Can a Hybrid Line Break a Selection Limit on Behavioral Evolution in Mice?

Artificial selection yielded four replicate high runner (HR) lines of mice that reached apparent selection limits (~ threefold increase in wheel revolutions per day vs. four control lines), despite maintenance of additive genetic variance. After 68 generations, we used animal models to test for changes in additive-genetic variances and covariance of the two measured components (average speed and duration) of running distance. We also attempted to break the selection limit by crossing two HR lines, then continuing directional selection on this hybrid line and on the two parental lines for nine generations. The genetic correlation between speed and duration was positive in the base population, but evolved to be negative in the two parental HR lines. Although heritability for both speed and duration (but not distance) increased in the hybrid line, their genetic correlation remained negative. Hybrid F1 mice from generation 68 parents showed heterosis for running distance, which was lost in subsequent generations, and the hybrid line did not exceed the limit. Both male and female hybrids ran faster than parental lines for most generations, but running duration was intermediate or reduced, reflecting their negative genetic correlation. The evolved genetic trade-off between speed and duration may explain the inability for the hybrid line to break the selection limit for distance run, despite renewed additive genetic variance for at least one of its component traits.

Muscle-Tendon Unit Properties in Mice Bred for High Levels of Voluntary Running: Novel Physiologies, Coadaptation, Trade-Offs, and Multiple Solutions in the Evolution of Endurance Running

AbstractMuscle-tendon unit (MTU) morphology and physiology are likely major determinants of locomotor performance and therefore Darwinian fitness. However, the relationships between underlying traits, performance, and fitness are complicated by phenomena such as coadaptation, multiple solutions, and trade-offs. Here, we leverage a long-running artificial selection experiment in which mice have been bred for high levels of voluntary running to explore MTU adaptation, as well as the role of coadaptation, multiple solutions, and trade-offs, in the evolution of endurance running. We compared the morphological and contractile properties of the triceps surae complex, a major locomotor MTU, in four replicate selected lines to those of the triceps surae complex in four replicate control lines. All selected lines have lighter and shorter muscles, longer tendons, and faster muscle twitch times than all control lines. Absolute and normalized maximum shortening velocities and contractile endurance vary across selected lines. Selected lines have similar or lower absolute velocities and higher endurance than control lines. However, normalized shortening velocities are both higher and lower in selected lines than in control lines. These findings potentially show an interesting coadaptation between muscle and tendon morphology and muscle physiology, highlight multiple solutions for increasing endurance running performance, demonstrate that a trade-off between muscle speed and endurance can arise in response to selection, and suggest that a novel physiology may sometimes allow this trade-off to be circumvented.

Selectively breeding for high voluntary physical activity in female mice does not bestow inherent characteristics that resemble eccentric remodeling of the heart, but the mini-muscle phenotype does

Physical activity engagement results in a variety of positive health outcomes, including a reduction in cardiovascular disease risk partially due to eccentric remodeling of the heart. The purpose of this investigation was to determine if four replicate lines of High Runner mice that have been selectively bred for voluntary exercise on wheels have a cardiac phenotype that resembles the outcome of eccentric remodeling. Adult females (average age 55 days) from the 4 High Runner and 4 non-selected control lines were anaesthetized via vaporized isoflurane, then echocardiographic images were collected and analyzed for structural and functional differences. High Runner mice in general had lower ejection fractions compared to control mice lines (2-tailed p ​= ​0.023 6) and tended to have thicker walls of the anterior portion of the left ventricle (p ​= ​0.065). However, a subset of the High Runner individuals, termed mini-muscle mice, had greater ejection fraction (p ​= ​0.000 6), fractional shortening percentage (p ​< ​0.000 1), and ventricular mass at dissection (p ​< ​0.002 7 with body mass as a covariate) compared to non-mini muscle mice. Mice from replicate lines bred for high voluntary exercise did not all have inherent positive cardiac functional or structural characteristics, although a genetically unique subset of mini-muscle individuals did have greater functional cardiac characteristics, which in conjunction with their previously described peripheral aerobic enhancements (e.g., increased capillarity) would partially account for their increased V ˙ O2max.

Trade-offs in muscle physiology in selectively bred high runner mice

A trade-off between locomotor speed and endurance occurs in various taxa, and is thought to be underpinned by a muscle-level trade-off. Among four replicate high runner (HR) lines of mice, selectively bred for voluntary wheel-running behavior, a negative correlation between average running speed and time spent running has evolved. We hypothesize that this trade-off is due to changes in muscle physiology. We studied the HR lines at generation 90, at which time one line (L3) is fixed for the mini-muscle phenotype, another is polymorphic (L6) and the others (L7, L8) lack mini-muscle individuals. We used in situ preparations to quantify the contractile properties of the triceps surae muscle complex. Maximal shortening velocity varied significantly, being lowest in mini-muscle mice (L3 mini=25.2 mm s-1, L6 mini=25.5 mm s-1), highest in normal-muscle mice L6 and L8 (40.4 and 50.3 mm s-1, respectively) and intermediate in normal-muscle L7 mice (37.2 mm s-1). Endurance, measured both as the slope of the decline in force and the proportion of initial force that could be sustained, also varied significantly. The slope was shallowest in mini-muscle mice (L3 mini=-0.00348, L6 mini=-0.00238), steepest in lines L6 and L8 (-0.01676 and -0.01853), and intermediate in L7 (-0.01145). Normalized sustained force was highest in mini-muscle mice (L3 mini=0.98, L6 mini=0.92) and lowest in L8 (0.36). There were significant, negative correlations between velocity and endurance metrics, indicating a muscle-level trade-off. However, this muscle-level trade-off does not seem to underpin the organismal-level speed and endurance trade-off previously reported as the ordering of the lines is reversed: the lines that run the fastest for the least time have the lowest muscle complex velocity and highest endurance.

Mice from lines selectively bred for voluntary exercise are not more resistant to muscle injury caused by either contusion or wheel running

Muscle injury can be caused by strenuous exercise, repetitive tasks or external forces. Populations that have experienced selection for high locomotor activity may have evolutionary adaptations that resist exercise-induced injury and/or enhance the ability to cope with injury. We tested this hypothesis with an experiment in which mice are bred for high voluntary wheel running. Mice from four high runner lines run ~three times more daily distance than those from four non-selected control lines. To test recovery from injury by external forces, mice experienced contusion via weight drop on the calf. After injury, running distance and speed were reduced in high runner but not control lines, suggesting that the ability of control mice to run exceeds their motivation. To test effects of injury from exercise, mice were housed with/without wheels for six days, then trunk blood was collected and muscles evaluated for injury and regeneration. Both high runner and control mice with wheels had increased histological indicators of injury in the soleus, and increased indicators of regeneration in the plantaris. High runner mice had relatively more central nuclei (regeneration indicator) than control in the soleus, regardless of wheel access. The subset of high runner mice with the mini-muscle phenotype (characterized by greatly reduced muscle mass and type IIb fibers) had lower plasma creatine kinase (indicator of muscle injury), more markers of injury in the deep gastrocnemius, and more markers of regeneration in the deep and superficial gastrocnemius than normal-muscled individuals. Contrary to our expectations, high runner mice were not more resistant to either type of injury.

Myogenic Precursor Cells Show Faster Activation and Enhanced Differentiation in a Male Mouse Model Selected for Advanced Endurance Exercise Performance

Satellite cells (SATC), the most abundant skeletal muscle stem cells, play a main role in muscle plasticity, including the adaptive response following physical activity. Thus, we investigated how long-term phenotype selection of male mice for high running performance (Dummerstorf high Treadmill Performance; DUhTP) affects abundance, creatine kinase activity, myogenic marker expression (Pax7, MyoD), and functionality (growth kinetics, differentiation) of SATC and their progeny. SATC were isolated from sedentary male DUhTP and control (Dummerstorf Control; DUC) mice at days 12, 43, and 73 of life and after voluntary wheel running for three weeks (day 73). Marked line differences occur at days 43 and 73 (after activity). At both ages, analysis of SATC growth via xCELLigence system revealed faster activation accompanied by a higher proliferation rate and lower proportion of Pax7+ cells in DUhTP mice, indicating reduced reserve cell formation and faster transition into differentiation. Cultures from sedentary DUhTP mice contain an elevated proportion of actively proliferating Pax7+/MyoD+ cells and have a higher fusion index leading to the formation of more large and very large myotubes at day 43. This robust hypertrophic response occurs without any functional load in the donor mice. Thus, our selection model seems to recruit myogenic precursor cells/SATC with a lower activation threshold that respond more rapidly to external stimuli and are more primed for differentiation at the expense of more primitive cells.

Trade-Offs (and Constraints) in Organismal Biology

AbstractTrade-offs and constraints are inherent to life, and studies of these phenomena play a central role in both organismal and evolutionary biology. Trade-offs can be defined, categorized, and studied in at least six, not mutually exclusive, ways. (1) Allocation constraints are caused by a limited resource (e.g., energy, time, space, essential nutrients), such that increasing allocation to one component necessarily requires a decrease in another (if only two components are involved, this is referred to as the Y-model, e.g., energy devoted to size versus number of offspring). (2) Functional conflicts occur when features that enhance performance of one task decrease performance of another (e.g., relative lengths of in-levers and out-levers, force-velocity trade-offs related to muscle fiber type composition). (3) Shared biochemical pathways, often involving integrator molecules (e.g., hormones, neurotransmitters, transcription factors), can simultaneously affect multiple traits, with some effects being beneficial for one or more components of Darwinian fitness (e.g., survival, age at first reproduction, fecundity) and others detrimental. (4) Antagonistic pleiotropy describes genetic variants that increase one component of fitness (or a lower-level trait) while simultaneously decreasing another. (5) Ecological circumstances (or selective regime) may impose trade-offs, such as when foraging behavior increases energy availability yet also decreases survival. (6) Sexual selection may lead to the elaboration of (usually male) secondary sexual characters that improve mating success but handicap survival and/or impose energetic costs that reduce other fitness components. Empirical studies of trade-offs often search for negative correlations between two traits that are the expected outcomes of the trade-offs, but this will generally be inadequate if more than two traits are involved and especially for complex physiological networks of interacting traits. Moreover, trade-offs often occur only in populations that are experiencing harsh environmental conditions or energetic challenges at the extremes of phenotypic distributions, such as among individuals or species that have exceptional athletic abilities. Trade-offs may be (partially) circumvented through various compensatory mechanisms, depending on the timescale involved, ranging from acute to evolutionary. Going forward, a pluralistic view of trade-offs and constraints, combined with integrative analyses that cross levels of biological organization and traditional boundaries among disciplines, will enhance the study of evolutionary organismal biology.

Genetic Basis of Aerobically Supported Voluntary Exercise: Results from a Selection Experiment with House Mice

The biological basis of exercise behavior is increasingly relevant for maintaining healthy lifestyles. Various quantitative genetic studies and selection experiments have conclusively demonstrated substantial heritability for exercise behavior in both humans and laboratory rodents. In the "High Runner" selection experiment, four replicate lines of Mus domesticus were bred for high voluntary wheel running (HR), along with four nonselected control (C) lines. After 61 generations, the genomes of 79 mice (9-10 from each line) were fully sequenced and single nucleotide polymorphisms (SNPs) were identified. We used nested ANOVA with MIVQUE estimation and other approaches to compare allele frequencies between the HR and C lines for both SNPs and haplotypes. Approximately 61 genomic regions, across all somatic chromosomes, showed evidence of differentiation; 12 of these regions were differentiated by all methods of analysis. Gene function was inferred largely using Panther gene ontology terms and KO phenotypes associated with genes of interest. Some of the differentiated genes are known to be associated with behavior/motivational systems and/or athletic ability, including Sorl1, Dach1, and Cdh10 Sorl1 is a sorting protein associated with cholinergic neuron morphology, vascular wound healing, and metabolism. Dach1 is associated with limb bud development and neural differentiation. Cdh10 is a calcium ion binding protein associated with phrenic neurons. Overall, these results indicate that selective breeding for high voluntary exercise has resulted in changes in allele frequencies for multiple genes associated with both motivation and ability for endurance exercise, providing candidate genes that may explain phenotypic changes observed in previous studies.

Effects of voluntary exercise on spontaneous physical activity and food consumption in mice: Results from an artificial selection experiment

We evaluated the effect of voluntary exercise on spontaneous physical activity (SPA) and food consumption in mice from 4 replicate lines bred for 57 generations for high voluntary wheel running (HR) and from 4 non-selected control (C) lines. Beginning at ~24 days of age, mice were housed in standard cages or in cages with attached wheels. Wheel activity and SPA were monitored in 1-min intervals. Data from the 8th week of the experiment were analyzed because mice were sexually mature and had plateaued in body mass, weekly wheel running distance, SPA, and food consumption. Body mass, length, and masses of the retroperitoneal fat pad, liver, and heart were recorded after the 13th week. SPA of both HR and C mice decreased with wheel access, due to reductions in both duration and average intensity of SPA. However, total activity duration (SPA+wheel running; min/day) was ~1/3 greater when mice were housed with wheels, and food consumption was significantly increased. Overall, food consumption in both HR and C mice was more strongly affected by wheel running than by SPA. Duration of wheel running had a stronger effect than average speed, but the opposite was true for SPA. With body mass as a covariate, chronic wheel access significantly reduced fat pad mass and increased heart mass in both HR and C mice. Given that both HR and C mice housed with wheels had increased food consumption, the energetic cost of wheel running was not fully compensated by concomitant reductions in SPA. The experiment demonstrates that both duration and intensity of both wheel running and SPA were significant predictors of food consumption. This sort of detailed analysis of the effects of different aspects of physical activity on food consumption has not previously been reported for a non-human animal, and it sets the stage for longitudinal examination of energy balance and its components in rodent models.

IL-15 is required for postexercise induction of the pro-oxidative mediators PPARδ and SIRT1 in male mice

Physical exercise induces transient upregulation of the pro-oxidative mediators peroxisome proliferator-activated receptor-δ (PPARδ), silent information regulator of transcription (sirtuin)-1 (SIRT1), PPARγ coactivator 1α (PGC-1α), and PGC-1β in skeletal muscle. To determine the role of the cytokine IL-15 in acute postexercise induction of these molecules, expression of these factors after a bout of exhaustive treadmill running was examined in the gastrocnemius muscle of untrained control and IL-15-knockout (KO) mice. Circulating IL-15 levels increased transiently in control mice after exercise. Control mice, but not IL-15-KO mice, upregulated muscle PPARδ and SIRT1 protein after exercise, accompanied by a complex pattern of mRNA expression for these factors. However, in exhaustive exercise, control mice ran significantly longer than IL-15-KO mice. Therefore, in a second experiment, mice were limited to a 20-minute run, after which a similar pattern of induction of muscle PPARδ and SIRT1 protein by control mice only was observed. In a separate experiment, IL-15-KO mice injected systemically with recombinant IL-15 upregulated muscle PPARδ and SIRT1 mRNA within 30 minutes and also exhibited increased muscle PPARδ protein levels by 3 hours. After exercise, both control and IL-15-KO mice downregulated IL-15 receptor-α (IL-15Rα) mRNA, whereas IL-15Rα-deficient mice exhibited constitutively elevated circulating IL-15 levels. These observations indicate IL-15 release after exercise is necessary for induction of PPARδ and SIRT1 at the protein level in muscle tissue and suggest that exercise releases IL-15 normally sequestered by the IL-15Rα in the resting state. These findings could be used to develop an IL-15-based strategy to induce many of the metabolic benefits of physical exercise.

A novel intronic single nucleotide polymorphism in the myosin heavy polypeptide 4 gene is responsible for the mini-muscle phenotype characterized by major reduction in hind-limb muscle mass in mice

Replicated artificial selection for high levels of voluntary wheel running in an outbred strain of mice favored an autosomal recessive allele whose primary phenotypic effect is a 50% reduction in hind-limb muscle mass. Within the High Runner (HR) lines of mice, the numerous pleiotropic effects (e.g., larger hearts, reduced total body mass and fat mass, longer hind-limb bones) of this hypothesized adaptive allele include functional characteristics that facilitate high levels of voluntary wheel running (e.g., doubling of mass-specific muscle aerobic capacity, increased fatigue resistance of isolated muscles, longer hind-limb bones). Previously, we created a backcross population suitable for mapping the responsible locus. We phenotypically characterized the population and mapped the Minimsc locus to a 2.6-Mb interval on MMU11, a region containing ∼100 known or predicted genes. Here, we present a novel strategy to identify the genetic variant causing the mini-muscle phenotype. Using high-density genotyping and whole-genome sequencing of key backcross individuals and HR mice with and without the mini-muscle mutation, from both recent and historical generations of the HR lines, we show that a SNP representing a C-to-T transition located in a 709-bp intron between exons 11 and 12 of the Myosin heavy polypeptide 4 (Myh4) skeletal muscle gene (position 67,244,850 on MMU11; assembly, December 2011, GRCm38/mm10; ENSMUSG00000057003) is responsible for the mini-muscle phenotype, Myh4(Minimsc). Using next-generation sequencing, our approach can be extended to identify causative mutations arising in mouse inbred lines and thus offers a great avenue to overcome one of the most challenging steps in quantitative genetics.

Immune response to a Trichinella spiralis infection in house mice from lines selectively bred for high voluntary wheel running

Four lines of mice bred for high voluntary wheel running (HR lines) have high baseline circulating corticosterone levels and increased daily energy expenditure as compared with four non-selected control (C) lines. High corticosterone may suppress immune function and competing energy demands may limit ability to mount an immune response. We hypothesized that HR mice have a reduced immune response and therefore a decreased ability to fight an infection by Trichinella spiralis, an ecologically relevant nematode common in mammals. Infections have an acute, intestinal phase while the nematode is migrating, reproducing and traveling throughout the bloodstream, followed by a chronic phase with larvae encysted in muscles. Adult males (generation 55 of the selection experiment) were sham-infected or infected by oral gavage with ~300 J1 T. spiralis larvae. During the chronic phase of infection, mice were given wheel access for 6 days, followed by 2 days of maximum aerobic performance trials. Two weeks post-infection, infected HR had significantly lower circulating immunoglobulin E levels compared with infected C mice. However, we found no statistical difference between infected HR and C mice in numbers of encysted larvae within the diaphragm. As expected, both voluntary running and maximum aerobic performance were significantly higher in HR mice and lower in infected mice, with no line type-by-infection interactions. Results complement those of previous studies suggesting decreased locomotor abilities during the chronic phase of T. spiralis infection. However, despite reduced antibody production, breeding for high voluntary wheel exercise does not appear to have a substantial negative impact on general humoral function.

Functional significance of genetic variation underlying limb bone diaphyseal structure

Limb bone diaphyseal structure is frequently used to infer hominin activity levels from skeletal remains, an approach based on the well-documented ability of bone to adjust to its loading environment during life. However, diaphyseal structure is also determined in part by genetic factors. This study investigates the possibility that genetic variation underlying diaphyseal structure is influenced by the activity levels of ancestral populations and might also have functional significance in an evolutionary context. We adopted an experimental evolution approach and tested for differences in femoral diaphyseal structure in 1-week-old mice from a line that had been artificially selected (45 generations) for high voluntary wheel running and non-selected controls. As adults, selected mice are significantly more active on wheels and in home cages, and have thicker diaphyses. Structural differences at 1 week can be assumed to primarily reflect the effects of selective breeding rather than direct mechanical stimuli, given that the onset of locomotion in mice is shortly after Day 7. We hypothesized that if genetically determined diaphyseal structure reflects the activity patterns of members of a lineage, then selected animals will have relatively larger diaphyseal dimensions at 1 week compared to controls. The results provide strong support for this hypothesis and suggest that limb bone cross sections may not always only reflect the activity levels of particular fossil individuals, but also convey an evolutionary signal providing information about hominin activity in the past.

Locomotor trade-offs in mice selectively bred for high voluntary wheel running

We investigated sprint performance and running economy of a unique`mini-muscle' phenotype that evolved in response to selection for high voluntary wheel running in laboratory mice (Mus domesticus). Mice from four replicate selected (S) lines run nearly three times as far per day as four control lines. The mini-muscle phenotype, resulting from an initially rare autosomal recessive allele, has been favoured by the selection protocol,becoming fixed in one of the two S lines in which it occurred. In homozygotes,hindlimb muscle mass is halved, mass-specific muscle oxidative capacity is doubled, and the medial gastrocnemius exhibits about half the mass-specific isotonic power, less than half the mass-specific cyclic work and power, but doubled fatigue resistance. We hypothesized that mini-muscle mice would have a lower whole-animal energy cost of transport (COT), resulting from lower costs of cycling their lighter limbs, and reduced sprint speed, from reduced maximal force production. We measured sprint speed on a racetrack and slopes(incremental COT, or iCOT) and intercepts of the metabolic rate versus speed relationship during voluntary wheel running in 10 mini-muscle and 20 normal S-line females. Mini-muscle mice ran faster and farther on wheels, but for less time per day. Mini-muscle mice had significantly lower sprint speeds, indicating a functional trade-off. However,contrary to predictions, mini-muscle mice had higher COT, mainly because of higher zero-speed intercepts and postural costs (intercept–resting metabolic rate). Thus, mice with altered limb morphology after intense selection for running long distances do not necessarily run more economically.