Download Compare the origin and subsequent evolution of mitochondria and

Jessica Griffiths
Compare the origin and subsequent evolution of mitochondria and chloroplasts.
Mitochondria and chloroplasts, the ‘powerhouses’ of eukaryotic cells, are absolutely fundamental to
understanding the sequence of events underpinning eukaryotic evolution. The endosymbiosis theory
proposes that chloroplasts and mitochondria found in present-day eukaryotic cells evolved from
bacteria endocytosed by ancient eukaryotes. Molecular evidence suggests that mitochondria evolved
from endocytosed proteobacteria, whilst chloroplasts originate from an oxygen-producing
photosynthetic organism resembling a cyanobacterium.
The symbiotic event giving rise to mitochondria is thought to have occurred over one and a half
billion years ago, following a substantial increase in atmospheric oxygen levels. The chloroplast
appears to have been similarly derived around one billion years ago, after mitochondria, since all
eukaryotic cells contain mitochondria, but not chloroplasts. This is serial endosymbiosis: an ancient
eukaryote engulfed the mitochondrion ancestor, and some descendants of it then engulfed the
chloroplast ancestor, giving rise to a cell containing both chloroplasts and mitochondria.
The mitochondria and chloroplasts were endocytosed by the ancient eukaryote either as food, or an
internal parasite. They managed to escape the phagocytic vacuole within which they were contained.
The double lipid-bilayer membranes that surround all chloroplasts correspond to the gram-negative
cell walls of the ancestral cyanobacterium, not the host’s phagosomal membrane, which was likely
lost throughout evolution. Ancient eukaryotes were able to subvert the oxidative phosphorylation
systems of the endocytosed bacteria in the endosymbiosis giving rise to mitochondria. Similarly, the
endosymbiotic relationship giving rise to chloroplasts allowed the ancient eukaryotes to harness the
photophosphorylation mechanism employed by the ancestral cyanobacteria. The resulting increase
in ATP production in both cases made the symbiosis energetically favourable for the host eukaryotes.
In return, the endocytosed bacteria were provided with protection, nutrients and a stable
environment. These relationships were therefore mutually-beneficial, and so were selected for in the
course of evolution.
Studies have revealed that mitochondria and chloroplasts contain their own genetic systems, which
are separate and distinct from the eukaryotic nuclear genome. Mitochondria and chloroplasts divide
by binary fission. Before each division, their DNA is replicated, and the genes encoded are
transcribed within the organelle, and translated on the organelle ribosomes. Mitochondrial and
chloroplast DNA is circular and similar in size and structure to bacterial plasmids. Their 70S
ribosomes and ribosomal RNA also more closely resemble those in bacteria than those in eukaryotes.
These similarities in the biochemistry and molecular biology of mitochondria and plastids in modernday eukaryotes and ancestral bacteria provide further evidence for the endosymbiosis theory.
There are, however, many significant differences that exist between the mitochondria and
chloroplasts, and the ancestral bacteria from which they were obtained. These differences can
provide insight into the evolution of modern eukaryotes. Sequencing of the mitochondrial genome
has revealed that the coding capacity of the modern day mitochondrial genome is much smaller than
their closest eubacterial relatives. Sequenced plastid genomes encode from 20 to 200 proteins, and
mitochondrial genomes encode anywhere from 3 to 67 proteins. The closest relatives of
mitochondrial and chloroplast genomes are alpha-proteobacteria and cyanobacteria respectively.
The modern alpha-proteobacteria Mesorhizobium loti contains 7 Mb of DNA encoding over 6,700
proteins, whilst the cyanobacterium Nostoc PCC 7,120 has a 6.4 Mb genome, which encodes
approximately 5,400 proteins. Comparison of the genome sizes of these bacterial ancestors and
mitochondria and chloroplasts illustrates the magnitude of organelle genome reduction. [2]
Jessica Griffiths
During eukaryote evolution, there therefore must have been an extensive transfer of genes from the
organelles to the nuclear genome. This is consistent with increased dependence on the eukaryotic
host following endosymbiosis. However, successful transfers of this type are rare, due to the fact
that the organelle gene needs to change to become a functional nuclear gene – it must adapt to the
nuclear and cytoplasmic and nuclear transcription and translation requirements, and must obtain a
signal sequence so that the encoded protein can be transported back to the chloroplast or
mitochondria following synthesis, with the help of transit peptides and protein-import machinery.
Nevertheless, it is possible; through experimentation, Guang et al. made a minimum estimate that
one pollen grain in 16,000 in tobacco undergoes a chloroplast-to-nucleus transfer. This demonstrates
that endosymbiotic gene transfer is possible, and that such gene transfers continue to occur today.
Endosymbiotic gene transfer is advantageous for a number of reasons. The nucleus is a sexual
population whereas the chloroplasts and mitochondria are not, because of uniparental inheritance.
In asexual reproduction, genomes are inherited as indivisible blocks, meaning that irreversible
deleterious mutations are inherited by future generations, and so eventually accumulate. This
genetic load eventually becomes so great that the population goes extinct. However, in sexual
populations, genetic recombination ensures that the genomes of the progeny differ from the
parental genomes, allowing progeny genomes with fewer mutations to be generated. Transfer of
genes from chloroplasts and mitochondria to the sexually-recombining gene pool of the nucleus is
therefore favourable. Another benefit of endosymbiotic gene transfer is that the nucleus reduces
the exposure of DNA to reactive oxygen species generated by oxidative phosphorylation in
mitochondria and photosynthesis in chloroplasts. This helps to protect the DNA from damage. DNA
transfer from the organelles to the nucleus also helps to ensure that the division of the cell and
organelles (which reproduce by binary fission) are synchronous.
Despite these advantages, some genes are still retained within mitochondria and chloroplasts. This
enables the control of the expression of genes encoding components of their electron transport
chain, allowing them to synthesise these components when needed. This helps to maintain redox
balance, thereby decreasing production of toxic reactive oxygen species. Local transcription and
translation of organelle genes allows for a quicker and more direct response. It also allows organelles
to produce personalised responses specific to them – proteins encoded by nuclear genes would be
delivered to all mitochondria or chloroplasts. Another plausible explanation for the retention of
certain genes in mitochondria and chloroplasts is the existence of a limited transfer window – the
possibility of gene transfer may have ended before everything could be transferred.
DNA sequencing also shows that deletion of certain genes within the genomes occurred. For
example, complex I genes in Saccharomyces cerevisciae have been lost, leading to the loss of the first
coupling site in the yeast’s electron-transport chain. [1] In some of these cases, the deleted gene’s
function may be carried out by unrelated nuclear genes instead. Despite these changes in the
mitochondrial and chloroplast genomes, in a large number of cases, the protein-coding genes are in
the same order as their bacterial homologues, providing evidence for a single, endosymbiotic origin.
Strong evidence for a single endosymbiotic origin of mitochondria is also provided by the presence of
hydrogenosomes and mitosomes in amitochondriate organisms. The presence of N-terminal
targeting peptides – conserved protein import machinery - in these organelles shows that they are in
fact mitochondrial remnants. All known lineages of eukaryotes containing mitosomes or
hydrogenosomes also branch as sisters to mitochondrion-bearing lineages. It is therefore widely
accepted that amitochondriate organisms lost their mitochondria during the course of evolution, as
opposed to forming a monophyletic group before the establishment of mitochondria.
Jessica Griffiths
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It has been debated and disputed whether chloroplasts also originated from a single endosymbiotic
event, or many independent engulfments in different eukaryotic lineages. It is most widely accepted
that they share a single ancestor resembling a cyanobacterium (with one exception of one species Paulinella chromatophora - which arose from an independent primary endosymbiosis separate from
land plants and green-algae). Despite this single eukaryotic origin, chloroplasts are found in an
extremely large variety of organisms – this is a consequence of many secondary and tertiary
endosymbiotic events. This can be seen in cryptophyte algae, which are surrounded by four
membranes – the two additional membranes corresponding to the plasma membrane of the
engulfed cell, and the phagosomal membrane of the host cell. The remnant nucleus – the
nucleomorph – from an intermediate photosynthetic eukaryote, is present between the two of the
plastid membranes. The topography of plastid membranes is an evolutionary signature of secondary
endosymbiosis.
The endosymbiotic relationships giving rise to chloroplasts and mitochondria are some of the most
important organisational events in the history of life. The origin and subsequent evolution of
mitochondria and chloroplasts demonstrates the importance of co-operation in evolutionary change.
They have far-reaching evolutionary implications, and are absolutely essential to the existence of all
higher life-forms on the planet Earth.
Bibliography:
1) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC138944/
2) http://www.nature.com/scitable/content/endosymbiotic-gene-transfer-organelle-genomesforge-eukaryotic-13997492 “Endosymbiotic Gene Transfer: Organelle Genomes Forge
Eukaryotic Chromosomes” by Jeremy N. Timmis, Michael A. Ayliffe, Chun Y. Huang and
William Martin