukaryotic cytoplasm

Environmental Implications?

• Because they have bigger cells, and because nearly all do aerobic respiration, Eukarya require higher levels of O2 to survive.

• Mitochondria - can do aerobic respiration when O2 is 2% of modern. • Chloroplasts - can do photosynthesis when O2 is 10% of modern.

• Many Archaea and Eubacteria can’t tolerate O2. • The rise in oxygen may have driven them into oxygen-poor zones where eukaryotes can’t go.

What does the fossil record say about this transition?   Sterols at 2.7 Bya. • Sterol, which are present in the cell walls of the Eukarya, may just indicate that the "host" cell that was eventually invaded by Purple Bacteria and Cyanobacteria had evolved. • They do not provide evidence that the key symbiotic events had taken place at 2.7 Bya.

• A better way to spot Eukarya in the fossil record is to look for big cells. • Bacteria and Archaea typically form small cells.  • If a cell is bigger than 60 microns, it was almost certainly was a eukaryote.

~ 2.0 Bya: The Gunflint Chert

• These rocks contain the first abundant and diverse record of unicellular organisms.  • The organisms are small, and all have morphologies very similar to living cyanobacteria.

Why have cyanbacteria failed to evolve for 2.0 billion years?

• Asexual/Clonal reproduction - No sexual mixing of the genome..• Cyanobacteria are very tolerant. They are relatively immune to selection driven by environmental change.

Grypania spp.

2.1 - 1.7 Bya: The first large fossil: Grypania

• It is not yet clear what Grypania is, but it is so large, it almost certainly is a eukaryote.

1.7 to 1.2 Bya: non-diverse communities of unicellular Eukarya

• Fossils are much more common in rocks of this age.  • Many contain unicellular organisms that are bigger than 60 microns, so we are probably dealing with eukaryotes.  • The number of different types of cells is low, however.

1.2 - 0.7 Bya:  Unicellular Eukarya diversify

•Acritarchs: a group of unicellular, eukaryotic fossils common at this time. • Based on biochemical and morphologic evidence, it is thought that they are the resting cysts of protozoans that are common in the ocean today.

Why the increase in diversity at 1.2 Bya?

Evolution of Sexual Reproduction

• Sexually reproducing organisms get copies of genes from each parent, then shuffle the genes from each parent before making gametes.  • This allows for the perpetual generation of new genetic combinations.  • Much higher levels of variation allows much more rapid evolutionary change.