Download Evolution

Supervisor: Dr. Justin Gerlach, Evolution and Behaviour
Set: Tuesday 11th November 2014
Compare the origin and subsequent evolution of mitochondria and chloroplasts.
Mitochondria are organelles that carry out oxygen-dependent ATP production,
while chloroplasts are organelles found within plant and algal cells, carrying out
oxygenic photosynthesis. A comparison of the similarities and differences in
their origins and subsequent evolution requires discussion of the primary
theories for how these events occurred, aided by illustration using the evidence
present to support these theories.
Mitochondria and chloroplasts are believed to have originated in the same way;
by the formation of an endosymbiotic relationship. Specifically, mitochondria are
theorised to have originated by symbiosis of an aerobic eubacterium into an
anerobic archaebacterium, while the chloroplasts are derived by endosymbiosis
of an oxygenic photosynthetic bacterium into a eukaryotic cell. According to the
endosymbiont hypothesis; in both cases the bacterium was engulfed by the other
cell or entered as a parasite, and an endosymbiotic relationship was developed.
The bacterial symbionts experienced similar advantages from this relationship,
as they were provided with substrates and protection from their host cells. The
benefits to the host cells of the presence of the symbionts was also similar in
each case; enabling them to carry out highly beneficial metabolic processes. The
chloroplasts provided the cells with the ability to photosynthesise, so they were
able to convert solar energy into chemical energy, and provided a source of fixed
carbon for the eukaryotic host. The mitochondrial endosymbionts enabled the
previously anaerobic archaea, generating ATP using glycolysis and fermentation,
to carry out aerobic respiration using their oxidative phosphorylation systems.
In both cases the ability to carry out these processes gave the cells a selective
advantage, so they proliferated.
Evidence to support the endosymbiotic theory in both cases comes from the
similarities between the organelles and bacteria, and provides evidence of the
specific organisms that the organelles were derived from. For example,
mitochondria and chloroplasts are reproduced via a process that is very similar
to binary fission, the method of reproduction of bacterial cells. Additionally, in
both mitochondria and chloroplasts genome sequence data supports this origin
theory. Most of eukaryotes’ nuclear DNA, particularly informational genes such
as those involved in translation, appear to be descended from archaebacterial
DNA, while mitochondrial DNA and genes involved in respiration appear to be
descended from eubacterial DNA. The bacterial and mitochondrial genomes
resemble one another closely. From sequence data mitochondria appear to be
derived from endosymbiosis of an alphaproteobacterium, very closely related to
members of the Rickettsiaceae group. A study published in Nature showed that
the function of many genes in Rickettsia prowazekii are similar to mitochondrial
genes, such as set of genes coding for protein components of the respiratorychain complex. The similarities between mitochondria and Rickettsia led to the
conclusion that ATP production is effectively the same in both. Further evidence
comes from a study which showed that from a series of 15 proteins encoded by
the mitochondria, which were unlikely to have undergone lateral gene transfer,
it could be inferred that Rickettsiales were a sister group of the mitochondria.
Similar evidence of the chloroplasts originating from bacterial symbionts comes
Supervisor: Dr. Justin Gerlach, Evolution and Behaviour
Set: Tuesday 11th November 2014
from the order in many clusters of chloroplast genes being the same as in
prokaryotes such as cyanobacteria. When phylogenetic trees were created using
gene sequencing data they suggested that chloroplasts evolved from the
ancestors of cyanobacteria, or from them directly. Therefore, despite there being
differences in the specific organisms that were taken in to become the
endosymbionts, the process through which mitochondria and chloroplasts
originated was very similar.
However, there may be a key difference in the origins of mitochondria and
chloroplasts in that the origin of mitochondria is believed to only have occurred
once, while there is some evidence to suggest that chloroplasts have formed
multiple times. Chloroplasts are found in green plants as well as red and brown
algae, in which they have some different photosynthetic pigments. This suggests
that there may have been multiple independent chloroplast origins, with
endosymbiosis involving symbionts with a different complement of
photopigments. Some further evidence to support this comes from the existence
of Paulinella chromataphora, which contains photosynthetic organelles called
chromatophores, that are also derived from cyanobacteria, but are distinct from
chloroplasts. This indicates that the origin of photosynthetic organelles could
occur multiple times, and may have occurred in the formation of chloroplasts. In
contrast, there is only believed to be a single origin of mitochondria. Genome
sequencing shows this. For example, clusters of genes for mitochondrial proteins
have the order of genes seen in equivalent clusters in bacteria, but with deletions
that are seen exclusively in mitochondria, that can be explained by occurring in a
common ancestral mitochondrion. This is a potential fundamental difference in
how chloroplasts and mitochondria originated. However, the hypothesis of
multiple chloroplast origins contends with the monophyletic hypothesis; that
there was a single endosymbiosis responsible for all types of chloroplasts, and
the other pigments developed in different lineages separately. This theory is
more supported by sequence data and more widely accepted. A better
characterised difference between their origins is that mitochondria are believed
to have originated before chloroplasts, since all cells containing chloroplasts also
contain mitochondria, but the reverse is not the case. This means that the
cyanobacteria were taken up into a eukaryotic cell.
In the subsequent evolution of both mitochondria and chloroplasts the loss or
degeneration of the organelles has been observed in some organisms, and this
has some significance in determining the origin of eukaryotic cells. In all
amitocondriate eukaryotic organisms, which superficially appear to have
diverged from eukaryotes before mitochondria were acquired, there is evidence
of their ancestors previously possessing mitochondria. For example, ATP
producing organelles called hydrogenosomes found in some anaerobic
organisms are thought to be mitochondrial remnants- evidence for this includes
that the hydrogenosomes have proteins and genes for proteins that are
mitochondrial. A specific example is that hydrogenosomes in organisms such as
Nyctotherus and Blastocystis have a genome that encodes subunits of NADH
hydrogenase, along with rRNAS and tRNAs all of mitochondrial origin. Every
eukaryote that has been examined has, or once had, mitochondria. This provides
evidence that all eukaryotes descend from an organism with mitochondria and
Supervisor: Dr. Justin Gerlach, Evolution and Behaviour
Set: Tuesday 11th November 2014
that the mitochondrial merger occurred at the start of the eukaryotic line.
Additionally, it suggests that the origin of mitochondria was from a symbiotic
relationship formed between eubacteria and archaebacteria, not eubacteria and
an amitochondriate eukaryote. Similar ancestral remnants of chloroplasts to the
hydrogenosomes have evolved, although their presence does not have such
significant implications. These include apicoplasts, descendants of chloroplasts
that do not photosynthesise, found in Apicomplexans, including Plasmodium
parasites. Some plants have also lost photosynthetic chloroplasts, including
those that have been found to retain part of the chloroplast DNA, such as
Epifagus virginiana, which retains many genes that code for gene expression
apparatus such as ribosomal proteins. In plants and other eukaryotes that
previously contained chloroplasts, even those that do not photosynthesise, they
retain some of the plasmid genome for use in essential cell function.
This introduces the second main way in which mitochondria and chloroplasts
have evolved since their origin; through loss and transfer to the nucleus of their
genes. The majority of the reduction in gene number from the original symbionts
is caused by gene loss, of genes that were redundant as their function could be
substituted for by genes already encoded in the nuclear DNA. In both
mitochondria and chloroplasts the genes remaining inside the organelles are
those coding for proteins involved in their function in serving the host cell;
carrying out respiration and photosynthesis respectively. The genes for other
functions have been lost or transferred to the nucleus, resulting in an inability
for the symbionts to survive independently of the host. The genes encoding most
mitochondrial and chloroplast proteins are present in the eukaryotic cell
nucleus. Therefore extensive transfer of genes from the organelles to the nucleus
must have occurred, resulting in expansion of the nuclear genome and reduction
of the mitochondrial and chloroplast genomes. The result of mitochondrial to
nuclear gene transfer is shown in the variability in the gene content of
mitochondria, with 67 protein-coding genes in R. americana mtDNA, in
comparison to only three in the mitochondrial genome of apicomplexans.
Similarly, for chloroplasts the original cyanobacterial symbiont would have
contained several thousand genes (according to genome sequencing of present
day cyanobacteria) while chloroplasts contain only 60-100 genes. The same
possible selective advantages resulted in the transfer of genes from both the
mitochondria and chloroplasts to the nucleus. One example is detailed in
Muller’s ratchet, which has the idea that genes in an asexual population
irreversibly accumulate mutations. The nucleus contains a sexual population of
genes, while the chloroplast and mitochondrial DNA contains an asexual
population of genes, therefore it is advantageous to transfer genes to the nucleus
since in sexually recombining gene populations mutations can be eliminated.
Another advantage is that photosynthesis and respiration both generate reactive
species damaging to DNA, so movement to the nucleus provides protection from
mutation. The evolution by reduction in genome sizes in mitochondria and
chloroplasts is a very similar element of their post-origin evolution.
A key difference in the subsequent evolution of mitochondria and chloroplasts is
that for chloroplasts secondary and tertiary endosymbiosis may have occurred,
but this has not happened for mitochondria. For example in Cryptophyte algae,
Supervisor: Dr. Justin Gerlach, Evolution and Behaviour
Set: Tuesday 11th November 2014
the chloroplasts are surrounded by 4 membranes, and DNA is present in the
inner lumen of the chloroplast as well as in a nucleomorph structure between
two of the four chloroplast membranes. This nucleomorph is derived from the
nucleus of an intermediate photosynthetic eukaryote, which had a chloroplast
formed by endosymbiosis, and was then taken up into a non-photosynthetic host
itself. Therefore 2 of the membranes are from the primary chloroplast, and 2
from the photosynthetic eukaryotic cell that was engulfed to produce the
secondary chloroplast. This is not a unique occurrence; the algae
Chlorarachniophytes are also thought to have undergone secondary
endosymbiosis, and dinoflagellates are thought to have plastids from tertiary
endosymbiosis; where they replaced secondary endosymbiotic chloroplasts with
alternative ones. In contrast no secondary or tertiary mitochondria have been
observed.
Overall the origin and subsequent evolution of mitochondria and chloroplasts
are very similar, aside from the possible multiple origins and multiple
endosymbioses of chloroplasts. However, the subsequent evolution of both
mitochondria and chloroplasts were each much less significant than their
origins. The implications of the origins of both mitochondria and chloroplasts are
very significant. The mitochondrial origin was the trigger for the origin of
eukaryotic organisms, which began an entire domain on the tree of life, which
evolved to contain all multicellular organisms. The chloroplasts’ origin triggered
the formation of photosynthetic eukaryotes, which resulted in photosynthesis
being carried out on a much larger scale. This caused an increase in the
concentration of oxygen in the atmosphere, which may have made the aerobic
respiration of the eukaryotes more selectively advantageous, and enabled them
to become more established as a domain. Therefore, the origins of mitochondria
and chloroplasts together resulted in considerable changes in the composition of
organisms on Earth.
Bibliography
Prof. Christopsher Howe Lectures ‘Early Events in Cellular Evolution’
Mark Ridley (2004) ‘Evolution’ 3rd Edition Blackwells: Oxford
Alberts, B. et al (2008) Molecular Biology of the Cell, 5th Edition (Garland)
David A. Fitzpatrick, Christopher J. Creevey, James O. McInerney (January 2006)
‘Genome Phylogenies Indicate a Meaningful α-Proteobacterial Phylogeny and
Support a Grouping of the Mitochondria with the Rickettsiales’ Mol Biol Evol
Davidov et al. (2006) ‘A new α-proteobacterial clade of Bdellovibrio-like
predators: implications for the mitochondrial endosymbiotic theory’
Environmental Microbiology
Lane, Nick. (2005). ‘Power, sex, suicide : mitochondria and the meaning of
life.’ Oxford [England] ; New York :Oxford University Press
Nakayama, Takuro; Archibald, John M (2012).’Evolving a photosynthetic
organelle’. BMC Biology
Siv G. E. Andersson et al. (1998) ‘The genome sequence of Rickettsia
prowazekii and the origin of mitochondria’ Nature
Grey et al. (2001) ‘The origin and early evolution of mitochondria’ Genome Biol.
Supervisor: Dr. Justin Gerlach, Evolution and Behaviour
Set: Tuesday 11th November 2014