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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