Chapter 29 & 30

Endosymbiotic Theory

“No great discovery was ever made without a bold guess.”

--Isaac Newton

Atmospheric Oxygen

Most atmospheric O2 has been produced by the water-splitting step of photosynthesis.– Cyanobacteria.

Atmospheric Oxygen

When photosynthesis first evolved, the O2 produced dissolved into the surrounding water.

Eventually it reacted with dissolved iron and precipitated as iron ore.

Atmospheric Oxygen

After the iron had precipitated out, O2 continued to accumulate until the waterways became saturated and the remaining O2 then entered the atmosphere.

Atmospheric Oxygen

Atmospheric oxygen continued to accumulate gradually from about 2.7 bya until about 2.3 bya and then dramatically increased.

The increase was likely due to the evolution of more oxygen producing organisms.

http://www.nature.com/nature/journal/v451/n7176/fig_tab/nature06587_F2.html

Atmospheric Oxygen

The increasing O2 levels on the planet likely led to the extinction of numerous prokaryotic groups.

Oxygen is a highly reactive compound that damages cells and disrupts chemical bonds.

Atmospheric Oxygen

Some species of bacteria survived in habitats that remained anaerobic, and others adapted to the changing atmosphere.

The First Eukaryotes

About 2.1 bya, the first eukaryotic fossils began forming.

Eukaryotic cells have a number of complex features.

Three such evolutionary novelties came to define the early eukaryotes.

A Change in Cell Structure and Function Three evolutionary novelties:

– 1. The formation of ribosome studded internal membranes.

– 2. The appearance of a cytoskeleton.– 3. The evolution of digestive vesicles.

1. A Ribosome Studded Membrane

The ribosome-studded membrane assisted in the movement of protein products throughout the internal portion of the cell without harm to other cytoplasmic factors.

2. The Appearance of a Cytoskeleton

The cytoskeleton is comprised of actin fibers and microtubules.– Allows form movement of the cell and

movement of the internal contents.The development allows for

phagocytosis.

3. Digestive Vesicles

The formation of digestive vesicles allowed for membrane bound enzymes to form.– If unbound, these enzymes would destroy

the cell.

Endosymbiotic Theory

Where did the features of eukaryotic cells come from?

Endosymbiotic Theory

A wide variety of evidence supports the theory that small prokaryotes began living in larger (host) cells.

These cells likely gained entry to the host as undigested prey, or internal parasites.– This process has been observed by

scientists in as little as 5 years.

Endosymbiotic Theory

The benefits of the relationship are easy to see.– A photosynthetic endosymbiont would

provide nutrients to the heterotrophic host.– The host would provide shelter for the

anaerobic prokaryote from the increasingly aerobic environment.

Endosymbiotic Theory

Over time, this relationship would result in a situation where to two parts would become inseperable giving rise to a single organism.

Serial Endosymbiosis

All eukaryotes have mitochondria (or remnants of them), but not all have plastids.– Plastids are chloroplasts

or any related organelle. Chloroplasts: for photosyntheis Chromoplasts: for pigment synthesis and storage. Gerontoplasts: control the dismantling of the

photynthetic apparatus. Leucoplasts: for monoterpene (fragrance, etc.)

synthesis.– Amyloplasts: for starch storage and gravitropism.– Elaioplasts: for storing fat.– Proteionplasts: for storing and modifying proteins.

http://en.wikipedia.org/wiki/File:Plastids_types_en.svg

http://www.tutorvista.com/biology/biology-cytoplasm

Serial Endosymbiosis

Thus, according to the hypothesis of serial endosymbiosis, mitochondria evolved before plastids.– This was the result of

numerous symbiotic events.

Evidence for Endosymbiosis

The evidence is overwhelming:– Both organelles have circular chromosomes.

These chromosomes lack histones.– Both organelles have their own DNA.

Both organelles can perform transcription and translation of their own DNA.

– Both organelles can self-replicate—via binary fission—just like prokaryotes.

Evidence for Endosymbiosis

The evidence is overwhelming:– The inner membranes of both organelles

have enzymes and transport systems that are homologous to those found in the plasma membranes of living prokaryotes.

– Both organelles are approximately the same size as typical bacterium.

– Both organelles use many bacteria-like enzymes.

Evidence for Endosymbiosis

The evidence is overwhelming:– Both organelles are sensitive to certain

antibiotics.– Some antibiotics interfere with mitochondrial

protein synthesis.Rifampicin-binds to bacterial RNA polymerase

preventing transcription.Can prevent mitochondrial RNA synthesis, but

only at a very high concentration.

Evidence for Endosymbiosis

The evidence is overwhelming:– Both organelles contain ribosomes.

These ribosomes are very similar to bacterial ribosomes.

The ribosomes are nearly the same size, have very similar RNA sequences, and are sensitive to the same antibiotics as bacterial ribosomes.

The ribosomes are more similar to bacterial ribosomes than they are to eukaryotic ribosomes.

Secondary Endosymbiosis

Secondary endosymbiosis is another step in eukaryotic evolution.

In this process, a heterotrophic eukaryote engulfed an unrelated photosynthetic eukaryote (plastid).– The plastids were likely ingested into the

food vacuole, and over time formed a symbiotic relationship with the host.

Secondary Endosymbiosis

Studies of plastid bearing eukaryotes demonstrate how this process has taken place.– Red and green algae, produced from

primary endosymbiosis, provide a nice example of this process.Chlorarachinophytes are a specific example.

– Green algae engulfed by a heterotrophic eukaryote.

Secondary Endosymbiosis

Within the engulfed cell, we see lines of evidence for this process having taken place.– Within the cell is remnants of an engulfed

cell with a vestigal nucleus—called a nucleomorph.Nucleomorphic genes are still transcribed. Their DNA sequences are very similar to

those of green algae—further supporting the hypothesis that an ancestral eukaryote engulfed a green algae.

Secondary Endosymbiosis

The plastids are surrounded by four membranes.– The inner two membranes originated as

an inner and an outer membrane of an ancient cyanobacterium.

– The third membrane is derived from the engulfed alga’s plasma membrane.

– The outermost membrane is derived from the heterotrophic eukaryote’s food vacuole.

Click here for a video summary.

Secondary Endosymbiosis Summary

Could it Really Occur?

It is now…Some eukaryotes live in low O2

environments and lack mitochondria.– They have endosymbionts that live within

them and generate energy for them.

Could it Really Occur?

Protists live symbiotically in the hindgut of termites. – The protists, in turn, are colonized by

symbiotic bacteria similar in size and distribution to mitochondria.

– These bacteria function well in low O2 environments--unlike mitochondria.They oxidize food and create ATP for the

protist.

Could it Really Occur?

A study of Pelomyxa palustris provides some interesting insight:– This ameoba lacks mitochondria.– It contains at least 2 kinds of

endosymbiotic bacteria.– Killing the bacteria with antibiotics causes

an increase in lactic acid.– This suggests that the bacteria oxidize the

end products of glucose fermentation—something mitochondria normally do.