Flexibility underlies differences in mitochondrial respiratory performance between migratory and non-migratory White-crowned Sparrows (Zonotrichia leucophrys)

Migration is one of the most energy-demanding behaviors observed in birds. Mitochondria are the primary source of energy used to support these long-distance movements, yet how mitochondria meet the energetic demands of migration is scarcely studied. We quantified changes in mitochondrial respiratory performance in the White-crowned Sparrow (Zonotrichia leucophrys), which has a migratory and non-migratory subspecies. We hypothesized that the long-distance migratory Gambel's subspecies (Z. l. gambelii) would show higher mitochondrial respiratory performance compared to the non-migratory Nuttall's subspecies (Z. l. nuttalli). We sampled Gambel's individuals during spring pre-migration, active fall migration, and a period with no migration or breeding (winter). We sampled Nuttall's individuals during periods coinciding with fall migration and the winter period of Gambel's annual cycle. Overall, Gambel's individuals had higher citrate synthase, a proxy for mitochondrial volume, than Nuttall's individuals. This was most pronounced prior to and during migration. We found that both OXPHOS capacity (state 3) and basal respiration (state 4) of mitochondria exhibit high seasonal flexibility within Gambel's individuals, with values highest during active migration. These values in Nuttall's individuals were most similar to Gambel's individuals in winter. Our observations indicate that seasonal changes in mitochondrial respiration play a vital role in migration energetics.

Mitochondrial stress in the spaceflight environment

Space is a challenging environment that deregulates individual homeostasis. The main external hazards associated with spaceflight include ionizing space radiation, microgravity, isolation and confinement, distance from Earth, and hostile environment. Characterizing the biological responses to spaceflight environment is essential to validate the health risks, and to develop effective protection strategies. Mitochondria energetics is a key mechanism underpinning many physiological, ecological and evolutionary processes. Moreover, mitochondrial stress can be considered one of the fundamental features of space travel. So, we attempt to synthesize key information regarding the extensive effects of spaceflight on mitochondria. In summary, mitochondria are affected by all of the five main hazards of spaceflight at multiple levels, including their morphology, respiratory function, protein, and genetics, in various tissues and organ systems. We emphasize that investigating mitochondrial biology in spaceflight conditions should become the central focus of research on the impacts of spaceflight on human health, as this approach will help resolve numerous challenges of space health and combat several health disorders associated with mitochondrial dysfunction.

Graduate Student Literature Review: Mitochondrial response to heat stress and its implications on dairy cattle bioenergetics, metabolism, and production

The dairy industry depends upon the cow's successful lactation for economic profitability. Heat stress compromises the economic sustainability of the dairy industry by reducing milk production and increasing the risk of metabolic and pathogenic disease. Heat stress alters metabolic adaptations, such as nutrient mobilization and partitioning, that support the energetic demands of lactation. Metabolically inflexible cows are unable to enlist the necessary homeorhetic shifts that provide the needed nutrients and energy for milk synthesis, thereby impairing lactation performance. Mitochondria provide the energetic foundation that enable a myriad of metabolically demanding processes, such as lactation. Changes in an animal's energy requirements are met at the cellular level through alterations in mitochondrial density and bioenergetic capacity. Mitochondria also act as central stress modulators and coordinate tissues' energetic responses to stress by integrating endocrine signals, through mito-nuclear communication, into the cellular stress response. In vitro heat insults affect mitochondria through a compromise in mitochondrial integrity, which is linked to a decrease in mitochondrial function. However, limited evidence exists linking the in vivo metabolic effects of heat stress with parameters of mitochondrial behavior and function in lactating animals. This review summarizes the literature describing the cellular and subcellular effects of heat stress, with a focus on the effect of heat stress on mitochondrial bioenergetics and cellular dysfunction in livestock. Implications for lactation performance and metabolic health are also discussed.

Understanding Patterns of Life History Trait Covariation in an Untapped Resource, the Lab Mouse

AbstractThrough artificial selection and inbreeding, strains of laboratory mice have been developed that vary in the expression of a single or suite of desired traits valuable to biomedical research. In addition to the selected trait(s), these strains also display variation in pelage color, body size, physiology, and life history. This article exploits the broad phenotypic variation across lab mouse strains to evaluate the relationships between life history and metabolism. Life history variation tends to exist along a fast-slow continuum. There has been considerable interest in understanding the ecological and evolutionary factors underlying life history variation and the physiological and metabolic processes that support them. Yet it remains unclear how these key traits scale across hierarchical levels, as ambiguous empirical support has been garnered at the intraspecific level. Within-species investigations have been thwarted by methodological constraints and environmental factors that obscure the genetic architecture underlying the hypothesized functional integration of life history and metabolic traits. In this analysis, we used the publicly available Mouse Phenome Database by the Jackson Laboratory to investigate the relationships among life history traits (e.g., body size, reproduction, and life span) and metabolic traits (e.g., daily energy expenditure and insulin-like growth factor 1 concentration). Our findings revealed significant variation in reproductive characteristics across strains of mice as well as relationships among life history and metabolic traits. We found evidence of variation along the fast-slow life history continuum, though the direction of some relationships among these traits deviated from interspecific predictions laid out in previous literature. Furthermore, our results suggest that the strength of these relationships are strongest earlier in life.

Solving the conundrum of intra-specific variation in metabolic rate: A multidisciplinary conceptual and methodological toolkit: New technical developments are opening the door to an understanding of why metabolic rate varies among individual animals of a species: New technical developments are opening the door to an understanding of why metabolic rate varies among individual animals of a species

Researchers from diverse disciplines, including organismal and cellular physiology, sports science, human nutrition, evolution and ecology, have sought to understand the causes and consequences of the surprising variation in metabolic rate found among and within individual animals of the same species. Research in this area has been hampered by differences in approach, terminology and methodology, and the context in which measurements are made. Recent advances provide important opportunities to identify and address the key questions in the field. By bringing together researchers from different areas of biology and biomedicine, we describe and evaluate these developments and the insights they could yield, highlighting the need for more standardisation across disciplines. We conclude with a list of important questions that can now be addressed by developing a common conceptual and methodological toolkit for studies on metabolic variation in animals.

How does density of the inner mitochondrial membrane influence mitochondrial performance?

Our current understanding of variation in mitochondrial performance is incomplete. The production of ATP via oxidative phosphorylation is dependent, in part, on the structure of the inner mitochondrial membrane. Morphology of the inner membrane is crucial for the formation of the proton gradient across the inner membrane and, therefore, ATP synthesis. The inner mitochondrial membrane is dynamic, changing shape and surface area. These changes alter density (amount per volume) of the inner mitochondrial membrane within the confined space of the mitochondrion. Because the number of electron transport system proteins within the inner mitochondrial membrane changes with inner mitochondrial membrane area, a change in the amount of inner membrane alters the capacity for ATP production within the organelle. This review outlines the evidence that the association between ATP synthases, inner mitochondrial membrane density, and mitochondrial density (number of mitochondria per cell) impacts ATP production by mitochondria. Furthermore, we consider possible constraints on the capacity of mitochondria to produce ATP by increasing inner mitochondrial membrane density.

The persistent effects of corticosterone administration during lactation on the physiology of maternal and offspring mitochondria

Reproduction and environmental stressors are generally thought to be associated with a cost to the individual experiencing them, but the physiological mechanisms mediating costs of reproduction and maternal effects remain poorly understood. Studies examining the effects of environmental stressors on a female's physiological state and body condition during reproduction, as well as the physiological condition of offspring, have yielded equivocal results. Mitochondrial physiology and oxidative stress have been implicated as important mediators of life-history trade-offs. The goal of this investigation was to uncover the physiological mechanisms responsible for the enhanced trade-off between self-maintenance and offspring investment when an animal is exposed to stressful conditions during reproduction. To that end, we manipulated circulating corticosterone (CORT) levels by orally supplementing lactating female mice with CORT and investigated mitochondrial physiology and oxidative stress of both the reproductive females and their young. We found that maternal CORT exposure resulted in lower litter mass at weaning, but mitochondrial performance and oxidative status of females were not impacted. We also found potential beneficial effects of maternal CORT on mitochondrial function (e.g. higher respiratory control ratio) and oxidative stress (e.g. lower reactive oxygen species production) of offspring in adulthood, suggesting that elevated maternal CORT may be a signal for early-life adversity and prepare the organism with a predictive, adaptive response to future stressors.

Flying on empty: Reduced mitochondrial function and flight capacity in food-deprived monarch butterflies

Mitochondrial function is fundamental to organismal performance, health, and fitness - especially during energetically challenging events, such as migration. With this investigation, we evaluated mitochondrial sensitivity to ecologically relevant stressors. We focused on an iconic migrant, the North American monarch butterfly (Danaus plexippus), and examined the effects of two stressors: seven days of food deprivation, and infection by the protozoan parasite Ophryocystis elektroscirrha (known to reduce survival and flight performance). We measured whole-animal resting (RMR) and peak flight metabolic rate, and mitochondrial respiration of isolated mitochondria from the flight muscles. Food deprivation reduced mass-independent RMR and peak flight metabolic rate, whereas infection did not. Fed monarchs used mainly lipids in flight (respiratory quotient 0.73), but the respiratory quotient dropped in food-deprived individuals, possibly indicating switching to alternative energy sources, such as ketone bodies. Food deprivation decreased mitochondrial maximum oxygen consumption but not basal respiration, resulting in lower respiratory control ratio (RCR). Furthermore, food deprivation decreased mitochondrial complex III activity, but increased complex IV activity. Infection did not result in any changes in these mitochondrial variables. Mitochondrial maximum respiration rate correlated positively with mass-independent RMR and flight metabolic rate, suggesting a link between mitochondria and whole-animal performance. In conclusion, low food availability negatively affects mitochondrial function and flight performance, with potential implications on migration, fitness, and population dynamics. Although previous studies have reported poor flight performance in infected monarchs, we found no differences in physiological performance, suggesting that reduced flight capacity may be due to structural differences or low energy stores.

Comparative transcriptomics reveals circadian and pluripotency networks as two pillars of longevity regulation

Mammals differ more than 100-fold in maximum lifespan. Here, we conducted comparative transcriptomics on 26 species with diverse lifespans. We identified thousands of genes with expression levels negatively or positively correlated with a species' maximum lifespan (Neg- or Pos-MLS genes). Neg-MLS genes are primarily involved in energy metabolism and inflammation. Pos-MLS genes show enrichment in DNA repair, microtubule organization, and RNA transport. Expression of Neg- and Pos-MLS genes is modulated by interventions, including mTOR and PI3K inhibition. Regulatory networks analysis showed that Neg-MLS genes are under circadian regulation possibly to avoid persistent high expression, whereas Pos-MLS genes are targets of master pluripotency regulators OCT4 and NANOG and are upregulated during somatic cell reprogramming. Pos-MLS genes are highly expressed during embryogenesis but significantly downregulated after birth. This work provides targets for anti-aging interventions by defining pathways correlating with longevity across mammals and uncovering circadian and pluripotency networks as central regulators of longevity.

A combination of red structural and pigmentary coloration in the eyespot of a copepod

While the specific mechanisms of colour production in biological systems are diverse, the mechanics of colour production are straightforward and universal. Colour is produced through the selective absorption of light by pigments, the scattering of light by nanostructures or a combination of both. When Tigriopus californicus copepods were fed a carotenoid-limited diet of yeast, their orange-red body coloration became faint, but their eyespots remained unexpectedly bright red. Raman spectroscopy indicated a clear signature of the red carotenoid pigment astaxanthin in eyespots; however, refractive index matching experiments showed that eyespot colour disappeared when placed in ethyl cinnamate, suggesting a structural origin for the red coloration. We used transmission electron microscopy to identify consecutive nanolayers of spherical air pockets that, when modelled as a single thin film layer, possess the correct periodicity to coherently scatter red light. We then performed microspectrophotometry to quantify eyespot coloration and confirmed a distinct colour difference between the eyespot and the body. The observed spectral reflectance from the eyespot matched the reflectance predicted from our models when considering the additional absorption by astaxanthin. Together, this evidence suggests the persistence of red eyespots in copepods is the result of a combination of structural and pigmentary coloration.

Postnatal expression of IGF2 is the norm in amniote vertebrates

The insulin and insulin-like signalling (IIS) network plays an important role in mediating several life-history traits, including growth, reproduction and senescence. Although insulin-like growth factors (IGFs) 1 and 2 are both key hormones in the vertebrate IIS network, research on IGF2 in juveniles and adults has been largely neglected because early biomedical research on rodents found negligible IGF2 postnatal expression. Here, we challenge this assumption and ask to what degree IGF2 is expressed during postnatal life across amniotes by quantifying the relative gene expression of IGF1 and IGF2 using publicly available RNAseq data for 82 amniote species and quantitative polymerase chain reaction on liver cDNA at embryonic, juvenile and adult stages for two lizard, bird and mouse species. We found that (i) IGF2 is expressed postnatally across amniote species and life stages-often at a higher relative expression than IGF1, contradicting rodent models; (ii) the lack of rodent postnatal IGF2 expression is due to phylogenetic placement, not inbreeding or artificial selection; and (iii) adult IGF2 expression is sex-biased in some species. Our results demonstrate that IGF2 expression is typical for amniotes throughout life, suggesting that a comprehensive understanding of the mechanisms mediating variation in life-history traits will require studies that measure both IGFs.

Short and long-term effect of reproduction on mitochondrial dynamics and autophagy in rats

We evaluated mitochondrial dynamics and autophagy by investigating the acute and long-term changes in the liver and skeletal muscle of rats in multiple reproductive stages. A total of 48 rats were used. Rats were randomly assigned to three groups (n = 16 per group): nonreproductive females; females that became pregnant, gave birth, but had their pups removed at birth, and thus, did not lactate; and females that experienced pregnancy, gave birth, and were allowed to lactate. Each group was further divided into two-time subgroups (n = 8 per subgroup) and data were collected at a time-point corresponding to 1) peak lactation (day 14 of lactation) in the lactating animals (4 months of age) and 2) 15 weeks after parturition (12 weeks post-weaning in lactating animals; 7 months of age). Levels of several proteins involved in mitochondrial dynamics and the autophagy system were measured in the liver and skeletal muscle. Beclin1 protein levels in the liver were higher in non-lactating rats two weeks after parturition, while Beclin1 protein levels were highest in 7-month-old animals that had previously experienced a standard reproductive event that included pregnancy and a full 3 week of lactation. These animals also exhibited higher protein levels of the mitochondrial fusion marker Mfn2 in the liver. In skeletal muscle, we also observed increased protein levels of the mitochondrial fission marker DRP1 in non-lactating animals compared to animals that lactated. In summary, our data provide insightful information on the mechanisms that influence liver and skeletal muscle remodeling in response to the metabolic challenges of reproduction, and lactation in particular. Autophagy remodeling and mitochondrial fusion seem to coincide with liver mass size during the lactation stage of reproduction. Our findings highlight the complex changes that occur in the liver and skeletal muscle during reproduction, and highlights the remarkable plasticity required during this demanding metabolic feat.

Development of a Mobile Mitochondrial Physiology Laboratory for Measuring Mitochondrial Energetics in the Field

Mitochondrial energetics is a central theme in animal biochemistry and physiology, with researchers using mitochondrial respiration as a metric to investigate metabolic capability. To obtain the measures of mitochondrial respiration, fresh biological samples must be used, and the entire laboratory procedure must be completed within approximately 2 h. Furthermore, multiple pieces of specialized equipment are required to perform these laboratory assays. This creates a challenge for measuring mitochondrial respiration in the tissues of wild animals living far from physiology laboratories as live tissue cannot be preserved for very long after collection in the field. Moreover, transporting live animals over long distances induces stress, which can alter mitochondrial energetics. This manuscript introduces the Auburn University (AU) MitoMobile, a mobile mitochondrial physiology laboratory that can be taken into the field and used on-site to measure mitochondrial metabolism in tissues collected from wild animals. The basic features of the mobile laboratory and the step-by-step methods for measuring isolated mitochondrial respiration rates are presented. Additionally, the data presented validate the success of outfitting the mobile mitochondrial physiology laboratory and making mitochondrial respiration measurements. The novelty of the mobile laboratory lies in the ability to drive to the field and perform mitochondrial measurements on the tissues of animals captured on site.

Graduate Student Literature Review: Mitochondrial adaptations across lactation and their molecular regulation in dairy cattle

As milk production in dairy cattle continues to increase, so do the energetic and nutrient demands on the dairy cow. Difficulties making the necessary metabolic adjustments for lactation can impair lactation performance and increase the risk of metabolic disorders. The physiological adaptations to lactation involve the mammary gland and extramammary tissues that coordinately enhance the availability of precursors for milk synthesis. Changes in whole-body metabolism and nutrient partitioning are accomplished, in part, through the bioenergetic and biosynthetic capacity of the mitochondria, providing energy and diverting important substrates, such as AA and fatty acids, to the mammary gland in support of lactation. With increased oxidative capacity and ATP production, reactive oxygen species production in mitochondria may be altered. Imbalances between oxidant production and antioxidant activity can lead to oxidative damage to cellular structures and contribute to disease. Thus, mitochondria are tasked with meeting the energy needs of the cell and minimizing oxidative stress. Mitochondrial function is regulated in concert with cellular metabolism by the nucleus. With only a small number of genes present within the mitochondrial genome, many genes regulating mitochondrial function are housed in nuclear DNA. This review describes the involvement of mitochondria in coordinating tissue-specific metabolic adaptations across lactation in dairy cattle and the current state of knowledge regarding mitochondrial-nuclear signaling pathways that regulate mitochondrial proliferation and function in response to shifting cellular energy need.

Methodological Considerations for Assessing Immune Defense in Reproductive Females

One of the key foci of ecoimmunology is understanding the physiological interactions between reproduction and immune defense. To assess an immune challenge, investigators typically measure an immune response at a predetermined time point that was selected to represent a peak response. These time points often are based on the immunological responses of nonreproductive males. Problematically, these peaks have been applied to studies quantifying immune responses of females during reproduction, despite the fact that nonreproductive males and reproductive females display fundamentally different patterns of energy expenditure. Previous work within pharmacological research has reported that the response to the commonly-used antigen keyhole limpet hemocyanin (KLH) varies among individuals and between females and males. In this heuristic analysis, we characterize antibody responses to KLH in females with varying reproductive demands (nonreproductive, lactating, concurrently lactating, and pregnant). Serum was taken from one animal per day per group and assessed for general and specific Immunoglobulins (Igs) G and M. We then used regression analysis to characterize the antibody response curves across groups. Our results demonstrate that the antibody response curve is asynchronous among females with varying maternal demands and temporally differs from the anticipated peak responses reflected in standardized protocols. These findings highlight the importance of multiple sampling points across treatment groups for a more integrative assessment of how reproductive demand alters antibody responses in females beyond a single measurement.

Integrating Mitochondrial Aerobic Metabolism into Ecology and Evolution

Biologists have long appreciated the critical role that energy turnover plays in understanding variation in performance and fitness among individuals. Whole-organism metabolic studies have provided key insights into fundamental ecological and evolutionary processes. However, constraints operating at subcellular levels, such as those operating within the mitochondria, can also play important roles in optimizing metabolism over different energetic demands and time scales. Herein, we explore how mitochondrial aerobic metabolism influences different aspects of organismal performance, such as through changing adenosine triphosphate (ATP) and reactive oxygen species (ROS) production. We consider how such insights have advanced our understanding of the mechanisms underpinning key ecological and evolutionary processes, from variation in life-history traits to adaptation to changing thermal conditions, and we highlight key areas for future research.

Effects of a Bacterial Infection on Mitochondrial Function and Oxidative Stress in a Songbird

AbstractAs a major physiological mechanism involved in cellular renewal and repair, immune function is vital to the body's capacity to support tissue maintenance and organismal survival. Because immune defenses can be energetically expensive, the activities of metabolically active organs, such as the liver, are predicted to increase during infection by most pathogens. However, some pathogens are immunosuppressive, which might reduce the metabolic capacities of select organs to suppress immune response. Mycoplasma gallisepticum (MG) is a well-known immunosuppressive bacterium that infects domestic chickens and turkeys as well as songbirds. In the house finch (Haemorhous mexicanus), which is the primary host for MG among songbird species, MG infects both the respiratory system and the conjunctiva of the eye, causing conspicuous swelling. To study the effect of a systemic bacterial infection on cellular respiration and oxidative damage in the house finch, we measured mitochondrial respiration, mitochondrial membrane potential, reactive oxygen species production, and oxidative damage in the livers of house finches that were wild caught and either infected with MG, as indicated by genetic screening for the pathogen, or free of MG infection. We observed that MG-infected house finches showed significantly lower oxidative lipid and protein damage in liver tissue compared with their uninfected counterparts. Moreover, using complex II substrates, we documented a nonsignificant trend for lower state 3 respiration of liver mitochondria in MG-infected house finches compared with uninfected house finches (P=0.07). These results are consistent with the hypothesis that MG suppresses organ function in susceptible hosts.

Ultraviolet irradiation alters the density of inner mitochondrial membrane and proportion of inter-mitochondrial junctions in copepod myocytes

The efficient production of energy via oxidative phosphorylation is essential to the growth, survival, and reproduction of eukaryotes. The behavior (position of, and communication between, mitochondria) and morphology of mitochondria play key roles in efficient energy production and are influenced by oxidative stressors such as ultraviolet (UV) radiation. We tested the hypothesis that mitochondria change their behavior and morphology to meet energetic demands of responding to changes in oxidative stress. Specifically, we predicted that UV irradiation would increase the density of inner mitochondrial membrane and proportion of inter-mitochondrial junctions to influence whole-animal metabolic rate. Using transmission electron microscopy, we found that both three and six hours of UV-A/B irradiation (0.5 W/m²) increased the proportion of inter-mitochondrial junctions (with increasing mitochondrial aspect ratio) and the density of inner mitochondrial membrane in myocytes of Tigriopus californicus copepods. Mitochondrial density increased following both irradiation treatments, but mitochondrial size decreased under the six hour treatment. Metabolic rate was maintained under three hours of irradiation but decreased following six hours of exposure. These observations demonstrate that the density of inner mitochondrial membrane and proportion of inter-mitochondrial junctions can play formative roles in maintaining whole-animal metabolic rate, and ultimately organismal performance, under exposure to an oxidative stressor.

Evaluating endoplasmic reticulum stress and unfolded protein response through the lens of ecology and evolution

Considerable progress has been made in understanding the physiological basis for variation in the life-history patterns of animals, particularly with regard to the roles of oxidative stress and hormonal regulation. However, an underappreciated and understudied area that could play a role in mediating inter- and intraspecific variation of life history is endoplasmic reticulum (ER) stress, and the resulting unfolded protein response (UPR^ER ). ER stress response and the UPR^ER maintain proteostasis in cells by reducing the intracellular load of secretory proteins and enhancing protein folding capacity or initiating apoptosis in cells that cannot recover. Proper modulation of the ER stress response and execution of the UPR^ER allow animals to respond to intracellular and extracellular stressors and adapt to constantly changing environments. ER stress responses are heritable and there is considerable individual variation in UPR^ER phenotype in animals, suggesting that ER stress and UPR^ER phenotype can be subjected to natural selection. The variation in UPR^ER phenotype presumably reflects the way animals respond to ER stress and environmental challenges. Most of what we know about ER stress and the UPR^ER in animals has either come from biomedical studies using cell culture or from experiments involving conventional laboratory or agriculturally important models that exhibit limited genetic diversity. Furthermore, these studies involve the assessment of experimentally induced qualitative changes in gene expression as opposed to the quantitative variations that occur in naturally existing populations. Almost all of these studies were conducted in controlled settings that are often quite different from the conditions animals experience in nature. Herein, we review studies that investigated ER stress and the UPR^ER in relation to key life-history traits including growth and development, reproduction, bioenergetics and physical performance, and ageing and senescence. We then ask if these studies can inform us about the role of ER stress and the UPR^ER in mediating the aforementioned life-history traits in free-living animals. We propose that there is a need to conduct experiments pertaining to ER stress and the UPR^ER in ecologically relevant settings, to characterize variation in ER stress and the UPR^ER in free-living animals, and to relate the observed variation to key life-history traits. We urge others to integrate multiple physiological systems and investigate how interactions between ER stress and oxidative stress shape life-history trade-offs in free-living animals.

Mitochondrial physiology varies with parity and body mass in the laboratory mouse (Mus musculus)

The life-history patterns that animals display are a product of their ability to maximize reproductive performance while concurrently balancing numerous metabolic demands. For example, the energetic costs of reproduction may reduce an animal's ability to support self-maintenance and longevity. In this work, we evaluated the impact of parity on mitochondrial physiology in laboratory mice. The theory of mitohormesis suggests that modest exposure to reactive oxygen species can improve performance, while high levels of exposure are damaging. Following this theory, we hypothesized that females that experienced one bout of reproduction (primiparous) would display improved mitochondrial capacity and reduced oxidative damage relative to non-reproductive (nulliparous) mice, while females that had four reproductive events (multiparous) would have lower mitochondrial performance and greater oxidative damage than both nulliparous and primiparous females. We observed that multiple reproductive events enhanced the mitochondrial respiratory capacity of liver mitochondria in females with high body mass. Four-bout females showed a positive relationship between body mass and mitochondrial capacity. In contrast, non-reproductive females showed a negative relationship between body mass and mitochondrial capacity and primiparous females had a slope that did not differ from zero. Other measured variables, too, were highly dependent on body mass, suggesting that a female's body condition has strong impacts on mitochondrial physiology. We also evaluated the relationship between how much females allocated to reproduction (cumulative mass of all young weaned) and mitochondrial function and oxidative stress in the multiparous females. We found that females that allocated more to reproduction had lower basal respiration (state 4), lower mitochondrial density, and higher protein oxidation in liver mitochondria than females that allocated less. These results suggest that, at least through their first four reproductive events, female laboratory mice may experience bioenergetic benefits from reproduction but only those females that allocated the most to reproduction appear to experience a potential cost of reproduction.

Beyond the Powerhouse: Integrating Mitonuclear Evolution, Physiology, and Theory in Comparative Biology

Eukaryotes are the outcome of an ancient symbiosis and as such, eukaryotic cells fundamentally possess two genomes. As a consequence, gene products encoded by both nuclear and mitochondrial genomes must interact in an intimate and precise fashion to enable aerobic respiration in eukaryotes. This genomic architecture of eukaryotes is proposed to necessitate perpetual coevolution between the nuclear and mitochondrial genomes to maintain coadaptation, but the presence of two genomes also creates the opportunity for intracellular conflict. In the collection of papers that constitute this symposium volume, scientists working in diverse organismal systems spanning vast biological scales address emerging topics in integrative, comparative biology in light of mitonuclear interactions.

Mitochondrial behaviour, morphology, and animal performance

We have a limited understanding of the proximate mechanisms that are responsible for the development of variation in animal performance and life-history strategies. Provided that components of an organism's successful life history - for example, mate competition, gestation, lactation, etc. - are energetically demanding, increased energy production within mitochondria is likely the foundation from which organisms are able to perform these tasks. Mitochondrial behaviour (positioning within the cell and communication between mitochondria) and morphology affect variation in energy production at the molecular, cellular, and organismal levels. Therefore, adaptations in mitochondrial behaviour and morphology that favour efficient energy production likely influence variation in animal performance. Previous work has linked greater proportions of inter-mitochondrial junctions and density of the inner mitochondrial membrane, among other traits, with increased energetic demand. Future research should focus on how inter-mitochondrial junctions and morphology of the inner mitochondrial membrane, in particular, influence animal performance in accordance with mitochondrial density, fission, and fusion.

Plumage redness signals mitochondrial function in the house finch

Carotenoid coloration is widely recognized as a signal of individual condition in various animals, but despite decades of study, the mechanisms that link carotenoid coloration to condition remain unresolved. Most birds with red feathers convert yellow dietary carotenoids to red carotenoids in an oxidation process requiring the gene encoding the putative cytochrome P450 enzyme CYP2J19. Here, we tested the hypothesis that the process of carotenoid oxidation and feather pigmentation is functionally linked to mitochondrial performance. Consistent with this hypothesis, we observed high levels of red ketolated carotenoids associated with the hepatic mitochondria of moulting wild house finches (Haemorhous mexicanus), and upon fractionation, we found the highest concentration of ketolated carotenoids in the inner mitochondrial membrane. We further found that the redness of growing feathers was positively related to the performance of liver mitochondria. Structural modelling of CYP2J19 supports a direct role of this protein in carotenoid ketolation that may be functionally linked to cellular respiration. These observations suggest that feather coloration serves as a signal of core functionality through inexorable links to cellular respiration in the mitochondria.

Maternal Dietary Protein Intake Influences Milk and Offspring Gut Microbial Diversity in a Rat (Rattus norvegicus) Model

Historically, investigators have assumed microorganisms identified in mother’s milk to be contaminants, but recent data suggest that milk microbiota may contribute to beneficial maternal effects. Microorganisms that colonize the gastrointestinal tracts of newborn mammals are derived, at least in part, from the maternal microbial population. Milk-derived microbiota is an important source of this microbial inocula and we hypothesized that the maternal diet contributes to variation in this microbial community. To evaluate the relationship between a mother’s diet and milk microbiome, we fed female rats a low- or high-protein diet and mated all individuals. Milk and cecal contents were collected from dams at peak lactation (14-day post-partum), and the bacterial composition of each community was assessed by 16S rRNA gene amplicon sequencing. Our findings revealed higher dietary protein intake decreased fecal microbial diversity but increased milk microbial and pup cecum diversity. Further, the higher dietary protein intake resulted in a greater abundance of potentially health-promoting bacteria, such as Lactobacillus spp. These data suggest that dietary protein levels contribute to significant shifts in the composition of maternal milk microbiota and that the functional consequences of these changes in microbial inocula might be biologically important and should be further explored.

Ultraviolet irradiation increases size of the first clutch but decreases longevity in a marine copepod

An important component of life history theory is understanding how natural variation arises in populations. Both endogenous and exogenous factors contribute to organism survival and reproduction, and therefore, it is important to understand how such factors are both beneficial and detrimental to population dynamics. One ecologically relevant factor that influences the life history of aquatic organisms is ultraviolet (UV) radiation. While the majority of research has focused on the potentially detrimental effects that UV radiation has on aquatic organisms, few studies have evaluated hormetic responses stimulated by radiation under select conditions. The goal of this study was to evaluate the impact of UV-A/B irradiation on life history characteristics in Tigriopus californicus copepods. After exposing copepods to UV-A/B irradiation (control, 1-, and 3-hr UV treatments at 0.5 W/m2), we measured the impact of exposure on fecundity, reproductive effort, and longevity. We found that UV irradiation increased the size of the first clutch among all reproducing females in both the 1- and 3-hr experimental groups and decreased longevity among all females that mated in the 1-hr treatment. UV irradiation had no effect on the number of clutches females produced. These findings indicate a potential benefit of UV irradiation on reproductive performance early in life, although the same exposure came at a cost to longevity.

An Ecologist’s Guide to Mitochondrial DNA Mutations and Senescence

Longevity plays a key role in the fitness of organisms, so understanding the processes that underlie variance in senescence has long been a focus of ecologists and evolutionary biologists. For decades, the performance and ultimate decline of mitochondria have been implicated in the demise of somatic tissue, but exactly why mitochondrial function declines as individual’s age has remained elusive. A possible source of decline that has been of intense debate is mutations to the mitochondrial DNA. There are two primary sources of such mutations: oxidative damage, which is widely discussed by ecologists interested in aging, and mitochondrial replication error, which is less familiar to most ecologists. The goal of this review is to introduce ecologists and evolutionary biologists to the concept of mitochondrial replication error and to review the current status of research on the relative importance of replication error in senescence. We conclude by detailing some of the gaps in our knowledge that currently make it difficult to deduce the relative importance of replication error in wild populations and encourage organismal biologists to consider this variable both when interpreting their results and as viable measure to include in their studies.

The Mitochondrial Contribution to Animal Performance, Adaptation, and Life-History Variation

Animals display tremendous variation in their rates of growth, reproductive output, and longevity. While the physiological and molecular mechanisms that underlie this variation remain poorly understood, the performance of the mitochondrion has emerged as a key player. Mitochondria not only impact the performance of eukaryotes via their capacity to produce ATP, but they also play a role in producing heat and reactive oxygen species and function as a major signaling hub for the cell. The papers included in this special issue emerged from a symposium titled "Inside the Black Box: The Mitochondrial Basis of Life-history Variation and Animal Performance." Based on studies of diverse animal taxa, three distinct themes emerged from these papers. (1) When linking mitochondrial function to components of fitness, it is crucial that mitochondrial assays are performed in conditions as close as the intracellular conditions experienced by the mitochondria in vivo. (2) Functional plasticity allows mitochondria to retain their performance, as well as that of their host, over a range of exogenous conditions, and selection on mitochondrial and nuclear-derived proteins can optimize the match between the environment and the bioenergetic capacity of the mitochondrion. Finally, (3) studies of wild and wild-derived animals suggest that mitochondria play a central role in animal performance and life history strategy. Taken as a whole, we hope that these papers will foster discussion and inspire new hypotheses and innovations that will further our understanding of the mitochondrial processes that underlie variation in life history traits and animal performance.

Prior reproduction alters how mitochondria respond to an oxidative event

An animal's pace of life is mediated by the physiological demands and stressors it experiences (e.g., reproduction) and one likely mechanism that underlies these effects is oxidative stress. Reproduction has been shown to increase or reduce oxidative stress under different conditions and modify mitochondrial performance. We hypothesized that the changes associated with reproduction can alter how animals respond to future oxidative stressors. We tested this theory by comparing the organ-specific mitochondrial response in female wild-derived house mice. Specifically, we compared mice that reproduced or were virgins to mice that were exposed to an oxidant (i.e., radiation) or not-exposed to radiation. We measured liver and skeletal muscle mitochondrial density, respiratory performance, enzyme activity, and oxidant production, as well as markers of oxidative damage to tissues. In the liver, prior reproduction prevented a radiation-induced reduction in mitochondrial density and increased mitochondrial respiratory performance. In skeletal muscle, prior reproduction resulted in a radiation-induced decline in mitochondrial density which could reduce the bioenergetic capacity of skeletal muscle mitochondria. Yet, electron transport chain complex I activity in skeletal muscle, which dropped with reproduction, returned to control levels following oxidant exposure. The results of this investigation indicate that prior reproduction alters the response of mitochondria to an oxidative challenge in an organ-specific manner. Such changes could have differential effects on future reproductive performance and risk of death.

Life History Trade-offs within the Context of Mitochondrial Hormesis

Evolutionary biologists have been interested in the negative interactions among life history traits for nearly a century, but the mechanisms that would create this negative interaction remain poorly understood. One variable that has emerged as a likely link between reproductive effort and longevity is oxidative stress. Specifically, it has been proposed that reproduction generates free radicals that cause oxidative stress and, in turn, oxidative stress damages cellular components and accelerates senescence. We propose that there is limited support for the hypothesis because reactive oxygen species (ROS), the free radicals implicated in oxidative damage, are not consistently harmful. With this review, we define the hormetic response of mitochondria to ROS, termed mitochondrial hormesis, and describe how to test for a mitohormetic response. We interpret existing data using our model and propose that experimental manipulations will further improve our knowledge of this response. Finally, we postulate how the mitohormetic response curve applies to variation in animal performance and longevity.

The effects of parity and litter size on bone metabolic activity in pregnant and lactating sows

During gestation and lactation, female mammals often mobilize endogenous nutrient reserves to meet the resource demands of offspring production. These mobilized stores include calcium, phosphorous and other minerals that are resorbed from maternal bone to facilitate rapid mineralization of offspring bones. The extent to which bone mineral is resorbed is governed by the total amount of mineral taken in from the diet, but also by the competing demands of offspring and the minimum level of bone density that a female must sustain to support self-maintenance. The maximum amount of bone that a female may mobilize is undoubtedly dependent a variety of maternal traits, including age and reproductive experience (i.e., parity). We evaluated changes in serum concentrations of biomarkers of metabolic activity (total deoxypyridinoline [tDPD] and osteocalcin [OC]) of maternal bone and its relationship to reproductive output and parity throughout pregnancy and lactation in Yorkshire sows. Litter size did not affect bone metabolism; however, serum concentrations of both tDPD and OC were significantly higher in sows with little or no reproductive experience when compared to sows that had produced at least 3 litters prior to the current reproductive bout. This suggests a shift in ability or physiological strategy to meet offspring mineral demands that is acquired or associated with reproductive experience.

High activity before breeding improves reproductive performance by enhancing mitochondrial function and biogenesis

Understanding of physiological responses of organisms is typically based on data collected during an isolated event. Although many fundamental insights have been gained from these studies, evaluating the response to a single event ignores the fact that each individual has experienced a unique set of events throughout its life that may have altered its physiology. The idea that prior experiences can influence subsequent performance is known as a carry-over effect. Carry-over effects may explain much of the variation in performance found among individuals. For example, high physical activity has been shown to improve mitochondrial respiratory function and biogenesis and reduce oxidative stress, and has been linked to improved health and longevity. In this study, we asked whether the bioenergetic differences between active and inactive individuals carry over to impact performance in a subsequent reproductive event and alter a female's reproductive outcome. Female mice that had access to a running wheel for a month before mating gave birth to a larger litter and weaned a heavier litter, indicating that high physical activity had a positive carry-over effect to reproduction. Mice that ran also displayed higher mitochondrial respiration and biogenesis with no changes in endogenous antioxidant enzymes. These results provide a mechanistic framework for how the conditions that animals experience before breeding can impact reproductive outcomes.

Changes in Metabolism, Mitochondrial Function, and Oxidative Stress Between Female Rats Under Nonreproductive and 3 Reproductive Conditions

Women who do not lactate display increased incidence of obesity, type II diabetes, and cancer. Stuebe and Rich-Edwards proposed that these effects occur because physiological changes that ensue during pregnancy are not reversed without lactation. To empirically test this hypothesis, we compared markers of metabolism, mitochondrial function, and oxidative stress between 4 groups of Sprague-Dawley rats: (1) nonreproductive (NR) rats, (2) rats killed at day 20 of gestation, (3) rats that gave birth but were not allowed to suckle their pups (nonlactating), and (4) rats that suckled their young for 14 days. Nonlactating females displayed higher body fat compared to all other groups. Peroxisome proliferator-activated receptor δ (PPARδ) in skeletal muscle and white adipose tissue of nonlactating rats was lower than the other groups. The PPARδ is associated with lipid metabolism suggesting that the higher fat mass in nonlactating females was not associated with the retention of a physiological state that was set during pregnancy but instead an independent drop in PPARδ. Relative mitochondrial respiratory function and complex activity in the liver and skeletal muscle of nonlactating mice were not predictive of higher body mass, and measures of oxidative stress displayed minimal variation between groups.

Physiological, mitochondrial, and oxidative stress differences in the presence or absence of lactation in rats

BACKGROUND

Human epidemiological data show that breastfeeding reduces the mother's probability of developing several disease conditions, including obesity and type II diabetes compared to mothers that give birth but do not breastfeed. The goal of this investigation was to characterize how lactation changes a rat's body composition, metabolism, mitochondrial function, and oxidative stress.

METHODS

Ten-week old female Sprague-Dawley rats were divided into three groups (n = 8 per group): 1) non-reproductive (NR), 2) those that were allowed to mate and give birth, but were not allowed to suckle their pups (PP), and 3) those that were allowed to mate and give birth, and suckled their young until weaning at 21 days (PL). All animals were sacrificed at a time corresponding to 7 days following the weaning of pups (i.e., day 28 postpartum).

RESULTS

The body mass of PL rats was similar to NR rats, but the body mass of PP rats was higher than NR rats. Importantly, PL rats had lower retroperitoneal white adipose tissue mass compared to both NR and PP rats. The difference in fat mass was accompanied by higher protein levels of PPARδ, SOD2, and reduced oxidative damage. Furthermore, the liver of PL rats had higher mitochondrial function with NADH-linked substrates, and higher expression of PGC-1α, PPARδ, and SOD2.

CONCLUSIONS

These acute differences observed between female rats that did and did not suckle their young could be used as the foundation for future research investigating the prolonged and sustained benefits of lactation.

A mitohormetic response to pro-oxidant exposure in the house mouse

Mitochondria are hypothesized to display a biphasic response to reactive oxygen species (ROS) exposure. In this study, we evaluated the time course changes in mitochondrial performance and oxidative stress in house mice following X-irradiation. Forty-eight mice were equally divided among six groups, including a nonirradiated control and five experimental groups that varied in time between X-ray exposure and euthanasia (1 h and 1, 4, 7, and 10 days after X-irradiation). We measured parameters associated with mitochondrial respiratory function and ROS emission from isolated liver and skeletal muscle mitochondria and levels of oxidative damage and antioxidants in liver, skeletal muscle, and heart tissues. Mitochondrial function dropped initially after X-irradiation but recovered quickly and was elevated 10 days after the exposure. Hydrogen peroxide production, lipid peroxidation, and protein carbonylation showed inverse U-shaped curves, with levels returning to control or lower than control, 10 days after X-irradiation. Enzymatic antioxidants and markers for mitochondrial biogenesis exhibited a tissue-specific response after irradiation. These data provide the first chronological description of the mitohormetic response after a mild dose of irradiation and highlight the protective response that cells display to ROS exposure. This study also provides valuable information and application for future mitochondrial and oxidative stress studies in numerous physiological settings.

Current versus future reproduction and longevity: a re-evaluation of predictions and mechanisms

Oxidative damage is predicted to be a mediator of trade-offs between current reproduction and future reproduction or survival, but most studies fail to support such predictions. We suggest that two factors underlie the equivocal nature of these findings: (1) investigators typically assume a negative linear relationship between current reproduction and future reproduction or survival, even though this is not consistently shown by empirical studies; and (2) studies often fail to target mechanisms that could link interactions between sequential life-history events. Here, we review common patterns of reproduction, focusing on the relationships between reproductive performance, survival and parity in females. Observations in a range of species show that performance between sequential reproductive events can decline, remain consistent or increase. We describe likely bioenergetic consequences of reproduction that could underlie these changes in fitness, including mechanisms that could be responsible for negative effects being ephemeral, persistent or delayed. Finally, we make recommendations for designing future studies. We encourage investigators to carefully consider additional or alternative measures of bioenergetic function in studies of life-history trade-offs. Such measures include reactive oxygen species production, oxidative repair, mitochondrial biogenesis, cell proliferation, mitochondrial DNA mutation and replication error and, importantly, a measure of the respiratory function to determine whether measured differences in bioenergetic state are associated with a change in the energetic capacity of tissues that could feasibly affect future reproduction or lifespan. More careful consideration of the life-history context and bioenergetic variables will improve our understanding of the mechanisms that underlie the life-history patterns of animals.

Change in the Lipid Transport Capacity of the Liver and Blood during Reproduction in Rats

To support the high energetic demands of reproduction, female mammals display plasticity in many physiological processes, such as the lipid transport system. Lipids support the energy demands of females during reproduction, and energy and structural demands of the developing offspring via the placenta in utero or milk during the suckling period. We hypothesized that key proteins supporting lipid transport in reproductive females will increase during pregnancy and lactation, but drop to non-reproductive levels shortly after reproduction has ended. We compared the relative protein levels of liver-type cytosolic fatty acid transporter (L-FABP_c), plasma membrane fatty acid transporter (FABPpm), fatty acid translocase (FAT/CD36) in the liver, a key site of lipid storage and synthesis, and free fatty acid transporter albumin and triglyceride transporter [represented by apolipoprotein B (apoB)] levels in serum in reproductive Sprague-Dawley rats during late pregnancy, peak-lactation, and 1-week post-lactation as well as in non-reproductive rats. We found that all lipid transporter levels were greater in pregnant rats compared to non-reproductive rats. Lactating rats also showed higher levels of FAT/CD36 and FABPpm than non-reproductive rats. Moreover, all fat transporters also dropped back to non-reproductive levels during post-lactation except for FAT/CD36. These results indicate that fat uptake and transport capacities in liver cells are elevated during late gestation and lactation. Liver lipid secretion is up-regulated during gestation but not during lactation. These data supported the plasticity of lipid transport capacities in liver and blood during reproductive stages.

Mitochondrial function and bioenergetic trade-offs during lactation in the house mouse (Mus musculus)

Energy allocation theory predicts that a lactating female should alter the energetic demands of its organ systems in a manner that maximizes nutrient allocation to reproduction while reducing nutrient use for tasks that are not vital to immediate survival. We posit that organ‐specific plasticity in the function of mitochondria plays a key role in mediating these energetic trade‐offs. The goal of this project was to evaluate mitochondrial changes that occur in response to lactation in two of the most energetically demanding organs in the body of a rodent, the liver and skeletal muscle. This work was conducted in wild‐derived house mice (Mus musculus) kept in seminatural enclosures that allow the mice to maintain a natural social structure and move within a home range size typical of wild mice. Tissues were collected from females at peak lactation and from age‐matched nonreproductive females. Mitochondrial respiration, oxidative damage, antioxidant, PGC‐1α, and uncoupling protein levels were compared between lactating and nonreproductive females. Our findings suggest that both liver and skeletal muscle downregulate specific antioxidant proteins during lactation. The liver, but not skeletal muscle, of lactating females displayed higher oxidative damage than nonreproductive females. The liver mass of lactating females increased, but the liver displayed no change in mitochondrial respiratory control ratio. Skeletal muscle mass and mitochondrial respiratory control ratio were not different between groups. However, the respiratory function of skeletal muscle did vary among lactating females as a function of stage of concurrent pregnancy, litter size, and mass of the mammary glands. The observed changes are predicted to increase the efficiency of skeletal muscle mitochondria, reducing the substrate demands of skeletal muscle during lactation. Differences between our results and prior studies highlight the role that an animals’ social and physical environment could play in how it adapts to the energetic demands of reproduction.

Female White-Footed Mice (Peromyscus leucopus) Trade Off Offspring Skeletal Quality for Self-Maintenance When Dietary Calcium Intake is Low

During gestation and lactation in mammals, calcium and other minerals are transferred from female to offspring to support skeletal ossification. To meet mineral requirements, females commonly mobilize mineral from their own skeleton to augment dietary intake. Because the fitness costs of bone loss are expected to limit the amount of endogenous mineral that females transfer to their young, the amount of mineral allocated to offspring is predicted to be influenced by the availability of mineral in the female's diet. Calcium is the most abundant element in bone, and exogenous calcium appears to be limiting for many species. Thus, we expected that females would adjust mineral allocation to offspring relative to calcium abundance in the diet. We provided breeding female white-footed mice (Peromyscus leucopus) with a low-calcium (0.1% Ca) or a standard diet (0.85% Ca) for approximately 1 year. Body mass and skeletal size of pups did not differ between diets. Relative to pups from females on the standard diet, pups from females on the low-calcium diet had less calcium and phosphorus in their femurs and humeri, less body calcium content, reduced mass of their femurs and humeri, and had femurs with a reduced width. Reproducing white-footed mice mobilize more bone when calcium intake is low; however, our results suggest that this does not completely compensate for a reduction in calcium intake. Thus, it appears that when calcium availability is low, female white-footed mice reduce the quantity of mineral allocated per offspring as a means of maintaining their own skeletal condition.

Reproduction Does Not Adversely Affect Liver Mitochondrial Respiratory Function but Results in Lipid Peroxidation and Increased Antioxidants in House Mice

Reproduction is thought to come at a cost to longevity. Based on the assumption that increased energy expenditure during reproduction is associated with increased free-radical production by mitochondria, oxidative damage has been suggested to drive this trade-off. We examined the impact of reproduction on liver mitochondrial function by utilizing post-reproductive and non-reproductive house mice (Mus musculus) living under semi-natural conditions. The age-matched post-reproductive and non-reproductive groups were compared after the reproductive females returned to a non-reproductive state, so that both groups were in the same physiological state at the time the liver was collected. Despite increased oxidative damage (p = 0.05) and elevated CuZnSOD (p = 0.002) and catalase (p = 0.04) protein levels, reproduction had no negative impacts on the respiratory function of liver mitochondria. Specifically, in a post-reproductive, maintenance state the mitochondrial coupling (i.e., respiratory control ratio) of mouse livers show no negative impacts of reproduction. In fact, there was a trend (p = 0.059) to suggest increased maximal oxygen consumption by liver mitochondria during the ADP stimulated state (i.e., state 3) in post-reproduction. These findings suggest that oxidative damage may not impair mitochondrial respiratory function and question the role of mitochondria in the trade-off between reproduction and longevity. In addition, the findings highlight the importance of quantifying the respiratory function of mitochondria in addition to measuring oxidative damage.

Relationship between maternal environment and DNA methylation patterns of estrogen receptor alpha in wild Eastern Bluebird (Sialia sialis) nestlings: a pilot study

There is mounting evidence that, across taxa, females breeding in competitive environments tend to allocate more testosterone to their offspring prenatally and these offspring typically have more aggressive and faster‐growing phenotypes. To date, no study has determined the mechanisms mediating this maternal effect's influence on offspring phenotype. However, levels of estrogen receptor alpha (ERα) gene expression are linked to differences in early growth and aggression; thus, maternal hormones may alter gene regulation, perhaps via DNA methylation, of ERα in offspring during prenatal development. We performed a pilot study to examine natural variation in testosterone allocation to offspring through egg yolks in wild Eastern Bluebirds (Sialia sialis) in varying breeding densities and percent DNA methylation of CG dinucleotides in the ERα promoter in offspring brain regions associated with growth and behavior. We hypothesized that breeding density would be positively correlated with yolk testosterone, and prenatal exposure to maternal‐derived yolk testosterone would be associated with greater offspring growth and decreased ERα promoter methylation. Yolk testosterone concentration was positively correlated with breeding density, nestling growth rate, and percent DNA methylation of one out of five investigated CpG sites (site 3) in the diencephalon ERα promoter, but none in the telencephalon (n = 10). Percent DNA methylation of diencephalon CpG site 3 was positively correlated with growth rate. These data suggest a possible role for epigenetics in mediating the effects of the maternal environment on offspring phenotype. Experimentally examining this mechanism with a larger sample size in future studies may help elucidate a prominent way in which animals respond to their environment. Further, by determining the mechanisms that mediate maternal effects, we can begin to understand the potential for the heritability of these mechanisms and the impact that maternal effects are capable of producing at an evolutionary scale.

The effect of locomotion on the mobilization of minerals from the maternal skeleton

Bone is a dynamic tissue from which minerals are deposited or withdrawn according to the body’s demand. During late pregnancy and lactation, female mammals mobilize mineral from bone to support the ossification of offspring skeleton(s). Conversely, in response to mechanical loading, minerals are deposited in bone enabling it to develop a stronger architecture. Despite their central importance to reproductive performance and skeletal integrity, the interactions between these potentially opposing forces remains poorly understood. It is possible that inter-individual differences in the loading imposed by different forms of locomotion may alter the amount of mineral mobilized during reproduction. Here, the impact of vertical versus horizontal locomotion on bone mobilization was examined during reproduction in the laboratory mouse. The vertical, or climbing, group had access to a 60-cm tower, increasing strain on their appendicular skeleton. The horizontal, or tunnel, group had access to a 100-cm tunnel, which encouraged movements within the horizontal plane. Form of locomotion did not impact the amount of bone females mobilized during reproduction or the amount of mineral females deposited in the litter, but maternal bone architecture differed between groups. The climbing group displayed more trabeculae than the tunnel group, whereas the tunnel group displayed greater cortical bone mineral density mid-shaft. Interestingly, pups born to mothers in the climbing group had a higher concentration of total body calcium at 16 days than pups of mothers in the tunnel group. As maternal total body calcium composition and the amount of calcium invested in the full litter were not different between groups, the difference in the relative calcium content of pups between groups is not suspected to reflect difference in mineral allocation. Future research should consider the impact of maternal activity on the efficiency of offspring skeletal ossification via hormones and other bioactive factors transferred in utero and in milk.

Prevalence of blood parasites in eastern versus Western house finches: are eastern birds resistant to infection?

The rapid spread of the bacterial disease, Mycoplasma gallisepticum (MG), throughout the introduced range of house finches (Carpodacus mexicanus) in eastern North America, compared to its slower spread through the native western range, has puzzled researchers and highlights the need to understand the relative differences in health state of finches from both populations. We conducted a light-microscope survey of hemoparasites in populations of finches from Arizona (within the western range) and from Alabama (within the eastern range), and compared our estimates of prevalence to published reports from house finches sampled in both ranges. Of the 33 Arizona birds examined, we recorded hematozoan infections in 16 (48.5%) individuals, compared to 1 infected Alabama bird out of 30 birds examined (3.3%). Based on independent surveys of seven western North American and five eastern North American populations of house finches the average prevalence of blood parasites in western populations is 38.8% (±17.9 SD), while the average prevalence within the eastern range is only 5.9% (±6.1 SD). The average rate of infection among all songbirds sampled in the east is 34.2% (±4.8 SD). Thus, our surveys of wild birds as well as previously published observations point to eastern house finches having a much lower prevalence of blood parasite infections than their western counterparts. Combined with the fact that eastern finches also tend to have lower rates of avian pox infections than do western birds (based on a literature review), these observations suggest that eastern birds have either strong resistance to these infections or high susceptibility and associated mortality.

The evolution of the nutrient composition of mammalian milks

1. In mammals, nutrient allocation during lactation is a critical component of maternal care as milk intake promotes juvenile growth and survival, and hence maternal and offspring fitness. 2. Milk composition varies widely across mammals and is hypothesized to have arisen via selection pressures associated with environment, diet and life history. These hypotheses have been proposed based on observations and/or cross-species comparisons that did not standardize for stage of lactation and did not consider evolutionary history of the species in analyses. 3. We conducted the largest comparative analysis of milk composition to date accounting for phylogenetic relationships among species in order to understand the selective advantage of producing milk with specific nutritional profiles. We examined four milk constituents in association with species ecology while incorporating phylogeny in analyses. 4. Phylogenetic signal was apparent for all milk constituents examined. After controlling for phylogeny, diet and relative lactation length explained the greatest amount of variation in milk composition. Several aspects of species' ecologies, including adaptation to arid environments, reproductive output and maternal body mass were not associated with milk composition after accounting for phylogeny. 5. Our results suggest that milk composition is largely a function of evolutionary history, maternal nutrient intake and duration of milk production. Arriving at these conclusions was made possible by including the evolutionary relationships among species.

Calcium availability influences litter size and sex ratio in white-footed mice (Peromyscus leucopus)

The production of offspring typically requires investment of resources derived from both the environment and maternal somatic reserves. As such, the availability of either of these types of resources has the potential to limit the degree to which resources are allocated to reproduction. Theory and empirical studies have argued that mothers modify reproductive performance relative to exogenous resource availability and maternal condition by adjusting size, number or sex of offspring produced. These relationships have classically been defined relative to availability of energy sources; however, in vertebrates, calcium also plays a critical role in offspring production, as a considerable amount of calcium is required to support the development of offspring skeleton(s). We tested whether the availability of calcium influences reproductive output by providing female white-footed mice with a low-calcium or standard diet from reproductive maturity to senescence. We then compared maternal skeletal condition and reproductive output, based on offspring mass, offspring number and litter sex ratio, between dietary treatments. Mothers on the low-calcium diet exhibited diminished skeletal condition at senescence and produced smaller and strongly female-biased litters. We show that skeletal condition and calcium intake can influence sex ratio and reproductive output following general theoretical models of resource partitioning during reproduction.

Chemical characterization of milk oligosaccharides of the island flying fox (Pteropus hypomelanus) (Chiroptera: Pteropodidae)

Although a considerable amount of information has accumulated about oligosaccharides in the milk and colostrum of representatives of various mammalian orders, nothing is so far known concerning these sugars in the milk of any bat species (order Chiroptera). In this study, we determined that the following oligosaccharides occur in milk of the island flying fox, Pteropus hypomelanus (Chiroptera: Pteropidae): Gal(α1-3)Gal(β1-4)Glc (isoglobotriose), Gal(β1-4)GlcNAc(β1-3)Gal(β1-4)Glc (lacto-N-neotetraose), Gal(β1-4)GlcNAc(β1-3)[Gal(β1-4)GlcNAc(β1-6)]Gal(β1-4)Glc (lacto-N-neohexaose) and Neu5Gc(α2-3)Gal(β1-4)Glc (3'-NGc-SL). However, lactose was found to be the dominant saccharide in this milk, as in most eutherian mammals. The biologic importance of oligosaccharides in Chiropteran milks warrants further study.