Defence against the intergenerational cost of reproduction in males: oxidative shielding of the germline

Reproduction is expected to carry an oxidative cost, yet in many species breeders appear to sustain lower levels of oxidative damage compared to non-breeders. This paradox may be explained by considering the intergenerational costs of reproduction. Specifically, a reduction in oxidative damage upon transitioning to a reproductive state may represent a pre-emptive shielding strategy to protect the next generation from intergenerational oxidative damage (IOD) - known as the oxidative shielding hypothesis. Males may be particularly likely to transmit IOD, because sperm are highly susceptible to oxidative damage. Yet, the possibility of male-mediated IOD remains largely uninvestigated. Here, we present a conceptual and methodological framework to assess intergenerational costs of reproduction and oxidative shielding of the germline in males. We discuss variance in reproductive costs and expected payoffs of oxidative shielding according to species' life histories, and the expected impact on offspring fitness. Oxidative shielding presents an opportunity to incorporate intergenerational effects into the advancing field of life-history evolution.

Metabolic stress as a driver of life-history plasticity: flight promotes longevity and antioxidant production in monarch butterflies

Life-history theory predicts that increased investment in traits related to reproduction will be associated with a reduced ability to invest in survival or longevity. One mechanistic explanation for this trade-off is that metabolic stress generated from current fitness activities (e.g. reproduction or locomotion) will increase somatic damage, leading to reduced longevity. Yet, there has been limited support for this damage-based hypothesis. A possible explanation is that individuals can respond to increases in metabolic stress by plastically inducing cellular maintenance responses, which may increase, rather than decrease, longevity. We tested this possibility by experimentally manipulating investment in flight activity (a metabolic stressor) in the migratory monarch butterfly (Danaus plexippus), a species whose reproductive fitness is dependent on survival through a period of metabolically intensive migratory flight. Consistent with the idea that metabolic stress stimulated investment in self-maintenance, increased flight activity enhanced monarch butterfly longevity and somatic tissue antioxidant capacity, likely at a cost to reproductive investment. Our study implicates a role for metabolic stress as a driver of life-history plasticity and supports a model where current engagement in metabolically stressful activities promotes somatic survival by stimulating investment in self-maintenance processes.

Alpha-ketoglutarate as a potent regulator for lifespan and healthspan: Evidences and perspectives

Aging is a natural process that determined by a functional decline in cells and tissues as organisms are growing old, resulting in an increase at risk of disease and death. To this end, many efforts have been made to control aging and increase lifespan and healthspan. These efforts have led to the discovery of several anti-aging drugs and compounds such as rapamycin and metformin. Recently, alpha-ketoglutarate (AKG) has been introduced as a potential anti-aging metabolite that can control several functions in organisms, thereby increases longevity and improves healthspan. Unlike other synthetic anti-aging drugs, AKG is one of the metabolites of the tricarboxylic acid (TCA) cycle, also known as the Krebs cycle, and synthesized in the body. It plays a crucial role in the cell energy metabolism, amino acid/protein synthesis, epigenetic regulation, stemness and differentiation, fertility and reproductive health, and cancer cell behaviors. AKG exerts its effects through different mechanisms such as inhibiting mTOR and ATP-synthase, modulating DNA and histone demethylation and reducing ROS formation. Herein, we summarize the recent findings of AKG-related lifespan and healthspan studies and discuss AKG associated cell and molecular mechanisms involved in increasing longevity, improving reproduction, and modulating stem cells and cancer cells behavior. We also discuss the promises and limitations of AKG for delaying aging and other potential applications.

Causes and consequences of telomere lengthening in a wild vertebrate population

Telomeres have been advocated to be important markers of biological age in evolutionary and ecological studies. Telomeres usually shorten with age and shortening is frequently associated with environmental stressors and increased subsequent mortality. Telomere lengthening - an apparent increase in telomere length between repeated samples from the same individual - also occurs. However, the exact circumstances, and consequences, of telomere lengthening are poorly understood. Using longitudinal data from the Seychelles warbler (Acrocephalus sechellensis), we tested whether telomere lengthening - which occurs in adults of this species - is associated with specific stressors (reproductive effort, food availability, malarial infection and cooperative breeding) and predicts subsequent survival. In females, telomere shortening was observed under greater stress (i.e., low food availability, malaria infection), while telomere lengthening was observed in females experiencing lower stress (i.e., high food availability, assisted by helpers, without malaria). The telomere dynamics of males were not associated with the key stressors tested. These results indicate that, at least for females, telomere lengthening occurs in circumstances more conducive to self-maintenance. Importantly, both females and males with lengthened telomeres had improved subsequent survival relative to individuals that displayed unchanged, or shortened, telomeres - indicating that telomere lengthening is associated with individual fitness. These results demonstrate that telomere dynamics are bidirectionally responsive to the level of stress that an individual faces, but may poorly reflect the accumulation of stress over an individuals lifetime.

The Mystery of Energy Compensation

AbstractThe received wisdom on how activity affects energy expenditure is that the more activity is undertaken, the more calories will have been burned by the end of the day. Yet traditional hunter-gatherers, who lead physically hard lives, burn no more calories each day than Western populations living in labor-saving environments. Indeed, there is now a wealth of data, both for humans and other animals, demonstrating that long-term lifestyle changes involving increases in exercise or other physical activities do not result in commensurate increases in daily energy expenditure (DEE). This is because humans and other animals exhibit a degree of energy compensation at the organismal level, ameliorating some of the increases in DEE that would occur from the increased activity by decreasing the energy expended on other biological processes. And energy compensation can be sizable, reaching many hundreds of calories in humans. But the processes that are downregulated in the long-term to achieve energy compensation are far from clear, particularly in humans-we do not know how energy compensation is achieved. My review here of the literature on relevant exercise intervention studies, for both humans and other species, indicates conflict regarding the role, if any, of basal metabolic rate (BMR) or low-level activity such as fidgeting play, particularly once changes in body composition are factored out. In situations where BMR and low-level activity are not major components of energy compensation, what then drives it? I discuss how changes in mitochondrial efficiency and changes in circadian fluctuations in BMR may contribute to our understanding of energy management. Currently unexplored, these mechanisms and others may provide important insights into the mystery of how energy compensation is achieved.

Exercise training has morph-specific effects on telomere, body condition and growth dynamics in a color-polymorphic lizard

Alternative reproductive tactics (ARTs) are correlated suites of sexually selected traits that are likely to impose differential physiological costs on different individuals. While moderate activity might be beneficial, animals living in the wild often work at the margins of their resources and performance limits. Individuals using ARTs may have divergent capacities for activity. When pushed beyond their respective capacities, they may experience condition loss, oxidative stress, and molecular damage that must be repaired with limited resources. We used the Australian painted dragon lizard that exhibits color polymorphism as a model to experimentally test the effect of exercise on body condition, growth, reactive oxygen species (ROS) and telomere dynamics - a potential marker of stress and aging and a correlate of longevity. For most males, ROS levels tended to be lower with greater exercise; however, males with yellow throat patches - or bibs - had higher ROS levels than non-bibbed males. At the highest level of exercise, bibbed males exhibited telomere loss, while non-bibbed males gained telomere length; the opposite pattern was observed in the no-exercise controls. Growth was positively related to food intake but negatively correlated with telomere length at the end of the experiment. Body condition was not related to food intake but was positively correlated with increases in telomere length. These results, along with our previous work, suggest that aggressive - territory holding - bibbed males suffer physiological costs that may reduce longevity compared with non-bibbed males with superior postcopulatory traits.

Early predictors of female lifetime reproductive success in a solitary hibernator: evidence for "silver spoon" effect

Fitness consequences of early-life conditions remain unclear and poorly studied in mammals. Based on long-term observations of yellow ground squirrels (Spermophilus fulvus), we identified early determinants of female fitness by analyzing the effects of early-life individual and environmental characteristics (weaning weight, weight gain rate, date of natal emergence, natal litter size, location of the natal burrow, local density of juveniles, population density and precipitation in the post-weaning period) on lifetime reproductive success (LRS). We found high variation and right-skewed distribution in all five LRS components (survival to adulthood, adult lifespan, and lifetime numbers of weaned litters, weanlings, and yearling offspring). Numbers of litters, weanlings, and adult offspring were correlated with each other and increased with lifespan, confirming that longevity is a better predictor of LRS than fecundity. Survival to adulthood was the most sensitive fitness component to early conditions and was higher in females (a) with greater weaning weight, (b) born further from human settlement and (c) born at lower population density. Population density at birth was the best early predictor of all LRS components and negatively influenced adult lifespan and numbers of weanlings and yearling offspring. Early growth rate positively affected the probability of reproducing after the first hibernation and the number of offspring weaned. Such syndrome of high-quality (heavy and fast-growing) young born in a favourable environment ("a silver spoon effect") with downstream damping fitness consequences has been observed so far in only a few mammalian species.

Muscle myonuclear domain, but not oxidative stress, decreases with age in a long-lived seabird with high activity costs

In birds, many physiological parameters appear to remain constant with increasing age, showing no deterioration until 'catastrophic' mortality sets in. Given their high whole-organism metabolic rate and the importance of flight in foraging and predator avoidance, flight muscle deterioration and accumulated oxidative stress and tissue deterioration may be an important contributor to physiological senescence in wild birds. As a by-product of aerobic respiration, reactive oxygen species are produced and can cause structural damage within cells. The anti-oxidant system deters oxidative damage to macromolecules. We examined oxidative stress and muscle ultrastructure in thick-billed murres aged 8 to 37 years (N=50) in pectoralis muscle biopsies. When considered in general linear models with body mass, body size and sex, no oxidative stress parameter varied with age. In contrast, there was a decrease in myonuclear domain similar to that seen in human muscle aging. We conclude that for wild birds with very high flight activity levels, muscle ultrastructural changes may be an important contributor to demographic senescence. Such gradual, linear declines in muscle morphology may eventually contribute to 'catastrophic' failure in foraging or predator avoidance abilities, leading to demographic senescence.

Traditional Indian sports - A case-control study on Kho Kho players investigating genomic instability and oxidative stress as a function of metabolic genotypes

The beneficial effects of physical exercise regularly for overall well being, or for recreational or professional purposes are widely accepted in clinical practice and have from time immemorial been the reasons for performing traditional sports. On the contrary, there is also evidence implying increased oxidative stress and genetic damage from intensive exercising. Depending on the intensity, time, frequency and characteristics of exercises, there can be differential induction of oxidative stress and provocation of oxidation of cellular macromolecules (including DNA) and cellular dysfunction which can likely accumulate with age, physical attributes and increase the susceptibility to disease on one hand, while stimulating cell signalling pathways leading to cell adaptation and improved resistance to stress, on the other. In order to observe if continuous sports activities as in Kho Kho increase oxidation capacity, which can also provoke oxidation of cellular macromolecules, the effects on oxidative/antioxidant changes and DNA damage in professional Kho Kho players modulated by individual genetic differences were assessed. Kho Kho, a traditional Indian game of ‘Tag’, is an all-time favourite which requires endurance, agility and strength. Healthy Kho Kho players (20.27 ± 0.28 y; sports age 6.78 ± 0.52 y) and controls (20.90 ± 0.45 y) were matched for age, gender, BMI, VO₂ max (maximal oxygen uptake), frequency of GSTT1 (present/null), M1 (present/null), SOD2 (C199T) polymorphisms but differed for variant allele frequencies of GSTP1 (A313G) and SOD2 (C47T). Players compared to controls had significantly increased levels of DNA damage (1.8x, 44.66 ± 1.68 vs. 23.85 ± 1.79 μm, p = 0.000), lipid (MDA) peroxidation (2x, 1.72 ± 0.06 vs. 0.83 ± 0.16 μmol/l, p = 0.000) and total antioxidant capacity (1.09x, 1.69 ± 0.06 vs. 1.11 ± 0.03 mmol Trolox equivalent/l, p = 0.000) but with no differences for SOD activity (94.99 ± 2.42 vs. 93.36 ± 2.54 U/ml, p = 0.935). These results suggest that the players have increased genetic damage and oxidative stress probably from the intense physical activity in the absence of other exposure(s) as other attributes were comparable in the study group. The players may therefore be at increased risk for susceptibility to cancer, various diseases and precocious age-related changes and should be sensitized to health risks related to regular intensive physical exercise.

Income and capital breeding in males: energetic and physiological limitations on male mating strategies

Income and capital breeding describe two dichotomous breeding strategies that characterise the allocation of resources to reproduction. Capital breeders utilise stored endogenous resources (typically lipids) to finance reproduction, whereas income breeders use exogenous resources (typically carbohydrates). The basis for such characterisation has mainly come from studying females, yet for many species, male and female reproductive success may be determined by substantially different factors. Females allocate resources to offspring production, whereas males typically allocate resources to accessing mating opportunities, e.g. from contests or displays. The primary metabolic fuel (lipids or carbohydrates) in males appears to be dependent on the type of activity being performed (i.e. high versus low intensity or long versus short duration), rather than capital or income breeding strategy per se. Males performing sustained, long-duration effort typically utilise lipids, whereas those undergoing intense activity more often utilise carbohydrates. As a result, either fuel type can be used in either strategy. Breeding season duration can constrain strategy choice; lipids and carbohydrates can be used in short breeding season species, but only lipids provide a viable fuel source for long breeding season capital breeders. Both capital- and income-breeding males must manage their resource use during the breeding season, but capital breeders must also cope with physiological stressors associated with extended fasting. Overall, the capital-income breeding concept applies equally to male reproduction, but compared with females, there are different physical and physiological constraints that shape choice of strategy. This Commentary also highlights some key future areas that need to be investigated to further understand how capital-income breeding strategies shape male mating strategies.