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ScienceWeek
EVOLUTIONARY BIOLOGY: ON ORGANELLE EVOLUTION
The following points are made by M. Lynch et al (Science 2006 311:1727):
1) The evolution of eukaryotes, and subsequently of multicellularity, was accompanied by dramatic changes in the nuclear genome, including expansions in sizes and numbers of introns, proliferation of mobile elements, and increases in lengths of intergenic regions. The continuity in scaling of these architectural features with genome size across major phylogenetic groups suggests that cellular and developmental features are not the primary driving forces in genome evolution, and the hypothesis has been raised that expansions in genome complexity are largely driven by two non-adaptive processes, random genetic drift and mutation [1,2]. If this hypothesis is correct, it ought to apply to all genomic regions.
2) However, in contrast to the shared patterns of evolution in the nuclear genomes of animals and plants, the organelle genomes of these lineages have evolved in radically different directions. Animal mitochondrial genomes are highly streamlined, whereas plant mitochondrial genomes contain large amounts of noncoding DNA. Is the theory less general than supposed, or do unique features of various organelle lineages encourage different evolutionary trajectories? The authors argue that when differences in mutation rates are accounted for, patterns of variation in organelle genome architecture support the theory that multiple aspects of genomic complexity owe their origins to non-adaptive processes.
3) Over the range of eukaryotic diversity, the scaling of mitochondrial genome content with genome size is quite similar to that in nuclear genomes [1,2]. The largest genome-size expansions are only weakly associated with gene number and primarily reflect increases in intronic and intergenic DNA. However, in contrast to the situation with nuclear genomes, animals and plants occupy positions at the opposite ends of this gradient. The diminutive mitochondrial genomes of animals generally fall in the range of 14 to 20 kb, whereas plant mitochondrial genome sizes range from ~180 to 600 kb. Most unicellular species have intermediate aspects of mitochondrial genomic architecture and contain many genes absent from animal mitochondria [4]. Thus, mitochondrial genomic architecture does not show overlap between animals and plants; this incongruity appears to be a consequence of contrasting evolutionary pressures unique to each lineage, with a strong ancestral component.
4) In summary: The nuclear genomes of multicellular animals and plants contain large amounts of noncoding DNA, the disadvantages of which can be too weak to be effectively countered by selection in lineages with reduced effective population sizes. In contrast, the organelle genomes of these two lineages evolved to opposite ends of the spectrum of genomic complexity, despite similar effective population sizes. This pattern and other puzzling aspects of organelle evolution appear to be consequences of differences in organelle mutation rates. These observations provide support for the hypothesis that the fundamental features of genome evolution are largely defined by the relative power of two non-adaptive forces: random genetic drift and mutation pressure.[3,5]
References (abridged):
1. M. Lynch, Mol. Biol. Evol. 23, 450 (2006)
2. M. Lynch, J. S. Conery, Science 302, 1401 (2003)
3. D. R. Denver, K. Morris, M. Lynch, L. L. Vassilieva, W. K. Thomas, Science 289, 2342 (2000)
4. M. W. Gray et al., Nucleic Acids Res. 26, 865 (1998).[Abstract/Free Full Text]
5. W.-H. Li, Molecular Evolution (Sinauer Associates, Sunderland, MA, 1997).
Science http://www.sciencemag.org
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CELL BIOLOGY: ON THE DIVISION OF ENDOSYMBIOTIC ORGANELLES
The following points are made by K.W. Osteryoung and J. Nunnari (Science 2003 302:1698):
1) Chloroplasts and mitochondria power eukaryotic cells (1). Chloroplasts fix carbon from CO2 into organic molecules that constitute the base of the global food chain. Mitochondria convert the energy stored in these compounds into adenosine triphosphate, the form of cellular energy used to power most of the processes required for growth and development. These organelles (2) also have many other metabolic functions essential to eukaryotic organisms (3). Mitochondria also play a key role in programmed cell death, a process essential to the development of multicellular organisms (4).
2) Mitochondria and chloroplasts are the descendants of serial endosymbiotic events (5). Mitochondria arose first from an alpha-proteobacterial ancestor that was acquired by either an archaeal or primitive eukaryotic host, and the transition from autonomous bacterium to host (nuclear)-controlled organelle was pivotal in the evolution of eukaryotic cells (5). Chloroplasts later arose from a cyanobacterial ancestor acquired by a eukaryote in which mitochondria were already established. Most of the bacterial genes were transferred to the nuclear genome or lost as the endosymbionts were subjugated by the host cell, but both organelles in present-day eukaryotes retain genes, metabolic activities, genetic mechanisms, and protein import complexes that clearly reflect their prokaryotic origins.
3) Like their free-living ancestors, both chloroplasts and mitochondria divide. Organelle division, segregation, and growth are often uncoupled from the cell division cycle, indicating that organelle and cell division are independent processes. Division of mitochondria and chloroplasts is orchestrated by multicomponent protein machines that assemble and drive the constriction and fission of the organellar membranes. Because both organelles are surrounded by inner and outer membranes that differ in composition, their division machines must accomplish the synchronized constriction of both membranes, the subsequent fusion of the four lipid bilayers, the final separation of the two daughter organelles, and possibly the resolution of the fused membranes back into two discrete bilayers.
4) In summary: Mitochondria and chloroplasts are essential eukaryotic organelles of endosymbiotic origin. Dynamic cellular machineries divide these organelles. The mechanisms by which mitochondria and chloroplasts divide were thought to be fundamentally different because chloroplasts use proteins derived from the ancestral prokaryotic cell division machinery, whereas mitochondria have largely evolved a division apparatus that lacks bacterial cell division components. Recent findings indicate, however, that both types of organelles universally require dynamin-related guanosine triphosphatases to divide. This mechanistic link provides fundamental insights into the molecular events driving the division, and possibly the evolution, of organelles in eukaryotes.
References (abridged):
1. J. F. Allen, Philos. Trans. R. Soc. London Ser. B 358, 19 (2003)
2. All subsequent references to organelles refer specifically to mitochondria and chloroplasts.
3. B. B. Buchanan, W. Gruissem, R. L. Jones, Biochemistry & Molecular Biology of Plants (American Society of Plant Physiologists, Rockville, MD, 2002)
4. D. D. Newmeyer, S. Ferguson-Miller, Cell 112, 481 (2003)
5. M. W. Gray, G. Burger, B. F. Lang, Science 283, 1476 (1999)
Science http://www.sciencemag.org
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CHLOROPLASTS AND GENES
The following points are made by Paul Jarvis (Current Biology 2003 13:R314):
1) Chloroplasts, like mitochondria, evolved from a free-living prokaryotic organism that entered the eukaryotic lineage through endosymbiosis. During the course of their evolution, chloroplasts relinquished most of their genes to the nucleus, and so became subservient to the eukaryotic host. Today, more than 90% of the 3000 or so proteins present in chloroplasts are encoded in the nucleus, translated in the cytosol and imported into the organelle post-translationally. The remainder are encoded and synthesized within the organelle itself by an endogenous genetic system.
2) One of the consequences of this partitioning of genetic information is that processes which take place inside chloroplasts necessarily require input from two different compartments. For example, the photosynthetic complexes of the thylakoid membranes comprise core subunits encoded by the chloroplast genome, and peripheral subunits encoded by the nuclear genome. To ensure that these complexes are assembled in stoichiometric fashion, and to enable their rapid reorganization in response to changing environmental cues, the activities of the nuclear and chloroplast genomes must be closely coordinated through intracellular signalling.
Current Biology http://www.current-biology.com
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Notes by ScienceWeek:
The term "thylakoid" refers to a sac-like vesicle containing the photosynthetic pigments in photosynthetic organisms. In prokaryotes, the thylakoids are of various shapes and are attached to the plasma membrane; in eukaryotes, the thylakoids are flattened and located in chloroplasts; in the chloroplasts of higher plants, the thylakoids form dense stacks called "grana". Isolated thylakoids preparations can carry out photosynthetic electron transport and associated phosphorylation.
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