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Life is the harnessing of chemical energy in such a way that the energy-harnessing device makes a copy of itself. No energy, no evolution. The ‘modern synthesis’ of the past century explained evolution in terms of genes, but this is only part of the story. While the mechanisms of natural selection are correct, and increasingly well understood, they do little to explain the actual trajectories taken by life on Earth. From a cosmic perspective—what is the probability of life elsewhere in the Universe, and what are its probable traits?—a gene-based view of evolution says almost nothing. Irresistible geological and environmental changes affected eukaryotes and prokaryotes in very different ways, ones that do not relate to specific genes or niches. Questions such as the early emergence of life, the morphological and genomic constraints on prokaryotes, the singular origin of eukaryotes, and the unique and perplexing traits shared by all eukaryotes but not found in any prokaryote, are instead illuminated by bioenergetics. If nothing in biology makes sense except in the light of evolution, nothing in evolution makes sense except in the light of energetics. This Special Issue of Philosophical Transactions examines the interplay between energy transduction and genome function in the major transitions of evolution, with implications ranging from planetary habitability to human health. We hope that these papers will contribute to a new evolutionary synthesis of energetics and genetics.

Hypoxic injury is a major cause of tubular necrosis in the corticomedullary junction of isolated perfused kidneys, and is ameliorated by inhibitors of active reabsorption, such as frusemide. Our objective was to determine whether frusemide has a similar effect on hypothermically stored transplanted kidneys and whether this effect is modulated by impermeant solutes included in the preservation solution. The effect of frusemide on cytochrome oxidase (cyt aa3) oxidation, renal hemodynamics, and morphology was investigated in the New Zealand White rabbit renal autograft model using near-infrared spectroscopy and light microscopy. A total of 30 kidneys were autografted in six groups. Kidneys were transplanted with or without frusemide either (1) without storage (groups 1 and 2) or after 72 hr of storage in: (2) hypertonic citrate containing mannitol (groups 3 and 4); and (3) hypertonic citrate containing polyethylene glycol (PEG) (groups 5 and 6). In unstored transplanted kidneys, frusemide infusion stimulated a significant (P < 0.05) increase in hemoglobin oxygenation, compared with untreated controls. There was a tendency for cyt aa3 to become reduced, but there were no significant differences between groups 1 and 2. After 72 hr of storage, frusemide infusion stimulated a significant increase in hemoglobin oxygenation in kidneys stored in mannitol (P < 0.005), but there was no significant change in the kidneys stored in PEG. There was a corresponding reduction in cyt aa3 in kidneys stored in mannitol (P < 0.05) but no change in those stored in PEG. These results suggest that frusemide has a significant effect on cortical hemoglobin oxygenation in transplanted kidneys and on active reabsorption in the corticomedullary junction. The selection of impermeant is important and mannitol is significantly superior to PEG in this model.

The significance of poor medullary reperfusion in the etiology of acute tubular necrosis during renal transplantation is poorly understood. Our objective was to determine the kinetics of renal hemoglobin oxygenation using near-infrared spectroscopy during renal transplantation, to provide a framework against which the timing of mitochondrial dysfunction could be considered. New Zealand White rabbit kidneys were flushed with hypertonic citrate solution (0-2 degrees C and autografted immediately (group 1) or stored at 0-2 degrees C for 72 hours before autografting (group 2). Changes in oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) were monitored by near-infrared spectroscopy for 3 hours of reperfusion. Intrarenal perfusion was evaluated separately by barium sulfate angiography. Reperfusion resulted in rapid increases in HbO2 within 1 minute in both groups. Group 1 HbO2 fell sharply to a minimum at 3 minutes but recovered by 20 minutes; group 2 changes were similar, but there was no recovery (P

Harnessing energy as ion gradients across membranes is as universal as the genetic code. We leverage new insights into anaerobe metabolism to propose geochemical origins that account for the ubiquity of chemiosmotic coupling, and Na + /H + transporters in particular. Natural proton gradients acting across thin FeS walls within alkaline hydrothermal vents could drive carbon assimilation, leading to the emergence of protocells within vent pores. Protocell membranes that were initially leaky would eventually become less permeable, forcing cells dependent on natural H + gradients to pump Na + ions. Our hypothesis accounts for the Na + /H + promiscuity of bioenergetic proteins, as well as the deep divergence between bacteria and archaea.

Mitochondria harbour some of the most critical functions of life [[1],[2],[3]]. As the site of ATP synthesis by oxidative phosphorylation, they are the primary energy‐generating system in almost all eukaryotic cells and are central to programmed cell death. Mitochondria also play major roles in a broad range of other key processes such as the synthesis of amino acids, haem, nucleotides and lipids, ion homeostasis, cell proliferation and motility. It is of major evolutionary and functional significance that they carry their own small DNA genome—a legacy of the endosymbiosis that created mitochondria, and possibly the eukaryotic cell, some 1.5–2 billion years ago [[4]]. In humans, mitochondrial DNA (mtDNA) is a double‐stranded circular molecule of 16.5 kilobase pairs that carries only 37 genes, 13 of …

The cellular energy produced by mitochondria is a fundamental currency of life. However, the extent to which mitochondrial (mt) performance (power and endurance) is adapted to habitats and life strategies of vertebrates is not well understood. A global analysis of mt genomes revealed that hydrophobicity (HYD) of mt membrane proteins (MMPs) is much lower in terrestrial vertebrates than in fishes and shows a strong negative correlation with serine/threonine composition (STC). Here, we present evidence that this systematic feature of MMPs was crucial for the evolution of large terrestrial vertebrates with high aerobic capacity. An Arrhenius-type equation gave positive correlations between STC and maximum life span (MLS) in terrestrial vertebrates (with a few exceptions relating to the lifestyle of small animals with a high resting metabolic rate [RMR]) and negative correlations in secondary marine vertebrates, such as cetaceans and alligators (which returned from land to water, utilizing buoyancy with increased body size). In particular, marked STC increases in primates (especially hominoids) among placentals were associated with very high MLS values. We connected these STC increases in MMPs with greater stability of respiratory complexes by estimating the degradation of the Arrhenius plot given by accelerating mtRMR up to mt maximum metabolic rate. Both mtRMR and HYD in terrestrial vertebrates decreased with increasing body mass. Decreases in mtRMR raise MMP stability when high mobility is not required, whereas decreased HYD may weaken this stability under the hydrophobic environment of lipid bilayer. High maximal metabolic rates (5-10 RMR), which we postulate require high MMP mobility, presumably render MMPs more unstable. A marked rise in STC may therefore be essential to stabilize MMPs, perhaps as dynamic supercomplexes, via hydrogen bonds associated with serine/threonine motifs.

Planctomycetes, Verrucomicrobia and Chlamydia are prokaryotic phyla, sometimes grouped together as the PVC superphylum of eubacteria. Some PVC species possess interesting attributes, in particular, internal membranes that superficially resemble eukaryotic endomembranes. Some biologists now claim that PVC bacteria are nucleus-bearing prokaryotes and are considered evolutionary intermediates in the transition from prokaryote to eukaryote. PVC prokaryotes do not possess a nucleus and are not intermediates in the prokaryote-to-eukaryote transition. Here we summarise the evidence that shows why all of the PVC traits that are currently cited as evidence for aspiring eukaryoticity are either analogous (the result of convergent evolution), not homologous, to eukaryotic traits; or else they are the result of horizontal gene transfers.

Energy-releasing chemical reactions are at the core of the living process of all organisms. These bioenergetic reactions have myriad substrates and products, but their main by-product today is adenosine triphosphate (ATP), life's primary currency of metabolic energy. Bioenergetic reactions have been running in a sequence of uninterrupted continuity since the first prokaryotes arose on Earth more than 3.5 billion years ago, long before there was oxygen to breathe ( 1 ). Under what conditions did these bioenergetic processes first evolve?

Despite thermodynamic, bioenergetic and phylogenetic failings, the 81-year-old concept of primordial soup remains central to mainstream thinking on the origin of life. But soup is homogeneous in pH and redox potential, and so has no capacity for energy coupling by chemiosmosis. Thermodynamic constraints make chemiosmosis strictly necessary for carbon and energy metabolism in all free-living chemotrophs, and presumably the first free-living cells too. Proton gradients form naturally at alkaline hydrothermal vents and are viewed as central to the origin of life. Here we consider how the earliest cells might have harnessed a geochemically created proton-motive force and then learned to make their own, a transition that was necessary for their escape from the vents. Synthesis of ATP by chemiosmosis today involves generation of an ion gradient by means of vectorial electron transfer from a donor to an acceptor. We argue that the first donor was hydrogen and the first acceptor CO2.

Ebselen (PZ51) was tested for its ability to inhibit oxidative membrane damage and improve outcome of rabbit kidneys rendered cold ischaemic for 72 hr. In view of the rapid metabolism of ebselen, the antioxidant capacities of its two principal metabolites were first compared with that of the parent drug in an in vitro hepatic microsomal lipid peroxidation system initiated by NADPH/Fe3+-ADP. The potent antioxidant activity of ebeselen was confirmed but metabolite I (2-glucuronylselenobenzanilide) exhibited no antioxidant potential up to a concentration of 50 μM; metabolite II (4-hydroxy-2-methyl-selenobenzanilide) did inhibit lipid peroxidation but was about 80 times less effective than the parent compound. The storage of rabbit kidneys in hypertonic citrate solution at 0° for 72 hr of cold ischaemia resulted in greatly increased susceptibility to oxidative membrane damage in both the cortex and medulla as determined by the subsequent in vitro formation of two markers of lipid peroxidation (Schiffs bases and thiobarbituric acid-reactive material). Inclusion of ebselen (50 μM) in the flush and storage solution led to a highly significant reduction in these oxidative markers in both regions of the kidney. Intracellular and interstitial oedema was noted in organs subjected to 72 hr cold ischaemia and was reduced by ebselen (50 μM in the flush/storage solution). The rate of post-ischaemic lipid peroxidation was found to correlate well with the extent of oedema in the renal medulla (r = 0.84, P < 0.001) but no such correlation was found in the cortex. Administration of ebselen (5.5 mg/kg i.v. and 100 μM in the flush/storage solution) did not improve the long-term survival of rabbits following autotransplantation of a single kidney stored for 48 or 72 hr. No protective effect of ebselen could be demonstrated either in terms of graded physiological function or histological outcome.

© Springer-Verlag Berlin Heidelberg 2007. All rights reserved. All known eukaryotic cells either have, or once had, and later lost, mitochondria (which is to say, the common ancestor of mitochondria and hydrogenosomes; Gray et al. 1999, 2001; Embley et al. 2003; Tielens et al. 2002; Boxma et al. 2005; Gray 2005). If this statement is upheld (discussed elsewhere in this volume), then possession of mitochondria could have been a sine qua non of the eukaryotic condition. That cannot be said of any other organelle. The eukaryotic cell apparently evolved only once all modern eukaryotes are descended from a single common ancestor and that ancestor had mitochondria (Martin 2005; Lane 2005; Martin and Muller 1998). By definition, there are no eukaryotic cells without a nucleus, but it is a surprise that there are no eukaryotes that did not have mitochondria in their past. The implications have not yet been properly digested. Why was this? What advantage did the mitochondria offer? Whatever the advantage, it was not trivial. Bacteria and archaea ruled the Earth for three billion years (Knoll 2003). During this time, they evolved a dazzling wealth of biochemical variety, making the eukaryotes look impoverished (Martin and Russell 2003). Yet the prokaryotes failed to evolve greater morphological complexity: although some bacteria might best be thought of as multicellular organisms, their degree of organisation falls far short of eukaryotic attainments (Kroos 2005; Velicer and Yu 2003). In general, bacteria today seem to be no more complex than in the earliest known fossils (Knoll 2003; Maynard Smith and Szathmry 1995). Such lack of progress seems to be true of their biochemistry too: all the most important geochemical cycles were apparently in place by 2.7 billion years ago, implying that prokaryotes had already by then evolved oxygenic photosynthesis, sulphate reduction, fermentation, oxidative phosphorylation, methanogenesis, denitrification and nitrification (Martin et al. 2003; Lane 2002; Anbar and Knoll 2002; Nisbet and Sleep 2001; Canfield et al. 2000; Castresana and Moreira 1999; Canfield 1998). The traditional long list of differences between prokaryotic and eukaryotic cells has been gradually eroded as exceptions are found to each. There are prokaryotes with structures resembling a nucleus, straight chromosomes, cytoskeleton, giant size, internal membranes, multiple replicons, introns, mitotic-like apparatus, gene regulation, inter-cellular signalling, genetic recombination, and indeed with endosymbionts (Margolin 2005; Jones et al. 2001; van den Ent et al. 2001; Vellai and Vida 1998; Fonstein and Haselkorn 1995). There has been a revolution in our perception of the prokaryotic state (Gitai 2005). But if all these supposedly eukaryotic traits are found in prokaryotes, why do we not see a continuum in complexity between prokaryotes and eukaryotes? Some might argue that we do, indeed that such a spectrum of eukaryotic traits in prokaryotes is already good evidence of a continuum. Even so, there is still a void although there is some degree of overlap in cell biology, no prokaryotes ever gave rise directly (without endosymbiosis) to organisms even of the complexity of protozoa. The distinction between prokaryotes and eukaryotes is essentially one of degree. Prokaryotes made a start with most aspects of molecular organisation, then stopped short. The eukaryotes took up the baton and ran. Picoeukaryotes, discovered at the turn of the millennium to thrive in surprising abundance in extreme environments, such as iron-rich rivers (Amaral Zettler et al. 2002) and deep oceans (Lpez-Garca et al. 2001), are similar to prokaryotes in their size and complexity. Even so, they have a nucleus, straight chromosomes, and organelles including tiny mitochondria (Baldauf 2003). In general, however, eukaryotes are 10,000100,000 times the size of prokaryotes, and have genomes to match. Including non-coding DNA, no known prokaryote has a genome larger than about 10 Mb (megabases), whereas eukaryotes have expanded their genome size up to an extraordinary 670,000 Mb in Amoeba dubia (200 times larger than the human genome), a range of more than 4 orders of magnitude (Cavalier-Smith 1985, 2005; Gregory 2001; Cavalier-Smith and Beaton 1999). This distinction is usually ascribed to nuclear factors, such as straight chromosomes, or to the invention of meiotic sex, which has the potential to postpone mutational meltdown, and so enable an expansion in genome size (Kondrashov 1988; Ridley 2001). On the other hand, if recombination can indeed prevent mutational meltdown (and there is little evidence to show it does; Keightley and Eyre-Walker 2000; Elena and Lenski 1997), then so too in principle could lateral gene transfer in bacteria. Despite the ubiquity of lateral gene transfer, indeed perhaps because of it, as I will argue, bacteria still do not expand their genome size (Konstantinidis and Tiedje 2004; Kunin and Ouzounis 2003; Gregory 2002). Similarly, if straight chromosomes were all that was needed to enable multiple origins of replication, then why could bacteria like Borrelia burgdorferi and several species of Streptomyces, which have straight chromosomes (Fonstein and Haselkorn 1995; Baril et al. 1989), not expand their own genome sizes up to eukaryotic proportions? Indeed, other bacteria, such as Pseudomonas cepacia and Rhodobacter sphaeroides, do have multiple replicons, but still do not expand their genome size (Vellai et al. 1998; Fonstein and Haselkorn 1995). Likewise, if the accumulation of DNA in eukaryotes was linked to the proliferation of selfishly replicating elements like transposable elements and retroviruses, as some have argued (Doolittle and Sapienza 1980; Orgel and Crick 1980), then why are bacteria virtually immune to this? If the duplication of selfish DNA has the power to expand genome size, and this is an advantage (Zuckerkandl 2002), why did prokaryotes not take advantage?

Renal preservation at for 24 hours at hypothermia was studied in a rabbit model after flush cooling with sucrose-based solution (SBS), compared with a standard preservation solution (in this case, Marshall's Hypertonic Citrate solution - HCA). Polyethylene glycol supplementation to SBS (SBS-PEG) was also investigated. Renal function was measured by plasma creatinine assays during 1 months post transplantation, and pathology of the explanted kidneys was undertaken. Results showed that survival at 28 days was similar in all groups, (HCA - 3 out of 6; SBS - 2 out of 5; SBS-PEG - 3 out of 5), and there were no differences in recovery of plasma creatinine values. Histopathological evaluation of the grafts indicated that SBS preservation resulted in more severe damage after transplantation (P less than 0.05 in both corticomedullary region and medulla compared to HCA), whilst addition of PEG reduced the damage score to that seen with HCA. SBS can be used as a simple, inexpensive preservation solution for kidney cold storage provided that PEG is used as an additional colloid.

The quest for therapies based on molecular genetics (pharmacogenomics, DNA microarrays, etc.) drives pharmaceutical research into individual diseases of old age, but has failed to deliver an unequivocal clinical breakthrough. Attempts to treat most age-related diseases using antioxidant supplements have been equally disappointing, despite the clear benefits of a healthy diet. The double-agent theory is a new, unifying synthesis that draws on flaws in three leading theories of ageing. It argues that there is a tradeoff between oxidative stress as a critical redox signal that marshals genetic defences against physiological stress (such as infection) and oxidative stress as a cause of ageing and age-related disease. The stress response and ageing are linked by redox-sensitive transcription factors, such as NFkappaB. Ageing is a function of rising intracellular oxidative stress, rather than chronological time, but this relationship is obscured because free-radical leakage from mitochondria also tends to rise with age. Mitochondrial leakage produces a genetic response which mirrors that following infection, but because mitochondrial leakage is continuous the shift in gene expression is persistent, leading to the chronic inflammation characteristic of old age. Age-related diseases are thus the price we pay for redox control of stress-gene expression. Because the selective pressure favouring the stress response in youth is stronger than that penalising degenerative diseases after reproductive decline, we may be homeostatically refractory to antioxidant supplements that 'swamp' the redox switch. Furthermore, because genetic selection takes place predominantly in the reductive homeostatic environment of youth, alleles associated with age-related diseases are not inherently damaging (they do not inevitably express a negative effect over time), but are simply less effective in the oxidising conditions of old age. Gene therapies for age-related diseases are unlikely to succeed unless oxidative stress can be controlled physiologically, thereby altering the activity and function of potentially hundreds of genes.

“There is something rather than nothing because something is more stable”, wrote Victor Stenger about the universe [1]. The same applies to the origin of life; but what sort of something, and where? Thermodynamics and life itself are the surest guides. Thermodynamics, because life is more stable than non-life only under certain farfrom-equilibrium conditions. Life, because the best explanation for the appearance of appropriate catalysts, whether enzymes or ribozymes, is selection, as noted by de Duve [2]. Catalysts can be selected only if they fit into protometabolism, predicting congruence between prebiotic conditions and biochemical pathways. Research on the origin of life has been marred by deep historical rifts. Genes first or metabolism first? Panspermia or terrestrial origins? Autotrophic origins in vents or heterotrophic origins in soup? Much fine experimental work has been done, but the gaps between laboratory chemistry, geological environments, and real biochemistry in living cells are still great. Closing those gaps is the task at hand, and we might be surprisingly close. All free-living cells combine six properties – carbon capture, energy transduction, heredity, metabolism, compartmentalisation and excretion. It is doubtful whether any of these traits is much use in isolation: life originated in a thermodynamic system capable of focusing abiotic equivalents of all six living processes. To my knowledge, the only system capable of doing that is a specific type of submarine hydrothermal vent, occupied by hot alkaline solutions rich in H2, accompanied by minor bisulfide and ammonia. This kind of system is maintained in disequilibrium by spontaneously precipitated inorganic osmotic barriers containing catalytic Fe( Ni Mo)S minerals separating the alkaline solution from the ambient CO2-bearing mildly acidic ocean. The juxtaposition of fluids on either side of these barriers imposes steep redox, proton and thermal gradients. These natural electrochemical reactors have been postulated as the ideal incubators of life by Russell and colleagues for two decades [3,4]; and the discovery of a modern analogue at Lost City, just over a decade ago, provided powerful support [5,6]. Remarkably, thermodynamic calculations under mild alkaline hydrothermal conditions indicate that the synthesis of all cellular materials, including amino acids, bases, sugars and lipids, is exergonic from H2 and CO2 between 50 ◦C and 125 ◦C [7]. What has been missing from this scenario is extensive experimental work. Russell himself has built a hydrothermal reactor and is exploring the geochemical origins of biochemistry [8,9]; and others, notably Braun and colleagues in Munich, have made headway on the origins of replication via thermal cycling [10]. But a detailed chemical simulation of autotrophic origins under warm alkaline conditions is missing. The next best thing, an impressive body of experimental work, is reported in this issue of Physics of Life Reviews by Di Mauro and colleagues in Rome [11].

Eukaryotic cell respiration depends on the interactions of proteins encoded by two genomes, mitochondrial and nuclear, which evolve in radically different ways. Mitochondrial genes evolve asexually (mitochondrial DNA is generally passed from mother to offspring without recombination), unlike nuclear genes, and their mutation rate can be orders of magnitude faster than the nuclear average ( 1 ). Despite these differences, the two genomes coadapt to each other over evolutionary time ( 2 ): Mutations in one genome are offset by changes in the other, preserving respiratory function and possibly adapting it to changes in diet and climate ( 3 ). The details of selection may hold surprising implications for fitness, fertility, and aging.

The media has recently been echoing a claim that medical statistics too frequently inspire cries of 'breakthrough!' later exposed in the cold light of experience, as nothing but mirages. The arbitrary parameters and potential subjectivity of conventional medical statistics have been debated, without consensus, in most of the major journals. The question stands: do we allow conventional medical statistics to bias our interpretation of the true significance of medical research? Undoubtedly, some form of statistics is needed to help interpret the results of clinical trials. The problem is the cut-off point: what qualifies as an important finding? For years, conference halls have rung with sceptical voices questioning the clinical relevance of statistically significant data. Neither P values nor confidence intervals seem consistently reliable as a guide to clinical significance. Moreover, the assumptions underlying parametric significance tests (such as equality of variance in group comparisons) may be untrue of some studies. A systematic approach often put forward to correct for subjectivity bias (prior probability) among clinical investigators is Bayesian analysis. We are told that elimination of subjectivity by use of Bayesian inference paves the way to truly objective, evidence-based medical practice. Yet who but a statistically minded minority can begin to interpret Bayesian analysis? Reading various exchanges of letters in the medical journals on statistical models, I am struck by the fact that almost all are written by biostatisticians. The opinions of practising physicians are deafeningly silent. In our mounting enthusiasm for evidence-based medicine, we may be giving too much weight to clumsy statistical formulations that should not hinge our judgment of clinical relevance. In this article I suggest, firstly, that conventional medical statistics are better than more sophisticated alternatives since they are better understood by practising physicians; and, secondly, that the problems we have with statistics derive less from our methodology than from our stiff conventions of interpretation.

Mitochondria and the W chromosome: Low variability on the W chromosome in birds is more likely to indicate selection on mitochondrial genes

The uniparental inheritance (UPI) of mitochondria is thought to explain the evolution of two mating types or even true sexes with anisogametes. However, the exact role of UPI is not clearly understood. Here, we develop a new model, which considers the spread of UPI mutants within a biparental inheritance (BPI) population. Our model explicitly considers mitochondrial mutation and selection in parallel with the spread of UPI mutants and self-incompatible mating types. In line with earlier work, we find that UPI improves fitness under mitochondrial mutation accumulation, selfish conflict and mitonuclear coadaptation. However, we find that as UPI increases in the population its relative fitness advantage diminishes in a frequency-dependent manner. The fitness benefits of UPI ‘leak’ into the biparentally reproducing part of the population through successive matings, limiting the spread of UPI. Critically, while this process favours some degree of UPI, it neither leads to the establishment of linked mating types nor the collapse of multiple mating types to two. Only when two mating types exist beforehand can associated UPI mutants spread to fixation under the pressure of high mitochondrial mutation rate, large mitochondrial population size and selfish mutants. Variation in these parameters could account for the range of UPI actually observed in nature, from strict UPI in someChlamydomonasspecies to BPI in yeast. We conclude that UPI of mitochondria alone is unlikely to have driven the evolution of two mating types in unicellular eukaryotes.

One of the major causes of dysfunction in transplanted organs is ischaemia-reperfusion (IR) injury. Impairment of mitochondrial function is likely to be central to many of the known consequences of ischaemia; these include loss of cellular homeostasis involving a fall in intracellular pH (Fuller et al., 1988), mitochondrial calcium loading and cellular swelling (Caiman et al., 1973), accumulation of reduced pyridine nucleotides, inhibition of mitochondrial electron transfer, and a fall in ATP levels (Hardy et al., 1991). In irreversibly damaged cells, respiratory control is lost and is accompanied by oxidation of cytochromes a and a3 and NADH (Taegtmeyer et al., 1985). The latter was attributed originally to substrate deficiency (Chance and Williams, 1955) but more recent studies indicate that an enzymological defect develops resulting in an inability to metabolise NADH-linked substrates (Taegtmeyer et al., 1985 and Hardy et al., 1991). In vitro studies of the respiratory chain (RC) complexes have been made in several tissues including cardiac and renal tissue, subjected to ischaemia-reperfusion injury and it was found that complexes I and IV are major defective sites (Hardy et al., 1991 and Veitch et al., 1992). Return of function may, therefore, relate to preservation of inner mitochondrial membrane integrity, and the structure and activities of the RC complexes. The integrity of oxidative metabolic pathways and capacity to resynthesise ATP rather than the immediate post-ischaemic ATP levels appears to determine the return of function (Taegtmeyer et al., 1985).

Background All complex life on Earth is eukaryotic. All eukaryotic cells share a common ancestor that arose just once in four billion years of evolution. Prokaryotes show no tendency to evolve greater morphological complexity, despite their metabolic virtuosity. Here I argue that the eukaryotic cell originated in a unique prokaryotic endosymbiosis, a singular event that transformed the selection pressures acting on both host and endosymbiont. Results The reductive evolution and specialisation of endosymbionts to mitochondria resulted in an extreme genomic asymmetry, in which the residual mitochondrial genomes enabled the expansion of bioenergetic membranes over several orders of magnitude, overcoming the energetic constraints on prokaryotic genome size, and permitting the host cell genome to expand (in principle) over 200,000-fold. This energetic transformation was permissive, not prescriptive; I suggest that the actual increase in early eukaryotic genome size was driven by a heavy early bombardment of genes and introns from the endosymbiont to the host cell, producing a high mutation rate. Unlike prokaryotes, with lower mutation rates and heavy selection pressure to lose genes, early eukaryotes without genome-size limitations could mask mutations by cell fusion and genome duplication, as in allopolyploidy, giving rise to a proto-sexual cell cycle. The side effect was that a large number of shared eukaryotic basal traits accumulated in the same population, a sexual eukaryotic common ancestor, radically different to any known prokaryote. Conclusions The combination of massive bioenergetic expansion, release from genome-size constraints, and high mutation rate favoured a protosexual cell cycle and the accumulation of eukaryotic traits. These factors explain the unique origin of eukaryotes, the absence of true evolutionary intermediates, and the evolution of sex in eukaryotes but not prokaryotes. Reviewers This article was reviewed by: Eugene Koonin, William Martin, Ford Doolittle and Mark van der Giezen. For complete reports see the Reviewers' Comments section.

Iatrogenic injury is an unfortunate reversal of the physician's role. To cause the death or brain damage of a patient has to be the physician's worst transgression, particularly if the causes are well known, simple, and reversible. Each is true of acute postoperative hyponatraemia, but, despite repeated warnings, the condition remains common. According to a recent estimate based on prospective and retrospective studies, 20% of women who develop symptomatic hyponatraemia die or suffer serious brain damage, totalling 10 000-15 0000 cases every year in the United States and Western Europe.1 An elderly female friend of ours is a classic example. Some months ago she underwent a routine knee replacement operation. Before the operation her blood sodium concentration was 134 mmol/l—borderline hyponatraemia—attributable to her long term use of thiazide diuretics. After the operation she vomited frequently and received 6 litres of 5% dextrose saline over two days before passing into a coma. Her blood sodium concentration measured on the second day after surgery was 115 mmol/l, but electrolyte disturbance was …

Damage incurred during ischaemia and reperfusion (IR) is a major cause of dysfunction in transplanted organs. It has been shown in freshly grafted livers and kidneys that the capacity to resynthesise ATP, rather than the immediate post-ischaemic ATP level determines the return of function [I]. Return of function may therefore relate to preservation of inner mitochondrial membrane integrity during reperfusion of ischaemic tissue. In this study our objective was to correlate in vivo measurements of respiratory chain function with histological and functional parameters and explore the possibility that surface fluorimetry might be useful for predicting the likely viability of an organ before it is transplanted into patients. Chance et a2 [2] reported on direct in vivo measurements of fluorescence from intracellular pyridine nucleotides in hypoxic brain and kidney. More recently, fluorescence measurements have been made of the reduction rate from NAD' to NADH, the energetically unfavourable reverse reaction, and therefore more critically dependent upon the integrity of the respiratory chain, in livers on an ex vivo circuit. NADH can also be correlated with other respiratory chain components, such as cyt aa,, the terminal electron carrier in the respiratory chain (Fig. 1). Cyt aa, can be measured using the non-invasive method of near infra-red spectroscopy (NIRS) [ 3 ] . In this study, measurements of NADH and cyt aa, have been made in unstored and stored transplanted rabbit kidneys. The effect of sodium pentobarbitone, thought to directly inhibit complex 1, on NADH levels was determined. Female NZW rabbits (2.5 kg) were sedated with an i.m. injection of ketamine (50 mg/kg) and xylazine (8 mg/kg), trachaeotomized and artificially ventilated with a 5050 0xygen:nitrous oxide mixture for periods of up to 8 hr. Anaesthesia was maintained by continuous i.v. infusion of ketamine:xylazine (50mg: 8mglkglhr). Continuous measurements of PO,, BP, core temperature, EtCO,, FiO, and ECG were made. Intermittent blood gas samples were taken for pC@, pOz, pH, and HcT determination. After 6 hr of reperfusion, rabbits were killed with an i.v. infusion of sodium pentobarbitone (200 mglkg). In Group 1, freshly nephrectomized left kidneys were flushed with 30 ml hypertonic citrate solution (HCA) at 1-2'C, and autografted immediately into the right renal bursa using standard microsurgical techniques. In Group 2, kidneys were flushed with HCA as in Group 1 and then stored for 72 hr ((P4°C) before autografting. SF measurements were made using a Perkin Elmer LS 50 with a fibreoptic probe placed gently on the surface of the kidney. Emission spectra of NADH (400-600 nm) were obtained by excitation at 366 nm. Measurements (both discrete and continuous) were made of the intensity and rate of change of NADH emission in response to: (i) reperfusion after transplantation and (ii) infusion of sodium pentobarbitone in both groups. NIRS measurements of cyt aa, were made using a NIRO monitor (Hamamatsu), for up to 6 hr, in which optrodes were placed on either side of the kidney. Reperfusion of Group 1 kidneys resulted in oxidation of NADH by 70% to 100% in all kidneys (Table 1) and thereafter values oscillated within this range. Sodium pentobarbitone infusion resulted in rapid formation of NADH, to pre reperfusion Table 1. Come lation of Resdratorv Chain Funct ion and Motwholory

Non-invasive measurement of respiratory chain dysfunction following hypothermic renal storage and transplantation. Ischemia/reperfusion (IR) damage is a major cause of dysfunction in transplanted organs. The objective of the present study was to correlate in vivo measurements of respiratory chain (RC) function with histological and physiological parameters. Non-invasive in situ (surface fluorescence) measurements of mitochondrial NADH and near infrared spectroscopic measurements of cyt aa 3 were made in unstored (Group 1) and 72 hour stored (1 to 2°C) (Group 2) autografted rabbit kidneys. The effect of sodium pentobarbitone on NADH levels was investigated. In Group 1, there was a significant change in the redox state of cyt aa 3 in all (N = 6) kidneys on reperfusion which correlated with organ viability and increased NADH oxidation and minimal edema on histological examination. In Group 2 there was no significant change in cyt aa 3 compared to baseline, and this correlated with poor long term organ viability, slower NADH oxidation, and severe cortical edema. Pentobarbitone inhibition of the RC resulted in rapid and complete reduction of NAD + in Group 1, but none or only a slight reduction in Group 2. The results demonstrate that it might be possible in future to predict organ viability and histological changes by non-invasive measurements of RC dysfunction in the clinical transplant situation.

The nature of the host that acquired the mitochondrion at the eukaryote origin is an important microbial evolutionary issue. Modern phylogenetics indicates that the host was an archaeon. The metagenome sequence of Candidatus Lokiarchaeon has identified it as being the closest relative of the host yet known. Here, we report comparative genomic evidence indicating that Lokiarchaeon is hydrogen dependent, as one theory for the eukaryote origin-the hydrogen hypothesis-predicts for the host lineage.

Maternal inheritance of mtDNA is the rule in most animals, but the reasons for this pattern remain unclear. To investigate the consequence of overriding uniparental inheritance, we generated mice containing an admixture (heteroplasmy) of NZB and 129S6 mtDNAs in the presence of a congenic C57BL/6J nuclear background. Analysis of the segregation of the two mtDNAs across subsequent maternal generations revealed that proportion of NZB mtDNA was preferentially reduced. Ultimately, this segregation process produced NZB-129 heteroplasmic mice and their NZB or 129 mtDNA homo-plasmic counterparts. Phenotypic comparison of these three mtDNA lines demonstrated that the NZB-129 heteroplasmic mice, but neither homoplasmic counterpart, had reduced activity, food intake, respiratory exchange ratio; accentuated stress response; and cognitive impairment. Therefore, admixture of two normal but different mouse mtDNAs can be genetically unstable and can produce adverse physiological effects, factors that may explain the advantage of uniparental inheritance of mtDNA.