Download Endosymbiosis And The Origin Of Photosynthetic Eukaryotes

Endosymbiosis and the origin
of photosynthetic eukaryotes
BY2204 Evolution
Trevor Hodkinson
Plant Sciences Moderatorship
Recommended reading:
Cambell and Reece “Biology” Origins of life; Endosymbiosis;
Protists.
New Scientist 17th Oct 2009 “Cradle of Life”
http://www.nick-lane.net/OriginOfLife.pdf - the detail of the alkaline
hydrothermal vents theory
New Scientist 6 Feb 2010 p36-39 Life s a Gas – current views
on how and when the levels of oxygen rose & allowed complex life.
Palmer, Soltis, Chase (2004) The plant tree of life: an overview and
some points of view. American Journal of Botany 91:1437-1445
Hodkinson & Parnell (2007) Reconstructing the tree of life,
Chapters 1, 19, 20.
1
The History of Life
4600mya
Earth
formed
4000mya
Life
4000
5000 million
years ago
2700mya
Photosynthesis
starts, producing
oxygen
3500mya
Oldest
prokaryote (?)
Read more about these
in the “Proof of Life”
and “Life’s a gas”
articles
3000
3800mya
Oldest
rock
Refer to 1st yr
notes for
differences
between
prokaryote and
eukaryote cells
2100mya
Eukaryotes
543mya
Cambrian
Explosion
1500mya
Multicellularity
2000
1000
Now
Snowball Earths
possibly until 650mya
3/4 of life’s history was single celled!
Prokaryotes dominated most of earth’s evolutionary
history
All the ways of extracting energy from the world were invented by
the prokaryotes
Heterotrophy (= eating other organisms)
Photo-autotrophy (=energy
from the sun)
Lithotrophy (=energy from
chemical reactions e.g. H2S)
Mixotrophy (= energy from any
combination of the above)
2
E.g. Cyanobacteria
Cyanobacteria evolved photosynthesis, i.e.
splitting water to make oxygen.
One group survives today as stromatolites
and thrombolites off Western Australia.
Fossilized ones exist from 2700mya.
Oxygen is very toxic to most other bacteria, so they poisoned almost
everything else. It also made an ozone atmosphere.
The ozone in the atmosphere
protected the land from UV
light and so made it habitable
They helped raise the oxygen levels to today’s levels (eventually –
see “Life’s a Gas” article), allowing larger animals onto land.
Zimmer 2001
3
Eukaryotes
Contain membrane bound structures
•  Nucleus
•  Organelles (mitochondria and plastids)
Characterised by endosymbiotic history
Endosymbiosis
•  Symbiotic incorporation of one organism by
another
Eg. Unicellular alga
(originaed from a eukaryotic cell engulfing a
photosynthetic cyanobacterium)
4
Protoeukaryotic cell
•  First endosymbiotic event –uptake of an
alpha-proteobacterium in to a host cell of
archaea
(not photosynthetic -they were heterotrophic
and gave rise to all subsequent eukaryotes)
Aerobic proteobacterium became
mitochondria (loss of genes to nucleus)
Mitochondria
•  Nearly all eukaryotes have them (for
respiration)
•  Some cells have a single large one, others
hundreds or thousands depending on
metabolic activity
•  Double membrane
(each a phospholipid
bilayer)
5
First endosymbiotic
event
Plants have three
genomes:
• nucleus
• mitochondria
• plastids (chloroplast)
Animals and fungi lack
plastids
6
PLASTIDS
plastids are structures within
cells called organelles
usually with lipid membranes
Chloroplasts photosynthesis
Chromoplasts pigment
synthesis and storage
Leucoplasts colourless and
give rise to:
Amyloplasts -starch storage･
Statoliths -for detecting gravity
Elaioplasts -for storing fat
Proteinoplasts -for storing and
modifying protein
Lots of different types
but all have identical
genomes
PLASTIDS
•  Have their own genetic system - non Mendelian
•  Have lost many genes to the nucleus but genes still
work and operate in the plastid (transit peptides help
transport of translated products from the nucleus to
plastid)
•  Plastids proliferate by division of pre-existing plastids
7
PLASTID DNA
•  c.113 genes
•  But represents
one non-recombining
genealogical unit
rbcL
•  Varies in size from
120 kb to 217 kb
Second endosymbiotic
event
cyanobacteria
8
First and second endosymbiotic
Plantae
events
(primoplantae)
Unikonts, rhizaria,
excavates,
chromalveolates
Evidence for endosymbiosis
•  DNA sequences of the organelle genomes
are homologous to bacteria
•  Replicate via splitting similar to certain
prokaryotes
•  Inner membranes of both mitochondria and
chloroplasts have enzymes and transport
systems homologous to those in plasma
membrane of prokaryotes
9
Endosymbiosis
Plants
After Lynn Margulis (1981)
Symbiosis in cell evolution
1.5 bybp
cyanobacteria
2.7 bybp
3.5 bybp
What is a plant?
Oxford dictionary definition:
A living organism of the kind exemplified by trees,
shrubs, herbs, grasses, ferns and mosses,
typically growing in a permanent site, absorbing
water and inorganic substances through its roots
and synthesizing nutrients in its leaves by
photosynthesis using the green pigment
chlorophyll
Outdated definition?
10
Dinoflagellate
Chlorachniophyte
Diatom
Gymnosperm
Apicomplexon
Cryptomonad
Haptophyte
Glaucophyte Euglinid Red alga
Green alga
11
Plants defined in a broad sense as:
Organisms containing plastids
How many times have plastids evolved?
-Single origin looks likely although this is
controversial (see Palmer et al. 2004)
Single cyanobacterial primary endosymbiosis
event established the three major Primoplantae
lineages (green algae, red algae, glaucophytes)
(not three independent events)
12
Eukaryotic tree
single origin of
plastids (thick green
arrow) with
subsequent
secondary
endosymbiotic events
(narrow green/red
arrows)
Green shading=green algal
symbionts
Red shading=red algal
symbionts
Endosymbiosis dominates the evolutionary history
of plants
A -Primary symbionts
(Primoplantae)
Loss of plastid genes to nucleus.
Primoplantae (=Plantae)
13
Primary plastid
endosymbiosis
B- Secondary symbiosis
1) Green algal symbionts*
*
*
14
primary
endosymbiosis
secondary endosymbiosis of
green algae
Chromalveolates
2-Red algae symbionts to create new types of plants
(photosynthetic eukaryotes)
Plants shaded by grey lack plastids (so did they once
contained them and then lose them?)
15
Hypotheses for the origins of red
algal plastids
a) Single origin of red algal plastids
and loss in some lineages (see
Keeling 2009. J. Eukaryotic Micro.)
b) Several independent origins of red
algal plastids via multiple
endosymbiotic events
primary
endosymbiosis
secondary endosymbiosis
of red algae
16
Chromalveolates
Primoplantae
Excavates
Rhizaria
Taxonomic implications:
Algae are not a natural group
(they are in 4 supergroups/divisions)
Supergroup
Excavates
Rhizaria
Primoplantae
Chromalveolates
Algal group
Est. species number
Euglenoids
800
Chlorarachniophytes
12
Charophytes
(stoneworts)
20,000
Chlorophytes (greens)
120,000
Rhodophytes
(reds)
20,000
Glycophytes
13
Phaeophytes (browns)
2,000
Chrysophytes (golden)
2,400
Bacillariophytes (diatoms)
200,000
Haptophytes (coccoliths)
2,000
Cryptophytes (crytomonads)
11,000
Source: Brodie and Zuccarello, in Hodkinson and Parnell (2007) (Chapter 20)
17
Green algae/green plants
•  Monophyletic and include embryophytes (land
plants, dependent embryo)
Green algae/plants
18
Conclusions
•  Mitochondria and chloroplasts have endosymbiotic origin
•  Heterotrophic eukaryotes lack chloroplasts eg.animals
•  Plants can be broadly defined as organisms with
chloroplasts and are found in all eukaryotic supergroups
(esp. Primoplantae and Chromalveolates) except unikonts
(animals, fungi, slime moulds)
•  Lineages of plants have resulted from primary (e.g.
Primoplantae) and secondary endosymbiosis of plastids
(Chromalveolates, Excavates, Rhizaria)
•  The green algal group includes embryophytes (seed plants)
Tree of life
Charles Darwin 1837
Ernst Haekel 1866
www.tolweb.org/tree/
Coordinated by
David Maddison
19
Land plant
(embryophyte)
evolution
c.500mya
Archaea now only survive in extreme environments where
nothing else competes with them
Bacteria are still common, and use a wide variety of ways of
making energy, scattered across their phylogenetic tree.
Only one made
oxygen cyanobacteria
20