Download 03/09/05, 1 Endosymbiosis Theory I. Introduction A. Evolution of

03/09/05, 1
Endosymbiosis Theory
I.
Introduction
A. Evolution of eukaryotic cell is result of multiple endosymbiotic events originally symbioses.
B. Provides broad relevance to this course
C. So symbiosis is really a driving force in evolution - and the evolution of
eukaryotic cell is one of the most important events that has allowed for modern
day diversity that we see
D. These symbioses evolved over time into organelles
E. Interesting story historically of how this area of science has developed
1. Lynn Margulis pioneered this work
a) In 1970’s
b) forceful personality - people didn’t accept her ideas - her assertions
based largely on morphology
c) possible that her ideas weren’t accepted in part because she is a woman
2. Now she is strongly backed up by evidence from modern molecular
techniques - we’ll go through that today in class
II. Evolution of eukaryotes
A. Progression of events: show slide
1. 3 bya: Evolution of photosynthetic bacteria - result in increase in O2 in
atmosphere by 2.5 bya
2. 2.5 – 1.5 bya first eukaryotic cells – formation of nucleus
3. 2.5 – 1.5 bya aerobic respiration becomes widespread
4. 1.5 bya: first endosymbiotic event – anaerobic eukaryote establishes a stable
symbiosis with bacteria with aerobic respiration (oxidative phosphorylation)
a) mitochondria are related to purple photosynthetic bacteria
5. 0.8 - 0.6 bya: origin of eukaryotic photosynthetic cells - by establishing a
stable symbiosis with photosynthetic bacteria.
a) chloroplasts arose from these photosynthetic bacteria
(1) Now well accepted that these endosymbioses occurred multiple
times
(2) some algal chloroplasts are surrounded by 3 or 4 membranes suggesting that they evolved from a symbiosis with a eukaryotic
alga: we’ll get to this later
6. 0.5 bya: These metabolic innovations due to symbionts allowed for
evolution of multicellularity and explosion of diversity that we see today.
B. This progression of events is linked to present day extant taxa show slides
1. Prokaryotes persist today as the eubacteria and the archaebacteria
2. Aerobic eukaryotes: animals and fungi
3. Aerobic photosynthetic orgs: likely that these symbioses occurred multiple
times
III. Model for occurrence of primary symbioses
A. Idea that primitive anaerobic eukaryote engulfed an aerobic bacterium.
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IV. Evidence linking organelles with prokaryotes.
A. Both prokaryotes and mitos/chloros have:
1. circular DNA molecule
2. similar ribosomes
3. sensitivity to antibiotics
4. no histones
5. same tRNAs: can use E coli t RNAs in protein synthesis in chloroplasts
6. transcription promoters and terminators identical
B. but most important – in age of genomics – high gene homologies with
prokaryotes
V. Cellular transition from symbiont to organelle
A. Symbiont is called an organelle if gene transfer from the symbiont to the host
nucleus has occurred
B. Genome sizes of bacteria vs organelles very different
1. bacteria 1-2 million
2. plastids: 200,000s
3. mitochondria: variability but 10,000s
4. so large difference in genome sizes between bacteria and symbionts
a) Where have all of the genes gone?
(1) Transferred to nucleus of host
C. Organellar molecular machinery and regulation is complex.
1. mitos and chloros have own molecular machinery; DNA, ribos, protein
2. BUT its regulation and synthesis is shared between nucleus and organelle
3. Evidence that over evolutionary time genes have been moved from
organelle to nucleus
D. How have scientists dissected apart this problem?
1. Biochemical approach: 80’s – 90’s
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a)
use cycloheximide to inhibit cytosolic protein synthesis doesn’t affect
organellar protein synthesis
b) use chloramphenicol to inhibit organellar protein synthesis not
cytosolic protein synthesis
c) What do these exp show?
(1) no proteins are exported from organelle to cytosol - only cytosol to
organelle.
(2) 80 - 90% of genes for mitos and chloros coded for in nucleus
Eukaryotic cell
cycloheximide
DNA
RNA
RNA
precursor protein
nucleus
mito/chloro
imported protein
DNA
RNA
organellar protein
chloramphenicol
2.
Molecular approach – age of genomics: 90’s – now. Despite their location,
many genes in nucleus
a) are homologous to bacterial genes
b) have no introns
c) Example 1: super oxide dismutase. In most eukaryotes there are two
different genes.
(1) 1 has a Cu/Zn cofactor
(2) Other has a Mn cofactor
(a) More closely resemble bacterial SOD than the Cu/Zn form
(b) This is thought to be the gene transferred in from the
mitochondria.
d) Example 2: Some proteins are coded for in part by nucleus and in part
by organelle - e.g. Rubisco in plants
(1) small subunits coded for in nucleus
(2) large subunits coded for in chloroplast
(3) Seems to be an evolutionary snapshot on the way to having the
entire protein coded for by the host.
e) Example 3: Evolutionary snapshot of gene transfer within a plant
lineage:
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(1) CoxII gene, important in electron transport
(2) 3 different patterns of occurrence in three closely related groups of
legumes
(a) Cowpea: present in mito only
(b) Soybean: 2 essentially identical copies – 1 in mito, 1 in
nucleus AND the mitochondrial form is inactive – it is not
transcribed!
(c) Mung bean: 1 copy in nucleus
VI. Secondary Symbioses
A. There is now very strong evidence that secondary symbioses have occurred
multiple times to give rise to the complexity of the unicellular algal groups
B. These groups have a variety of different chloroplast features that suggest that
they are at different evolutionary stages of the integration of organelles
1. It is now thought these are a result of two endosymbiotic events.
2. The first was a eukaryote, host I, in symbiosis with prokaryote
3. Second was a eukaryote, host II then engulfing host I.
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C. Cryptophytes and Chlorarchniophytes
1. Small freshwater and marine algae
2. Morphological evidence that they are a result of secondary symbioses:
a) 4 membranes surrounding the chloroplast
b) two cytosolic areas, one enclosed by former host I cell membrane, the
other enclosed by outer 2 chloroplast membranes
c) This 2nd cytosolic area has its own ribosomes - resemble eukaryotic
ribosomes, not prokaryotic
d) small degenerate nucleus, called nucleomorph, within these same 2
membranes, which includes a nucleolus and contains DNA, and has
double membrane with pores
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D. There are several other divisions of algae where the chloroplasts are even more
degenerate forms of the formerly eukaryotic selves
1. Dinoflagellates: chloroplast has 4 membranes
a) No evidence of nucleomorph or other symbiotic eukaryotic components
2. Diatoms and Euglenophytes: chloroplast has 3 membranes