Endosymbiosis and the evolution of eukaryotes Flashcards
What is the evidence that endosymbiosis happened?
- Size and behaviour is bacteria-like
- Number and arrangement of membranes and presence of nucleomorph
- Organelles contain ribosomes which are more bacterial than eukaryotic
- Organelles contain DNA which codes mainly for the transcription/ translation apparatus – all components show strong homology to eubacteria
How are mitochondrial and chloroplast ribosomes more bacterial-like than euk-like?
o Sedimentation closer to 70S (bacterial) form
o Inhibited by bacterial antibiotics eg. Streptomycin
o rRNA sequence homologous to bacteria rRNA
Endosymbiotic theory
• Mitochondria and chloroplasts did not evolve in situ but were acquired as free-living bacteria
• Two independent types of bacteria engulfed by host cell
- Cyanobacterium formed chloroplast
- A-proteobacterium formed mitochondrion
• Bacterial phylogeny from rRNA sequence only
- Shows that chloroplast and mitochondria are not in any way closely related
Things in euks and not in proks
nucleus, endoplasmic reticulum, golgi, organelles with double membranes
Why is it still relevant to know when and how it happened?
Understanding endosymbiosis in the past may make the NITROPLAST a more feasible solution to world hunger
Food security RELIES on NITROGEN FERTILISERS
o There’s been an exponential rise in human pop.
o Food production has managed to keep up with demand pretty well, but by
growing more food in the same area ie. increased intensification
o Increased yield has been matched with a much increased demand for N fertilisers
NITROGEN FERTILISERS come from OIL are UNSUSTAINABLE & energetically expensive to manufacture
o N demand so far met w/ the Haber process, but the symbiosis between leguminous plants and bacterial N2 fixers has been explored.
Legumes don’t need fertiliser as the bacteria fix nitrogen in a usable form for them.
WE CAN’T J USE LEGUMES
Legumes don’t have high carbohydrate content so are not very viable as alternatives to staple crops eg. Maize, wheat, rice, potato
Engineering stable crops to produce usable N?
o There has been a long history of trying to GM plants to express all the genes required for a staple crop to fix its own nitrogen, but it has had little success
The nitroplast (nitrogenosome) o Idea to encourage an endosymbiotic event between rice plant and a free-living N2-fixing bacteria to develop a N2-fixing organelle
How are mitochondrial and chloroplast size and behaviour bacteria-like?
- however…
o Organelles in eukaryotes resemble bacteria morphologically
o They replicate by binary fission like bacteria! New organelles only arise from existing ones – they are never formed de novo
However…
Mitochondria and most chloroplasts lack any cell walls and peptidoglycan
Mitochondria are surprisingly dynamic, showing morphological plasticity and with extensive fusion and fission (they change shape and fuse together!)
How are mitochondrial and chloroplast number and arrangement of membranes and presence of nucleomorph bacteria-like?
o Most organelles (nucleus and lysosome) are surrounded by a single membrane, but mit. have two membranes
• ‘chloroplast’ with 4 membranes is created by the existing double membrane of the red algal chloroplast + the double membrane of the red alga itself
What’s a nucleomorph? How formed?
▪ Theory that red alga underwent primary endosymbiosis, engulfing a bacteria and not eating it. This led to the production of a chloroplast with 2 membranes.
▪ Then red alga is itself engulfed by a protoeukaryotic host (cryptomonads) in secondary endosymbiosis.
• The nucleus of the red alga is nearly lost, becoming a nuleomorph
• The mitochondrion of the red alga is lost
• ‘chloroplast’ with 4 membranes is created by the existing double membrane of the red algal chloroplast + the double membrane of the red alga itself
name photosynthetic eukaryotes with…
single membrane
double membrane
triple membrane
quadruple membrane
single = cyanobacteria double = green and red algae after primary endosymbiosis quadruple = cryptomonod, brown algae, apicomplexa !! after secondary endosymbiosis
triple = two lineages have triple (dinoflagellates, euglenoids)
due to secondary loss of one membrane
if you test a cryptomonad with FISH, where would you see the eukaryotic and prokaryotic probes?
fluorescent probes are for in situ hybridisation to rRNA so will target pro/euk ribosomes
prokaryotic rRNA probe
o Gold atoms as stain, electron microscopy for detection
→ Gold label only found within the innermost compartment of the chloroplast, supporting cyanobacterial ancestry for chloroplast
eukaryotic rRNA probe
o There is labelling seen in the cytoplasm of the 1st and 2nd eukaryotic host ie. in the nucleomorph bit of the chloro and in the cytoplasm (as expected)
→ Labelling in both compartments supports eukaryotic ancestry of both hosts
How does organelle DNA show evidence of endosymbiosis?
Organelles contain DNA which codes mainly for the transcription/ translation apparatus – all components show strong homology to eubacteria
o The symbiosis has progressed - mitochondria and chloroplasts are now no longer capable of independent existence. The genes from them have migrated exclusively to the nucleus.
o Bacteria – 7000 genes
vs Mitochondrion – 35 genes
The proteins are made in the cytoplasm and are imported into organelles
What selective forces encouraged endosymbiotic events?
Name the 4 theories
- The Great Oxygenation event
- OxTox model (not correct)
- The hydrogen hypothesis
- Sulphur synotrophy
Basically… there are still lots of options on the table and it’s v much still an active research question
How did atmospheric oxygen levels over time encourage endosymbiotic events?
Rise in oxygen levels due to cyanobacterial photosynthesis was HIGHLY TOXIC to the surrounding biota, and that this selective pressure drove the evolutionary transformation of an archaeal lineage into the first eukaryotes.
The Great Oxygenation event
- what
Earth’s atmosphere and the shallow ocean experienced a rise in oxygen around 2.4 billion years ago
o For most of earth’s history (4.5-2billion years ago) O2 not present in atmosphere
o Life predates oxygen – before there was anaerobic fermentation and non-oxygenic photosynthesis
o There were no fossils for the first 3.5 billion years, until cyanobacterial stromatolites (the first aerobic prokaryotes)
The Great Oxygenation event
- caused by
Cyanobacteria produced and released oxygen as a by-product of photosynthesis, converting the early oxygen-poor, reducing atmosphere into an oxidising one
The Great Oxygenation event
Why did it take so long for oxygen levels to get high?
There was a huge amount of iron that first was oxidised from Fe2+ to Fe3+
The OxTox model
OxTox model - Metabolic integration during the evolutionary origin of mitochondria
o Eukaryotes evolved due to increasing oxygen tension
o Oxygen is actually very toxic – we have evolved a system to detoxify it but they didn’t
Mitochondria
+ provide eukaryotic cells with metabolic advantages (compartmentalisation, increased efficiency of ATP production)
- produce reactive oxygen species that damage the nucleocytoplasm and the mitochondria, causing mutations, diseases, and ageing
o Ancestor of the nucleocytoplasm was an O2-intolerant cell. It became associated with aerobic bacteria that served to consume any O2 present, protecting the cell from O2 toxicity. In return, the host provided fermentation product such as pyruvate to the bacteria
o Over time, bacteria became intracellular, ATP export added etc
Why is oxygen toxic?
Mitochondria produce reactive oxygen species that
- damage the nucleocytoplasm and the mitochondria
- causr mutations, diseases, and ageing
Criticism of the OxTox model
How could the symbiont shield the host from O2 once it had been internalised, as surely the O2 would have to pass through the cytoplasm?
There would be no advantage to internalising the bacteria as the protection would be lost!!
→ This theory ‘oxtox’ is NOT COMPATIBLE with symbiosis!!!!
The hydrogen hypothesis
- what
The hydrogen hypothesis – Methanogenic syntrophy. Symbiosis between archaeon and bacterium to allow archaea to use H produced by proteobacterium
o Metabolic symbiosis between methane-producing archaea and -proteobacteria (capable of aerobic and anaerobic resp)
o In the absence of oxygen, protobacterial cell metabolised organic compounds to H2 and CO2, and archaeal methanogen anaerobically converted those products into CH4
o Over time, the methanogenic species becomes increasingly dependent on fermenting bacteria, engulfing it.
o Methanogen evolved into the nucleocytoplasm and the H2-excreting bacteria became mitochondria
o Gene transfer between symbiont (bacteria) and host (archaea) to allow host to take up glucose needed by bacteria
Criticism of the hydrogen hypothesis
Methanogenic metabolism is incompatible with O2 – methanogens are strict anaerobes. There is no evolutionary pathway by which a methanogen can become aerobic
The Sulphur syntrophy hypothesis
- what
Sulphur syntrophy between archaeon and bacterium
o Neither cell in isolation can extract all the energy from glucose – requires anaerobic/aerobic interface
o Theory that an alpha-proteobacterium oxidised H2S to sulphur and heterotrophic archaeon reduced it back to H2S
o Archaea uses glucose and anaerobically respires sulphate to sulphide. Sulphide used as electron donor for energy
respiratory complexes.
The Sulphur syntrophy hypothesis
Evidence and criticism
Evidence:
- Mitochondria are closely related to purple sulphur bacteria (which oxidise H2S to sulphur either photosynthetically or by using O2)
Criticism
- H2S is highly toxic and inhibits mitochondria
When did endosymbiosis happen for mitochondrion?
The Old Theory – Mitochondrion-late school
- Theory that eukaryotes evolved and radiated a bit before the mitochondrion was acquired
- Later the mitochondrion was acquired by endosymbiosis by a protoeukaryote and there was further radiation into many different eukaryotic groups
Now seen as WRONG
When did endosymbiosis happen for mitochondrion?
Why is the OLD theory now seen as WRONG?
There’s a whole set of proteins (HSP70) that are present in ALL eukaryotes with mitochondria, but INITIALLY thought to be absent in all archaezoa
1) Now, mitochondrial HSP70 proteins are found in all key archezoal groups, suggesting they lost their mitochondria
2) Determined that archezoa actually had organelles surrounded by TWO membranes which were functionally derived from mitochondria – the hydrogenosome and the mitosome.
Now thought that mito were there in archaezoa but got diverted to other processes, evolving into H2 producing hydrogenosomes and mitochondrion-derived mitosomes
When did endosymbiosis happen for mitochondrion?
The New Theory – Mitochondrion early model
- Mitochondrial acquisition occurred before eukaryotic radiation
- All basal groups had mitochondria but either lost them or diverted them (hydrogenosomes/mitosomes)
ISSUE
• Only issue is that prokaryotes cannot phagocytose so the bacteria (proto) cannot be engulfed
Who participated in endosymbiosis?
chimeric theory
Currently thought
mitochondrion = rhodobacter-like
chloroplast = cyanobacteria-like
(unless their endosymbiosise were not unique events??)
Comparison of sequence homology suggest that eukaryotes are a chimera of bacteria AND archaea
o There are two sets of genes that imply different lineages (ie. Which are more closely related to eukaryotes?? We dunno)
o Recent evidence suggests that informational genes (RNA/DNA) are from archaea and operational (metabolic) genes are from bacteria
Problems with chimeric theory
- No prokaryote was known to allow phagocytosis. Therefore there cannot have been early mitochondrial engulfment using this host
- NOT TIME for eukaryotic signature proteins (present in all eukaryotes but not in any bacteria or archaea) to evolve in a common ancestor if mitochondria evolved early and mitochondria are monophyletic.
Possible solution to no bacterial/archaeal phagocytosis problem of chimeric theory
RUSSIAN DOLLS
• Current bacteria CAN endosymbiose, although mechanism is unknown
• Bacterial endosymbionts of mealybugs themselves contain bacterial endosymbionts!
• Red fluorescence shows FISH probe to gamma-proteobacteria
• Blue fluorescence shows FISH probe to beta-proteobacteria
• Composite shows and combined, with localised within
Solution to eukaryotic signature proteins problem of chimeric theory
• Recent analysis of a group of eukaryotic genes suggests that Eukaryota lie WITHIN the Archaea
ie. Eukaryotes are not a monophyletic group
• Eukaryotes now thought to nestle deeply within the Asgard archaea (found near to hydrothermal vents)
▪ The asgard group contain many genes previously only ever in eukaryotes!! Including genes coding for signature proteins
• This allows time for the development of signature proteins within an archaeal-related lineage
Order of events during endosymbiosis
- chimera ‘protoeukaryote’ formed from archaea engulfing a gram -ve α-proteobacteria
This THEN diversified into red and green algae (they resulted from the same endosymbiotic event)
- Chloroplasts
Primary endosymbiosis
- red/green algae engulfs ancestral prokaryotic cyanobacterium, forming chloro with 2 membranes
Secondary endosymbioses
- eukaryote engulfs red/green algae, forming chloro with 4 membranes
- within the first set of membranes there’s a nucleomorph and 80S ribosomes, consistent with the remnants of the red/green algae
engulfing red formed cryptomonads, brown algae, dinoflagellates
engulfing green formed apicomplexa, chlorarachniophytes, and euglenoids
Did the red and green algae arise from separate endosymbiotic events?
Molecular evidence points to a SINGLE endosymbiotic event
The chloroplast
- has a double membrane
- can self-replicate
- has its own circular DNA lacking introns and histone proteins
- has 70S ribosomes and synthesises some of its own proteins
RED algae have chlorophyll a & c and phycobiliproteins and a spiral thylakoid membrane
GREEN algae have chlorophyll a & b and stacked thylakoids
Chromist algae
4 membranes but no nucleomorph
- formed from engulfing of red algae in 2nd endosymbiosis event
Apicomplexan parasites
green algal plastid with 4 membranes
eg. T. gondii
algae that have chloroplasts with 3 membranes
dinoflagellate (red)
- complex skeleton made from cellulosic plates
euglenoid (green)
- no cell wall but a flexible, protinaceous pellicle
loss of 4th membrane!
support for serial endosymbiotic theory
EM ultrastructure
- look at no. membranes down microscope
- presence of nucleomorph
pigment analysis
- use stains
biochemical data
- Different rRNA species can be detected by FISH
- gold label for pro. only present in innermost part of chloroplast
sequence data
- Sequence comparisons group the nucleomorph with the red algae and the main nucleus with acanthamoeba
- substantial number of genes have been transferred to the host nucleus, even in multiple symbiotic associations
What are the algae?
photosynthetic autotrophs
- chlorophyll a is the major pigment
- they lack morphological differentiation into roots, stems and leaves
- they lack a sterile covering around their reproductive cells
- they are not monophyletic, but include prokaryote and eukaryote members
Endosymbiosis of mitochondria and chloroplasts not unique events??
o Even more evidence in insects of endosymbiotic BACTERIA!!
o Now thought that bacteria aren’t that uncommon as endosymbionts!
o Maybe the nitroplast is worth some research effort??
