Evolution of the eukaryotic cell - PowerPoint PPT Presentation

1 / 28
About This Presentation
Title:

Evolution of the eukaryotic cell

Description:

Evolution of the eukaryotic cell – PowerPoint PPT presentation

Number of Views:1093
Avg rating:3.0/5.0
Slides: 29
Provided by: borisst
Category:

less

Transcript and Presenter's Notes

Title: Evolution of the eukaryotic cell


1
Evolution of the eukaryotic cell
  • Protozoa evolving models on the origin of
    eukaryotes
  • Early branching eukaryotes primitive or
    specialized?
  • Primer on Giardia Trichomonas biology

2
protozoa
  • 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

3
From 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

4
the 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
5
the 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

6
Molecular 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.
7
Molecular phylogeny
8
Molecular 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

9
The three kingdoms of life(Mitch Sogins 16s RNA
tree)
10
The 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?

11
The 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

12
More 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
13
Archezoa 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

14
Is 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?

15
E. 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
16
E. 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
17
Mitosomes are also detectable in Giardia (lscU
staining)
http//www.natur.cuni.cz/parazit/tachezy_web/mito
some.htm
18
Mitochondrial proteins indentified in
amitochondriate organisms
T. Martin Embley and William Martin Nature 440,
623-630 (blue likely eubacterial, red
archaebacterial ancestry, green eukaryotic
inventions)
19
Trichomonas 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)

20
THE HYDROGENOSOME
Tissue and Cell, 1996 28287
21
Hydrogenosomes 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
22
PFO 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
23
Is (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

24
Is (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)
25
The 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

26
The Baldauf explosion of parallel crown groups
27
A similar effort by Simpson showing that after
all we might be early branching
28
Boris 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
Write a Comment
User Comments (0)
About PowerShow.com