Endosymbiosis Flashcards
Features of eukaryotes?
Nucleus, ER, Golgi, all organelles with double membranes.
What is the endosymbiotic theory?
Proposes that mitochondria and chloroplasts were acquired as free living bacteria.
In what order did photosynthetic cells develop>
Mitochondria (proteobacteria) Chloroplast (cyanobacteria)
Why is it important to understand how endosymbiosis occurred?
Not enough food to feed the growing population so need better nitrogen fertilisers.
What is the nitrogenosome?
Encourage an endosymbiosis event between a rice plant and a free living Nitrogen fixing bacteria to develop a Nitrogen fixing organelle.
Evidence for endosymbiosis?
- Size and behaviour of the chloroplast/mitochondria is bacterial like. Don’t self generate in cells, divide by binary fission.
- Number and arrangement of membranes, presence of nucleomorph. Primary involves one cell enveloping another. Secondary leads to a chloroplast with two membranes (mitochondria and chloroplasts)
What ribosomes do organelles contain?
Contain ribosomes which are more bacterial than eukaryotic
Sedimentation closer to 70S (bacterial form)
Inhibited by bacterial antibiotics
rRNA sequence is homologous to bacterial rRNA
Do organelles contain DNA?
Yes, it codes mainly for the transcription/translation apparatus.
How has the endosymbiosis progressed?
The organelles are now no longer capable of independent existence:
Mitochondria and chloroplast genes have migrated extensively to the nucleus.
Mitochondrion genomes code for 3-67 proteins compared to 6000-8000 likely alpha-protobacterial donor. Experiment in 2004 showed that chloroplast antibiotic resistance genes transfer to nucleus at high frequency
What selective forces encourage endosymbiotic events?
Atmospheric oxygen have risen over time and the first large rise coincided with the origin of the first eukaryotes.
- Sulphur Syntrophy* – neither cell in isolation can extract all energy from glucose, requires anaerobic/aerobic interface
- Hydrogen hypothesis/Anaerobic Syntrophy* – the host that acquired the mitochondrion was a hydrogen-dependent archaeon which used hydrogen and carbon dioxide to produce methane. The future mitochondrion was an anaerobic eubacterium which produced hydrogen and carbon dioxide as by-products of anaerobic respiration. A symbiotic relationship between the two started based on the host’s hydrogen dependence
When did endosymbiosis of mitochondria occur?
1. Mitochondria-late theory:
Deep routing group of Archezoa lack mitochondria and therefore the early evolution of eukaryotes took place after the initial eukaryote radiation. Ancestral host must have been capable of phagocytosis to allow the bacterium to have been engulfed, no prokaryote has been shown to phagocytose so this ability must have evolved in the eukaryotic host
However…
Was determined that Archezoa had organelles surrounded by two membranes and these were functionally derived from mitochondria
Mitochondrial heat shock proteins are transferred to the nucleus and these were found in all key groups of Archezoa suggesting that these groups had lost their mitochondria
2. Mitochondria-early theory: (current idea)
Mitochondrion acquisition must occur before eukaryotic radiation but prokaryotes can’t phagocytose so bacteria cannot be engulfed.
What was the host cell?
Recent evidence suggests that informational (RNA/DNA) genes are from Archaea and operational (metabolic) genes are from bacteria
Believed that the ancestral eukaryote arose from fusion of genomes, before endosymbiosis
However…
Eukaryotic signature proteins found in all eukaryotes but not in any bacteria or archaea. Must have had time to evolve in a common ancestor but there doesn’t seem time if mitochondria evolved early and are monophyletic. Recent evidence suggests that eukaryotes lie within the Archaea so the eukaryotes aren’t monophyletic
Endosymbiosis involves phagocytosis but no known prokaryote is capable of this function. Therefore cannot have early mitochondrial engulfment using this host. Solution to this fusion issue might be that it occurred via Lateral Gene Transfer, not really a fusion event.
