Title: Evolution of the eukaryotic cell
1Evolution of the eukaryotic cell
- Protozoa evolving models on the origin of
eukaryotes - Early branching eukaryotes primitive or
specialized? - Primer on Giardia Trichomonas biology
2protozoa
- Primary unicellular eukaryotes, often also called
protists - Many important human and veterinary pathogens
- It is important to understand that protozoa are
mostly a historic grouping and not a cohesive
biological group that contains closely related
organisms - A very diverse group with a vast variety of
morphological and biochemical adaptations to
almost any ecological niche
3From letter case of life to the tree of life
(Linneus to Haeckel)
- Taxonomy classifies organisms into meaningful
groups that help to conquer and understand the
massive diversity - The tree concept uses evolution as guiding
principle of taxonomy - No evolution no tree.
- Choosing the tree metaphor makes several
important assumptions - All life is related
- Life diversifies
- Life has a common origin
4the tree of life(Ernst Haeckel, 1874)
man
ungulates
- The tree of life (who is related and how did they
evolve) was initially based on morphological
characteristics - Complex organisms were viewed as derived and
highly evolved simple organisms as primitive - This scheme puts protozoa as a cohesive group to
the bottom of the tree
carnivores
whales
fish
reptiles
crustaceans
molluscs
worms
protozoa
5the tree of life(Ernst Haeckel, 1866)
- Monophyletic tree of organisms again by Haeckel
- Note that he divides life into three kingdoms
(plants, protists, and animals) - Note also that he hypothesizes a common root
(radix) for all organisms - Loss or gain of characters produces branching of
the tree - The advent of electron microscopy brought more
morphological characters even for the small
protists - However, reduction and simplifications (e.g. due
to parasitism) pose significant problems for
morphology based trees - Homology is not always discernable from analogy,
and characters are not always easily quantifiable
6Molecular phylogeny
- Uses the sequence of macromolecules (RNA, DNA
proteins) to measure similarity, and deduce
phylogenetic relation - The molecule has to present in all the organisms
you want to compare - Multiple sequences are aligned and relatedness is
inferred from the simple argument that two
molecules from two related organisms are likely
to be more similar than from two organisms that
branched a long time ago
30S ribosomal subunit, rRNA pink Schluenzen et
al. Cell 102 (5) 61523.
7Molecular phylogeny
8Molecular phylogeny
- Molecular phylogeny assumes that changes occur
over time and that these changes can be modeled
and used to infer a process (evolution) out of
the current pattern - A large number of statistical approaches has been
developed to model and weigh change, build trees
that depict the results, and evaluate the
significance of the tree topologies obtained - If you are interested in how this really works we
could ask Jessie Kissinger for a primer
9The three kingdoms of life(Mitch Sogins 16s RNA
tree)
10The archezoa hypothesis
- Several early branching protozoa appear to lack
classical mitochondria - These organisms were grouped as archezoa
- They were hypothesized to represent the
eukaryotic root predating the acquisition of
mitochondria and certain other advanced
eukaryotic organelles - How do you acquire an organelle?
11The Lynn Margulis model of the endosymbiotic
origin of mitochondria
- A free living alpha proteobacterium was engulfed
by a proto-eukaryote and subsequently
domesticated - This idea is now very well supported by numerous
phylogenetic and biochemical studies that show a
clear link between mitochondria and proteobacteria
12More good theories for eukaryotic origins than
good data
- Most models now assume that eukaryotes are a
merger of an archaebacterium and a eubacterium - Phylogenetic analyses of eukaryotes suggest that
informational proteins (DNA replication,
transcription, translation) are related to
archaea while many metabolic proteins appear
eubacterial - Who ate who and how and when is controversial
T. Martin Embley and William Martin Nature 440,
623-630
13Archezoa amoeba the most primitive eukaryotes?
- No mitochondrion and no typical mitochondrial
enzymes (Krebs cycle and oxidative
phosphorylation is missing) - A fermentative bacteria-like anaerobic
metabolism - It was assumed that archezoa and amoeba
represent the stage of early eukaryotes before
the endosymbiosis event that let to the
mitochondrion - An alternative hypothesis stated that these
organisms once had mitochondria and subsequently
lost them while adapting to parasitism and life
in anaerobic environments
14Is the absence of mitochondria a primary of
secondary trait?
- The genomes of most important protozoan parasites
are now fully sequenced - This provides the opportunity to hunt for
molecular fossils - No trace of a mitochondrial genome has been found
in Entamoeba, Giardia or Trichomonas - However, most proteins that do their job in the
mitochondrion are actually encoded in the nucleus
and are imported from the cytoplasm (gene
transfer from the endosymbiont to the host
represents an important element of control and
domestication) - So are there remnants of mitochondrial protein
genes in the nuclear genome?
15E. histolytica Cpn60 identifies the mitosome
- The E. histolytica genome encodes an ortholog of
the mitochondrial chaperon Cpn60 - Antibodies raised against this protein reveal
numerous small organelles - This has now been validated using a number of
additional proteins
Cpn60
DIC
Microbiology 150 (2004), 1245-1250
16E. histolytica mitosomes do not contain DNA
- DNA was detected by in situ nick translation in
E. histolytica (a, b) and in mammalian cells (c) - Note absence of labeling in amoeba
- DNA is equally absent in Giardia mitosomes and
trichomonas hydrogenosomes
Microbiology 150 (2004), 1245-1250
17Mitosomes are also detectable in Giardia (lscU
staining)
http//www.natur.cuni.cz/parazit/tachezy_web/mito
some.htm
18Mitochondrial proteins indentified in
amitochondriate organisms
T. Martin Embley and William Martin Nature 440,
623-630 (blue likely eubacterial, red
archaebacterial ancestry, green eukaryotic
inventions)
19Trichomonas hydrogenosomes
- 0.5-2 ?m double membrane organelle
- no genetic material
- Present in anaerobic/ aerotolerant organisms
- (Trichomonas, rumen-dwelling ciliates and several
other apparently unrelated species)
20THE HYDROGENOSOME
Tissue and Cell, 1996 28287
21Hydrogenosomes use protons as terminal electron
acceptors
Pyruvate from the cytosol is oxidizes do acetyl
coA by the Pyruvate Ferredoxin Oxidoreductase
(PFO (1)) in the Hydrogenosome. The enzyme
Hydrogenase (4) uses the electrons from
ferredoxin and transfers them to H to form
hydrogen gas. Acetyl CoA can be further
metabolized by the acetatesuccinate CoA
transferase (2) to form acetate and succiniyl-CoA
(2) which could be hydrolyzed into CoA and
succinate and the energy released used to make
ATP by the succinate thiokinase (3).
Int. J. Parasitol. 1999, 29 199
22PFO activates the prodrug metronidazole
In the presence of metronidazole, electrons
generated by PFO are transported by ferredoxin
2Fe2S to the drug (bold arrow) and not to
their natural acceptor hydrogenase (HY).
Metronidazole is reduced with one electron
forming a nitro anion free radical. The cytotoxic
radicals (RNO2-) are formed as intermediate
products of the drug reduction. PFO is not
limited to hydrogenosomes but also found in
mitosomes and in a variety of anaerobic
bacteria Metronidazole (Flagyl) is the standard
treatment for Trichomonas, Giardia and invasive
amebiasis
Int. J. Parasitol. 1999, 29 199
23Is (was) the hydrogenosome a mitochondrion or not?
- Hydrogenosomes share features with mitochondria
- They have a similar import machinery, they have
two membranes and harbor certain mitochondrial
proteins (e.g. the mitochondrial iron sulfur
cluster assembly machinery - There are some atypical features like lack of
DNA, PFOR, and hydrogenase, which has led some
authors to suggest an indpendent origin
24Is (was) the hydrogenosome a mitochondrion or not?
- Overall, the mitochondrial origin hypothesis
seems to gain more and more support - It is the most parsimonious, explaining emergence
of hs in different unrelated taxa - Also recent identification of a hydrogenosome
NADH dehydrogenase which shares a common ancestry
with mitochondrial enzymes
Nature 432, 618-622 (NADH reductase, green
hydrogenosome marker, red)
25The archezoa hypothesis is dead
- Lack of mitochondria in archezoa is secondary
not primary - Recent phylogenies based on multiple concatenated
proteins fail to clearly pin the root to one
primitive eukaryote and rather suggest an
explosion of several groups from a common yet
unknown ancestor
26The Baldauf explosion of parallel crown groups
27A similar effort by Simpson showing that after
all we might be early branching
28Boris simplified summary of it all
- Note that this is only a schematic tree
- Eubacteria, archea eukaryotes remain three
clearly distinguished groups - Eukaryotes have archeal eubacterial features
- Mitochondria evolved by endosymbiosis, we dont
know of any true amitochondriate eukaryotes
there might never have been one - The root of the eukaryotic tree remains in the
dark - There appears to have been a relatively early
split between opisthokonts (animals, fungi
ameba) and plants and the rest of protozoal
eukaryotic life on the other branch - Protozoa are not little animals, they are very
diverse and highly divergent from us and each
other