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Compare the origin and subsequent evolution of mitochondria and
chloroplasts
Energy conversion is the most essential process carried out by all cells, the more efficiently it can be
done, the higher the evolutionary advantage to any cell. Eukaryotes have two main energy converting
organelles, mitochondria which are complex organelles which host the site of aerobic respiration in
most eukaryotes. Also, chloroplasts, found in plant cells, where the light dependent reaction of
photosynthesis takes place. In the late nineteenth century, evidence was beginning to be collected that
appeared to show similarities between the ultrastructure of the organelles and bacteria and it was
suggested that chloroplasts and mitochondria were once free living bacteria that had formed a
symbiotic relationship with other cells. Both organelles show some of the same ultrastructure evidence
for this theory, such as their own circular DNA; 70s size ribosomes and an electron transport chain in
their membranes, features they share with modern day bacteria.
There are several theories describing the origin of the organelles and it is only through the study of
present day bacteria and the genomes of mitochondria and chloroplasts that we are able to deduce
which theory is most likely to have occurred. A theory unique to chloroplasts is that they were formed
by the partitioning of a specialized photosynthetic region of an existing eukaryote. Also partitioned was
the DNA which represented the genes required for photosynthesis (such as photosystem I an II) for
efficiency as each chloroplast could control it's own synthesis of proteins. However the sequencing of
the chloroplast genome provides stronger support for a bacterial precursor to the chloroplast.
A separate theory about mitochondria suggests that there was in existence an anaerobic eukaryote
which engulfed and digested prokaryotes and on one occasion engulfed an aerobic alpha
proteobacterium which remained intact and provided ATP to the eukaryote. This first aerobic eukaryote
would have had an evolutionary advantage over the anaerobic eukaryotes as it could produce ATP at a
much higher rate. This theory was supported by Amitochondriate eukaryotes (such as Giardia lambila)
which do not contain mitochondria and are possibly closely related to the anaerobic eukaryote.
However, it is now believed that Amitochondriates did originally contain mitochondria but lost them,
this was suggested when chaperone proteins normally involved in mitochondrial biogenesis were
discovered in a number of Amitochondriates. Also found were hydrogensomes and mitosomes which
are both thought to be remnants of lost mitochondria. A similar example of this with chloroplasts is
when chloroplast DNA is found in non-photosynthesising cells such as Plasmodium falciparum, the
causal agent of malaria which contains a 35kbp molecule of DNA with chloroplast origin, suggesting that
the bacterium was originally photosynthetic but has lost the rest of the genes.
The theory common to both organelles was championed by Lynn Margulis in 1970, Margulis published
her paper, ‘The Origin of Mitosing Eukaryotic Cells’. She described the process whereby one prokaryotic
organism engulfs another, leading to a mutually beneficial situation which allows both to survive and
evolve over around 2.7 billion years into what we describe today as eukaryotic cells. She compared the
process to the symbiotic relationship between termites and bacteria ,which Joseph Leidy observed in
the nineteenth century, where bacteria decompose the cellulose and lignin that the termites consume
into useful food products in the gut.
Endosymbiosis theory is now the widely accepted explanation for the origin of mitochondria. Clearly,
the ability to produce ATP at a rate up to 15 times higher than in anaerobic respiration would be
advantageous to a cell, however this cannot be the primary advantage engulfing a mitochondrion
precursor would have given the host. Once ATP was produced by the symbiont, it would have been used
up in metabolic processes, not excreted out into the host's cytoplasm. However, around 2.7 billion years
ago, when the first cells containing mitochondria appeared, the oxygen concentration in the atmosphere
significantly increased. As oxygen is highly toxic to most cells, it would have been advantageous for an
anaerobic prokaryote to take up an aerobic symbiont which would utilise the oxygen in the host's
cytoplasm and ensure oxygen concentrations inside the cell do not become toxic. In return the host cell
provided nutrients to the symbiont in order for it to continue respiring. Phosphate transporters called
anti-porters later developed in the membrane of the mitochondria which were synthesised by the host
cell and allowed the transportation of ATP out of the matrix and into the cell cytoplasm.
The sequencing of the individual genomes of both chloroplasts and mitochondria supports
endosymbiosis theory as each can be linked evolutionarily to different modern day bacteria.
Mitochondria have very similar genomes to the Rickettsiaceae prowazekii, interestingly an intracellular
parasite which causes epidemic louse-borne typhus. The close link here could suggest the precursor to
mitochondria was not engulfed but actually was a parasite of the first eukaryote.
Chloroplasts on the other hand, have been found to have many clusters of genes in the same order as
cyanobacterium, an oxygenic photosynthetic bacterium. The cyanobacterium would have given the host
a selective advantage over it's competitors as it would be able to fix carbon to form carbohydrates. The
ancient remains of these early autotrophs can be found today in the form of stromatolites, layers of
fossilised structures found at Lake Thetis in Western Australia. Both chloroplasts and cyanobacteria
contain chlorophyll a, suggesting that the chloroplast precursor may have been a cyanobacterium.
Unfortunately, chlorophyll b, which has a methyl group replacing an aldehyde group, is only present in
chloroplasts. Instead it was proposed in the 1970s that they are more closely related to a grou of
photosynthetic prokaryotes called Prochloropytes which contain both chlorophyll a and b. However
sequence based data shows that they aren't specifically related and so they can not represent the
chlorophyll precursors. The solution to the chlorophyll b issue therefore is either it evolved multiple
times independently in chloroplasts and Prochloropytes or evolved once, was deleted from the modern
day cyanobacterium's genome and passed around via lateral transfer. Both of these explanations would
explain why chloroplast's appear structurally and genetically most similar to cyanobacteria whilst
sharing the genetic sequence for chlorophyll b with the Prochloropytes.
Since the endocytosis of the precursors to mitochondria and chloroplasts occurred, the evolution of
their genomes has involved a dramatic reduction in the number of base pairs. This could be partly be
due to the fact that the organelles no longer need to produce certain proteins which the host cell
already produces. The main factor however, is that there has been a movement of genes from the
organelle genome to the cell nucleus. Prokaryotes have on average around 2mbp, whereas mammalian
mitochondria, for example, have just 16.6kbp. The same process has evolved in chloroplasts, which have
on average around 130kbp, assuming they have evolved from cyanobacteria their loss of base pairs is
even more significant. Synechocytstis sp. PCC6803 is a cyanobacterium with around 3700kbp. The genes
from the organelle can be inserted into the nuclear DNA and so the organelle proteins can be produced
by the nucleus and then re-imported back to the organelle using intra-cell signaling. A study by Huang et
al. in 2003 showed that this gene transfer between chloroplasts and the cell nucleus is surprisingly
often, with a minimum estimate of 1 in every 16,000 tobacco pollen grains having a chloroplast to
nucleus gene transfer.
One theory of how this could have occurred is the autophaging of the organelles did not fully break
down it's DNA, this DNA would then be in the cell's cytoplasm and it is possible it could have entered the
nucleus through the nuclear pores. If it was advantageous to have these genes in the nuclear DNA
instead, then this would be selected for.
There are several reasons that it could be advantageous, firstly it gives the host cell more control over
the symbiont's processes and so can increase their efficiency. Also, the presence of the nuclear
membrane means that the DNA will be more protected from damage. Respiration or photosynthesis
could produce reactive species which could damage the organelle DNA and so the DNA in the nucleus is
more stable. Another hypothesis is Muller's ratchet, this suggests that as the cell reproduces sexually,
recombination of DNA in the nucleus leads to higher variation, which leads to a higher probability of
advantageous alleles being produced. The DNA in the organelles does not undergo recombination as
they divide by binary fisson, therefore mitochondria and chloroplasts which transfer genes to the
nucleus will be selected for. Of course, there could be no advantage to the transfer of genes, it could
simply be caused by drift, purely because of the fact that genes can be transferred into the nucleus.
A reason for chloroplast DNA in particular to be transferred is that the chloroplast genome is becoming
very AT rich and so makes genes more rich in amino acids with adenine and thymine bases, this could
mean that amino acids without A or T residues are being deleted. This could significantly effect the
tertiary structure and function of proteins. As a result of this, it would be advantageous for sequences
lacking in A or T residues to become molecular refugees and transfer to the nucleus.
By studying the genome of chloroplasts, mitochondria and their modern day, bacterial relatives, it is
clear to see that the evolution of these two vital organelles has been very complex. It is believed that
the main reason this complex evolution could have happened separately but in such a similar way for
both organelles is simply because of the huge evolutionary advantage the organelles provide eukaryotes
with. However, if we look at the timescale of their evolution, it could be argued that the evolution of
chloroplasts actually caused the origin of the mitochondria within cells. It was around 2.7 bya that the
first cells with chloroplast precursors appeared: were hugely successful and so grew dramatically in
number in the Earth's oceans and lakes. This meant a much larger volume of oxygen was being produced
which lead to the Great Oxidation Event, the suggested cause for the endosymbiosis of anaerobic
prokaryotes and mitochondria precursors.