Lecture 10 Flashcards
Eukaryotes
- Protists, fungi, plants, animals
4 super groups of eukaryotes
- Unikonta: Including fungi and animals
- SAR: Stramenopiles, Alveolates, Rhizarians
- Archaeplastida: Including plants
- Excavata: protists
Protist
a polyphyletic group of eukaryotic organisms which are not plants, fungi or animals
* Used to be a Kingdom of Eukaryotes, but this classification is not used anymore
Observed in all four supergroups of eukaryotes
* Some are closer to plants, some are closer to fungi and animals
Many protists are chemoheterotrophs while others are photoautotrophs
Examples of protists
Various Diatoms, Stramenopile (SAR)
Volvox, Green algae, Archaeplastida
Stentor, ciliate, Alveolate (SAR)
Amoeba proteus, Unikonta
Stromatolites are the earliest
fossils of life, observed from about 3.5 billion years ago
* Ancient Greek, strôma-lithos, layered rocks
First cellular life on earth were
prokaryotes
Some prokaryotes build
thin, mineralized layers on top of another, which became stromatolites
Stromatolites were built mainly by cyanobacteria which performs oxidative photosynthesis
Earth at the beginning had minimum oxygen (O2) and therefore, the first cellular life on Earth were
anaerobes
Earth’s oxygen is produced mostly by
biological activity
via oxygenic photosynthesis
As cyanobacteria populated earth, it started to produce
oxygen
* Earliest oxygen production at 3.5 billion years ago
* Eventually, oxygen started to saturate water and the atmosphere
Huge rise in atmospheric O2 around
2.7 billion years ago after water was completely saturated with O 2
How did organisms react to the reactive and poisonous nature of O2
Huge impact on existing protists
* Many anaerobic organisms probably did not survive
* Some anaerobes found anaerobic niches to survive in, and their descendants still exist
* Many organisms adapted and took advantage of oxygen giving rise to aerobic respiration
Eukaryotes are thought to be descendants of an ancestral, possibly anaerobic
Archean
The ancestor engulfed an aerobic bacterium which begun co-existing inside the cytoplasm of the host
* Mutualistic relationship where the symbiont provides the host access to aerobic respiration, and the host provides nutrients, safety, etc
Two species eventually fused into one
Mitochondria is believed to be the descendant
of the aerobic prokaryote which got engulfed by the ancestral Archean
Mitochondria
- Organelle for aerobic respiration
- In eukaryotes, the TCA cycle occurs in the mitochondrial cytosol, and the ETC is located in the mitochondrial inner membrane
Evidence for the bacterial origin of mitochondria
Mitochondria is very similar to a Gram
negative bacteria
Gram negative bacteria and a mitochondria both have:
- Two membranes, inner and outer
- Circular genomes (bacterial chromosome vs. the mitochondrial DNA)
- Molecular machines inside the cytosol such as Ribosomes for protein translation
- Homologous proteins on the inner membrane
When using molecular phylogeny, the eukaryotic mitochondrial DNA is
placed within the Domain Bacteria
Molecular analysis suggest that the mitochondria is very close to (or within) the clade alpha-proteobacteria
* Exact relationship not yet determined
The bacterial origin of mitochondria is strongly supported by these morphological and genetic evidences
Primary endosymbiosis
ancestral, heterotrophic eukaryote engulfed a cyanobacteria as a symbiont
* Cyanobacteria gave the host oxygenic photosynthesis
Two species eventually fused, giving
rise to the ancestral algae as the cyanobacteria turned into chloroplast
- Occurred about 1 - 1.5 billion years ago
This ancestral algae gave rise to Archaeplastida, clade of land plants, Red algae and Green algae
Evidence for the cyanobacterial origin of chloroplasts
Primary endosymbiosis is supported by evidences similar to those supporting mitochondrion endosymbiosis
* Cyanobacteria and the chloroplast shares multiple morphological and genetic similarities, including:
* Photosynthetic pathways
* Systems for transcription and protein translation
The cyanobacterial origin of chloroplasts is well accepted, although there are debates of how exactly the process occurred
Secondary endosymbiosis spread photosynthesis to other eukaryotes
- Red algae and green algae has been engulfed by another eukaryote in multiple independent occasions, spreading oxygenic photosynthesis to other eukaryotic clades
Examples of organisms that resulted from secondary endosymbiosis
Red algae to some members of:
* Stramenopiles (Diatoms and Brown algae)
* Alveolates (Dinoflagellates)
* Cryptophytes
* Haptophytes
Green algae to some members of:
* Excavata (Euglenozoans)
* Rhizarians (Cercozoans)
Paramecium
non-photosynthetic, chemoheterotropic ciliates (Alveolates, SAR supergroup)
* There is no ‘paramecium algae’ known
to exist today
Some members, such as Paramecium
bursaria, are observed to host
symbiont Green algae
Traces of different endosymbiotic events can be observed in some algae such as the
cryptophytes: gained photosynthesis by engulfing Red algae
Nucleomorph:
Cryptophyte plastids have a ‘nucleus’ remaining from the original red-algae endosymbiont
* Nucleomorphs contain DNA surrounded by a nuclear membrane
Chlorarachniophyte algae (Rhizaria, SAR supergroup) also has a nucleomorph
cryptophytes have four separate genomes
- Nuclear DNA (DNA of the host protist which engulfed the red algae)
- Nucleomorph DNA (Remains of nuclear DNA of the red algae)
- Plastid DNA (Remains of cyanobacteria DNA, originally held by the red algae)
- Mitochondrial DNA (Remains of the alpha-proteobacteria)
Nuclear DNA
DNA of the host protist which engulfed the Red algae
Mitochondrial DNA
Remains of alpha-proteobacteria genome which got engulfed during Mitochondrion endosymbiosis
Plastid DNA
Remains of cyanobacterial genome which got engulfed during primary endosymbiosis
Nucleomorph DNA
Remains of red algae nuclear DNA which got engulfed during secondary endosymbiosis
Mitochondrion endosymbiosis
Archean engulfed an alphaproteobacteria
Primary endosymbiosis
Ancestral Archaeplastida engulfed a
cyanobacteria
Secondary endosymbiosis
Red algae engulfment gave rise to
photosynthetic Haptophytes, Cryptophytes,
Stramenopiles, Alveolates
* Green algae engulfment gave rise to
photosynthetic Rhizarians and Excavates
Eukaryotes have multiple genomes due to endosymbiosis
- Human
- Nuclear DNA (human original)
- Mitochondrial DNA (proteobacteria origin)
- Green algae and plants
- Nuclear DNA (human original)
- Mitochondrial DNA (proteobacteria origin)
- Plastid DNA (cyanobacteria origin)
- Cryptophytes
- Nuclear DNA (human original)
- Mitochondrial DNA (proteobacteria origin)
- Plastid DNA (cyanobacteria origin)
- Nucleomorph DNA (green algae origin)
Order of secondary endosymbiotic events transferring ‘Red algae’ to other eukaryotic clades are
arbitrary
